Re: why () is () and [] is [] work in other way?
In article 7xvckq4c2j@ruckus.brouhaha.com, Paul Rubin no.email@nospam.invalid wrote: Kiuhnm kiuhnm03.4t.yahoo.it writes: I can't think of a single case where 'is' is ill-defined. If I can't predict the output of print (20+30 is 30+20) # check whether addition is commutative print (20*30 is 30*20) # check whether multiplication is commutative by just reading the language definition and the code, I'd have to say is is ill-defined. The output depends whether the compiler is clever enough to realise that the outcome of the expressions is the same, such that only one object needs to be created. What is ill here is the users understanding of when it is appropriate to use is. Asking about identity of temporary objects fully under control of the compiler is just sick. Groetjes Albert -- -- Albert van der Horst, UTRECHT,THE NETHERLANDS Economic growth -- being exponential -- ultimately falters. albert@spearc.xs4all.nl =n http://home.hccnet.nl/a.w.m.van.der.horst -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
On 4/28/2012 16:18, Adam Skutt wrote: On Apr 28, 7:26 am, Kiuhnmkiuhnm03.4t.yahoo.it wrote: On 4/27/2012 19:15, Adam Skutt wrote: On Apr 27, 11:01 am, Kiuhnmkiuhnm03.4t.yahoo.itwrote: The abstraction is this: - There are primitives and objects. - Primitives are not objects. The converse is also true. - Primitives can become objects (boxing). - Two primitives x and y are equal iff x == y. - Two objects x and y are equal iff x.equals(y). - Two objects are the same object iff x == y. - If x is a primitive, then y = x is a deep copy. - If x is an object, then y = x is a shallow copy. - ... This is not an abstraction at all, but merely a poor explanation of how things work in Java. Your last statement is totally incorrect, as no copying of the object occurs whatsoever. The reference is merely reseated to refer to the new object. If you're going to chide me for ignoring the difference between the reference and the referent object, then you shouldn't ignore it either, especially in the one case where it actually matters! If we try to extend this to other languages, then it breaks down completely. With shallow copy I meant exactly that. I didn't think that my using the term with a more general meaning would cause such a reaction. It has a very strict, well-defined meaning in these contexts, especially in languages such as C++. In C++ it's called memberwise copy (see the C++ spec), which is not equivalent to Java's shallow copy, thanks to copy constructors. So you're saying that I said that Primitive constructs are references. Right... No, still wrong. You said: You have the first statement backwards. References are a primitive construct, not the other way around. The other way around is Primitive construct are references, but I never said that. What I said is correct, References are a form of primitive construct. In C, an int is a primitive but not a reference. An int* is a pointer (reference), and is also (essentially) a primitive. I never said it isn't. While true, it's still a bad way to think about what's going on. It breaks down once we add C++ / Pascal reference types to the mix, for example. ? Assignment to a C++ reference (T) effects the underlying object, not the reference itself. A reference can never be reseated once it is bound to an object. Comparing equality on two references directly is the same as comparing two values (it calls operator==). Comparing identity requires doing (x ==y), like one would do with a value. However, unlike a value, the object is not destroyed when the reference goes out of scope. Most importantly, references to base classes do not slice derived class objects, so virtual calls work correctly through references. As a result, normally the right way to think about a value is as a temporary name for an object and not worry about any of the details about how the language makes it work. If you add C++ references to Java, the abstraction breaks down too, so I don't see your point. Equality or equivalence is a relation which is: - reflexive - symmetric - transitive Everything else... is something else. Call it semi-equality, tricky-equality or whatever, but not equality, please. Sure, but then it's illegal to allow the usage of '==' with floating point numbers, which will never have these properties in any usable implementation[1]. ??? The operator == is called the equality operator. Floating-point numbers don't really obey those properties in any meaningful fashion. I say they do. Any counter-examples? The result is that portions of your view contradict others. Either we must give '==' a different name, meaning what you consider equality is irrelevant, or we must use method names like 'equals', which you find objectionable. If anything, you have that backwards. Look at Python: all variables in Python have pointer semantics, not value semantics. When everything is white, the word white becomes redundant. So the fact that everything in Python have reference semantics means that we can't stop thinking about value and reference semantics. Nope. The behavior of variables is absolutely essential to writing correct programs. If I write a program in Python that treats variables as if they were values, it will be incorrect. You misunderstood what I said. You wouldn't treat variables as if they were values because you wouldn't even know what that means and that that's even a possibility. Well, one hopes that is true. I think we have a misunderstanding over language: you said value and reference semantics when you really meant value vs. reference semantics. Ok. I've never heard an old C programmer talk about value semantics and reference semantics. When everything is a value, your world is pretty simple. Except if that were true, the comp.lang.c FAQ wouldn't have this question and answer: http://c-faq.com/ptrs/passbyref.html, and several others. That's why I said an /old/ C programmer. Much as
Re: why () is () and [] is [] work in other way?
On 4/28/2012 4:47 AM, Kiuhnm wrote: On 4/27/2012 17:39, Adam Skutt wrote: On Apr 27, 8:07 am, Kiuhnmkiuhnm03.4t.yahoo.it wrote: Useful... maybe, conceptually sound... no. Conceptually, NaN is the class of all elements which are not numbers, therefore NaN = NaN. NaN isn't really the class of all elements which aren't numbers. NaN is the result of a few specific IEEE 754 operations that cannot be computed, like 0/0, and for which there's no other reasonable substitute (e.g., infinity) for practical applications . In the real world, if we were doing the math with pen and paper, we'd stop as soon as we hit such an error. Equality is simply not defined for the operations that can produce NaN, because we don't know to perform those computations. So no, it doesn't conceptually follow that NaN = NaN, what conceptually follows is the operation is undefined because NaN causes a halt. Mathematics is more than arithmetics with real numbers. We can use FP too (we actually do that!). We can say that NaN = NaN but that's just an exception we're willing to make. We shouldn't say that the equivalence relation rules shouldn't be followed just because *sometimes* we break them. This is what programming languages ought to do if NaN is compared to anything other than a (floating-point) number: disallow the operation in the first place or toss an exception. If you do a signaling floating point comparison on IEEE floating point numbers, you do get an exception. On some FPUs, though, signaling operations are slower. On superscalar CPUs, exact floating point exceptions are tough to implement. They are done right on x86 machines, mostly for backwards compatibility. This requires an elaborate retirement unit to unwind the state of the CPU after a floating point exception. DEC Alphas didn't have that; SPARC and MIPS machines varied by model. ARM machines in their better modes do have that. Most game console FPUs do not have a full IEEE implementation. Proper language support for floating point exceptions varies with the platform. Microsoft C++ on Windows does support getting it right. (I had to deal with this once in a physics engine, where an overflow or a NaN merely indicated that a shorter time step was required.) But even there, it's an OS exception, like a signal, not a language-level exception. Other than Ada, which requires it, few languages handle such exceptions as language level exceptions. John Nagle -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
On 4/27/2012 19:15, Adam Skutt wrote: On Apr 27, 11:01 am, Kiuhnmkiuhnm03.4t.yahoo.it wrote: On 4/27/2012 1:57, Adam Skutt wrote: On Apr 26, 6:34 pm, Kiuhnmkiuhnm03.4t.yahoo.itwrote: If you understand that your 'a' is not really an object but a reference to it, everything becomes clear and you see that '==' always do the same thing. Yes, object identity is implemented almost? everywhere by comparing the value of two pointers (references)[1]. I've already said I'm not really sure how else one would go about implementing it. You might tell me that that's just an implementation detail, but when an implementation detail is easier to understand and makes more sense than the whole abstraction which is built upon it, something is seriously wrong. I'm not sure what abstraction is being built here. I think you have me confused for someone else, possibly Steven. The abstraction is this: - There are primitives and objects. - Primitives are not objects. The converse is also true. - Primitives can become objects (boxing). - Two primitives x and y are equal iff x == y. - Two objects x and y are equal iff x.equals(y). - Two objects are the same object iff x == y. - If x is a primitive, then y = x is a deep copy. - If x is an object, then y = x is a shallow copy. - ... This is not an abstraction at all, but merely a poor explanation of how things work in Java. Your last statement is totally incorrect, as no copying of the object occurs whatsoever. The reference is merely reseated to refer to the new object. If you're going to chide me for ignoring the difference between the reference and the referent object, then you shouldn't ignore it either, especially in the one case where it actually matters! If we try to extend this to other languages, then it breaks down completely. With shallow copy I meant exactly that. I didn't think that my using the term with a more general meaning would cause such a reaction. I don't agree on the other things you said, of course. The truth: - Primitives can be references. - Two primitives are equal iff x == y. - Operator '.' automatically derefences references. You have the first statement backwards. References are a primitive construct, not the other way around. So you're saying that I said that Primitive constructs are references. Right... While true, it's still a bad way to think about what's going on. It breaks down once we add C++ / Pascal reference types to the mix, for example. ? It's better to think about variables (names) and just recognize that not all variables have the same semantics. It avoids details that are irrelevant to writing actual programs and remains consistent. Maybe in your opinion. As I said, I don't agree with you. Equality or equivalence is a relation which is: - reflexive - symmetric - transitive Everything else... is something else. Call it semi-equality, tricky-equality or whatever, but not equality, please. Sure, but then it's illegal to allow the usage of '==' with floating point numbers, which will never have these properties in any usable implementation[1]. ??? So we're back to what started this tangent, and we end up needing 'equals()' methods on our classes to distinguish between the different forms of equality. That's precisely what you want to avoid. Or we can just accept that '==' doesn't always possess those properties, which is what essentially every programming language does, and call it (value) equality. As long as we don't cross incompatible meanings, it's hard to believe that this isn't the right thing to do. If anything, you have that backwards. Look at Python: all variables in Python have pointer semantics, not value semantics. When everything is white, the word white becomes redundant. So the fact that everything in Python have reference semantics means that we can't stop thinking about value and reference semantics. Nope. The behavior of variables is absolutely essential to writing correct programs. If I write a program in Python that treats variables as if they were values, it will be incorrect. You misunderstood what I said. You wouldn't treat variables as if they were values because you wouldn't even know what that means and that that's even a possibility. I've never heard an old C programmer talk about value semantics and reference semantics. When everything is a value, your world is pretty simple. In imperative languages, pointers have greater utility over value types because not all types can obey the rules for value types. For example, I don't know how to give value semantics to something like a I/O object (e.g, file, C++ fstream, C FILE), since I don't know how to create independent copies. By defining a copy constructor. Then write me a working one. I'll wait. To save yourself some time, you can start with std::fstream. Will you pay me for my time? Your problem is that you think that copy semantics requires real copying. I really don't
Re: why () is () and [] is [] work in other way?
On Sat, Apr 28, 2012 at 9:26 PM, Kiuhnm kiuhnm03.4t.yahoo...@mail.python.org wrote: Your problem is that you think that copy semantics requires real copying. I really don't see any technical difficulty in virtualizing the all thing. Copy semantics without real copying is an optimization that a program should never need to be aware of. For instance, you could have two 16GB strings share their buffers to avoid having to use 32GB of memory; but to demonstrate copy semantics, they would need to copy-on-write in some fashion. There's duplicate state but shared memory. The trouble with duplicating state of a std::fstream is that it's roughly impossible. You could perhaps simulate it with read-only file access, but when you write, somehow it has to affect the disk, and that means either you copy the file (but keep the same file name) or have both of them affect the same file (meaning we're on Borg semantics, not copying). ChrisA -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
On 4/27/2012 17:39, Adam Skutt wrote: On Apr 27, 8:07 am, Kiuhnmkiuhnm03.4t.yahoo.it wrote: Useful... maybe, conceptually sound... no. Conceptually, NaN is the class of all elements which are not numbers, therefore NaN = NaN. NaN isn't really the class of all elements which aren't numbers. NaN is the result of a few specific IEEE 754 operations that cannot be computed, like 0/0, and for which there's no other reasonable substitute (e.g., infinity) for practical applications . In the real world, if we were doing the math with pen and paper, we'd stop as soon as we hit such an error. Equality is simply not defined for the operations that can produce NaN, because we don't know to perform those computations. So no, it doesn't conceptually follow that NaN = NaN, what conceptually follows is the operation is undefined because NaN causes a halt. Mathematics is more than arithmetics with real numbers. We can use FP too (we actually do that!). We can say that NaN = NaN but that's just an exception we're willing to make. We shouldn't say that the equivalence relation rules shouldn't be followed just because *sometimes* we break them. This is what programming languages ought to do if NaN is compared to anything other than a (floating-point) number: disallow the operation in the first place or toss an exception. Any code that tries such an operation has a logic error and must be fixed. However, when comparing NaN against floating point numbers, I don't see why NaN == NaN returning false is any less conceptually correct than any other possible result. NaN's very existence implicitly declares that we're now making up the rules as we go along, so we might as well pick the simplest set of functional rules. Plus, floating point numbers violate our expectations of equality anyway, frequently in surprising ways. 0.1 + 0.1 + 0.1 == 0.3 is true with pen and paper, but likely false on your computer. Maybe wrong expectations of equality, since 0.1 (the real number) is /not/ a floating point. Don't confuse the representation of floating points with the floating point themselves. It's even potentially possible to compare two floating point variables twice and get different results each time[1]! We should look at the specification and not the single implementations. As such, we'd have this problem with defining equality even if NaN didn't exist. We must treat floating-point numbers as a special case in order to write useful working programs. This includes defining equality in a way that's different from what works for nearly every other data type. Adam [1] Due to register spilling causing intermediate rounding. This could happen with the x87 FPU since the registers were 80-bits wide but values were stored in RAM as 64-bits. This behavior is less common now, but hardly impossible. Kiuhnm -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
On Apr 28, 7:26 am, Kiuhnm kiuhnm03.4t.yahoo.it wrote: On 4/27/2012 19:15, Adam Skutt wrote: On Apr 27, 11:01 am, Kiuhnmkiuhnm03.4t.yahoo.it wrote: The abstraction is this: - There are primitives and objects. - Primitives are not objects. The converse is also true. - Primitives can become objects (boxing). - Two primitives x and y are equal iff x == y. - Two objects x and y are equal iff x.equals(y). - Two objects are the same object iff x == y. - If x is a primitive, then y = x is a deep copy. - If x is an object, then y = x is a shallow copy. - ... This is not an abstraction at all, but merely a poor explanation of how things work in Java. Your last statement is totally incorrect, as no copying of the object occurs whatsoever. The reference is merely reseated to refer to the new object. If you're going to chide me for ignoring the difference between the reference and the referent object, then you shouldn't ignore it either, especially in the one case where it actually matters! If we try to extend this to other languages, then it breaks down completely. With shallow copy I meant exactly that. I didn't think that my using the term with a more general meaning would cause such a reaction. It has a very strict, well-defined meaning in these contexts, especially in languages such as C++. So you're saying that I said that Primitive constructs are references. Right... No, still wrong. What I said is correct, References are a form of primitive construct. In C, an int is a primitive but not a reference. An int* is a pointer (reference), and is also (essentially) a primitive. While true, it's still a bad way to think about what's going on. It breaks down once we add C++ / Pascal reference types to the mix, for example. ? Assignment to a C++ reference (T) effects the underlying object, not the reference itself. A reference can never be reseated once it is bound to an object. Comparing equality on two references directly is the same as comparing two values (it calls operator==). Comparing identity requires doing (x == y), like one would do with a value. However, unlike a value, the object is not destroyed when the reference goes out of scope. Most importantly, references to base classes do not slice derived class objects, so virtual calls work correctly through references. As a result, normally the right way to think about a value is as a temporary name for an object and not worry about any of the details about how the language makes it work. Equality or equivalence is a relation which is: - reflexive - symmetric - transitive Everything else... is something else. Call it semi-equality, tricky-equality or whatever, but not equality, please. Sure, but then it's illegal to allow the usage of '==' with floating point numbers, which will never have these properties in any usable implementation[1]. ??? The operator == is called the equality operator. Floating-point numbers don't really obey those properties in any meaningful fashion. The result is that portions of your view contradict others. Either we must give '==' a different name, meaning what you consider equality is irrelevant, or we must use method names like 'equals', which you find objectionable. If anything, you have that backwards. Look at Python: all variables in Python have pointer semantics, not value semantics. When everything is white, the word white becomes redundant. So the fact that everything in Python have reference semantics means that we can't stop thinking about value and reference semantics. Nope. The behavior of variables is absolutely essential to writing correct programs. If I write a program in Python that treats variables as if they were values, it will be incorrect. You misunderstood what I said. You wouldn't treat variables as if they were values because you wouldn't even know what that means and that that's even a possibility. Well, one hopes that is true. I think we have a misunderstanding over language: you said value and reference semantics when you really meant value vs. reference semantics. I've never heard an old C programmer talk about value semantics and reference semantics. When everything is a value, your world is pretty simple. Except if that were true, the comp.lang.c FAQ wouldn't have this question and answer: http://c-faq.com/ptrs/passbyref.html, and several others. Much as you may not like it, most code doesn't care about a pointer's value, doesn't need to know anything about it, and would just as soon pretend that it doesn't exist. All it really wants is a controlled way to mutate objects in different scopes. Which is precisely why references are preferred over pointers in C++, as they're a better expression of programmer intent, and far safe as a result. Peaking under the covers in an attempt to simplify the definition of '==' is silly. As I've hopefully shown by now, it's pretty much a
Re: Python id() does not return an address [was Re: why () is () and [] is [] work in other way?]
Adam Skutt wrote: You can't treat id() as an address. Did you miss my post when I demonstrated that Jython returns IDs generated on demand, starting from 1? In general, there is *no way even in principle* to go from a Python ID to the memory location (address) of the object with that ID, because in general objects *may not even have a fixed address*. Objects in Jython don't, because the Java virtual machine can move them in memory. Yes, there is a way. You add a function deref() to the language. This is getting pretty absurd. By that logic you could say With Python, you can end all life on earth! You just add a function to the language called nuclear_winter() that remotely accesses warhead launch sites in the US and Russia, enters the appropriate launch codes, and launches the entire nuclear arsenal! -- --OKB (not okblacke) Brendan Barnwell Do not follow where the path may lead. Go, instead, where there is no path, and leave a trail. --author unknown -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
Steven D'Aprano steve+comp.lang.pyt...@pearwood.info writes:
I'm seeing code generated by the Haskell GHC compiler being 2-4 times
slower than code from the C gcc compiler, and on average using 2-3 times
as much memory (and as much as 7 times).
Alioth isn't such a great comparison, because on the one hand you get
very carefully tuned, unidiomatic code for each language; but on the
other, you're somewhat constrained by the benchmark specs. Obviously C
is not much above assembler, so you can write almost-optimal programs if
you code close enough to the metal and suffer enough. If you're talking
about coding reasonably straightforwardly, C usually does beat Haskell
(once you've debugged the core dumps...) but there are exceptions to
that.
Feel free to find your own set of benchmarks that show the opposite. I'd
be interested to see under what conditions Haskell might be faster than C.
Haskell wasn't included in this multi-way comparison, but Ocaml beat C
by a significant factor at a straightforward vector arithmetic loop,
because it didn't have to pessimize around possible pointer aliasing:
http://scienceblogs.com/goodmath/2006/11/the_c_is_efficient_language_fa.php
GHC should be able to do similar things.
Also, here's a sort of cheating Haskell example: the straightforward
Haskell Fibonacci code is slower than C, but just sprinkle in a few
parallelism keywords and run it on your quad core cpu:
http://donsbot.wordpress.com/2007/11/29/use-those-extra-cores-and-beat-c-today
Note the Haskell code in that example is using arbitrary-precision
integers while C is using int64's. Yes, you could beat the GHC speed by
writing a lot more C code to manage Posix threads, locks, etc., but in
Haskell two do two things in parallel you can just say par.
There is also work going on to support parallel listcomps (just like
regular ones but they run on multiple cores), and vector combinators
that offload the computation to a GPU. Those things are quite hard to
do in plain C, though there are some specialty libraries for it.
Finally, a less-cheating example (this is from 2007 and I think things
are even better now):
http://neilmitchell.blogspot.com/2007/07/making-haskell-faster-than-c.html
Gives a Haskell word count program
main = print . length . words = getContents
which could also be written (if the syntax looks better to you):
main = do
text - getContents
print (length (words text))
The comparison C code is:
int main() {
int i = 0;
int c, last_space = 1, this_space;
while ((c = getchar()) != EOF) {
this_space = isspace(c);
if (last_space !this_space)
i++;
last_space = this_space;
}
printf(%i\n, i);
return 0;
}
and GHC/Supero beats the C code by about 10% even though both use
getchar. The blog post explains, you could speed up the C code by
writing a rather contorted version, unrolling it into two separate
loops, one for sequences of spaces and one for non-spaces, and jumping
back and forth between the loops instead of using the last_space
variable. That is basically the code that Supero figures out how to
generate: two separate loops with transitions in the right places,
starting from very straightforward high-level input.
I'm not really good at Haskell even after fooling with it on and off for
several years now, and it certainly can't beat Python for ease-of-use
without a lot of experience. But in the hands of experts it is
incredibly powerful. It makes Python seem almost like a toy.
--
http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
On Apr 26, 10:56 pm, OKB (not okblacke) brennospamb...@nobrenspambarn.net wrote: Adam Skutt wrote: If I write a function that does a value comparison, then it should do value comparison on _every type that can be passed to it_, regardless of whether the type is a primitive or an object, whether it has value or reference semantics, and regardless of how value comparison is actually implemented. If I write some function: f(x : T, y : U) = x == y where T and U are some unknown types, then I want the function to do a value comparison for every type pairing that allows the function to compile. Likewise, if I write a function that does identity comparison, then it logically wants to do identity comparison on _every type that can be passed to it_. What you say here makes perfect sense, but also shows that you really shouldn't be using Python if you want stuff to work this way. In Python any value of any type can be passed to any function. The claims you are making about object identity and object equality are reasonable, but as you show here, to really handle them requires dragging in a huge amount of type-system baggage. So the check gets deferred to runtime, and the programmer may need to explictly throw 'NotImplemented' or something like that. Which is what happens in Python. Not type-checking arguments simply moves the burden from the language to the programmer, which is a standard consequence of moving from static to dynamic typing. Adam -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
On Thu, 26 Apr 2012 18:02:31 +0200, Kiuhnm wrote: On 4/26/2012 16:00, Adam Skutt wrote: C# and Python do have a misfeature: '==' is identity comparison only if operator== / __eq__ is not overloaded. Identity comparison and value comparison are disjoint operations, so it's entirely inappropriate to combine them. They're not disjoint, in fact one almost always implies the other (*). Python's idea is that, by default, any object is equal to itself and only itself. The fact that this is equivalent to identity comparison is just a coincidence, from a conceptual point of view. Define your terms: what do you mean by equal? The complication is that equal has many meanings. Does 1/2 equal 2/4? Well, yes, numerically, but numerical equality is not the only useful sense of equality -- not even for mathematicians! Although the convention to write 1/2 = 2/4 is too strong to discard, there are areas of mathematics where 1/2 and 2/4 are not treated as equal regardless of numerical equality. http://en.wikipedia.org/wiki/Mediant_%28mathematics%29 In Python, equal can have any meaning we like, because we can override __eq__. For most meaningful equality comparisons, we expect that X should always equal itself, even if it doesn't equal anything else, and so __eq__ defaulting to an identity comparison if you don't override it makes good sense. Some people (e.g. the creator of Eiffel, Bertrand Meyer) argue that identity should *always* imply equality (reflexivity). I disagree, but regardless, reflexivity is *almost always* the right thing to do. When it comes to equality, Python defaults to sensible behaviour. By default, any object supports equality. By default, a == a is true for any object a. If you want to define a non-reflexive type, you have to do so yourself. If you want to define a type that doesn't support equality at all, you have to do so yourself. But both use-cases are vanishingly rare, and rather troublesome to use. It would be stupid for Python to make them the default behaviour. After all, Python is a tool, not a philosophy. There's no need to force the user to start from a blank slate and define everything from first principles when you can start with the common tools you normally need, and add or subtract from it as needed. (*) nan == nan is false, but, at least conceptually, a 'NotComparable' exception should be raised instead. That wouldn't be very useful, though. NANs are comparable. By definition, NAN != x for every x. They're just not reflexive. I don't necessarily mind if the two operations have the same symbol, as long as there's some other way in-context to determine which operation is occurring. This is the case in C and C++, for example. Python's way is much much cleaner. Nope. Automatically substituting identity equality for value equality is wrong. While rare, there are legitimate reasons for the former to be True while the latter is False. There shouldn't be, to be fair. I disagree. Violating reflexivity has its uses. NANs are the classic example. Another example is if you redefine == to mean something other than equals. If your class treats == as something other than equality, there is no need for a==a to necessarily return True. -- Steven -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
On 4/27/12 12:07 AM, Paul Rubin wrote: Nobodynob...@nowhere.com writes: All practical languages have some implementation-defined behaviour, often far more problematic than Python's. The usual reason for accepting implementation-defined behavior is to enable low-level efficiency hacks written for specific machines. C and C++ are used for that sort of purpose, so they leave many things implementation-defined. Python doesn't have the same goals and should leave less up to the implementation. Java, Ada, Standard ML, etc. all try to eliminate implementation-defined behavior in the language much more than Python does. I don't have any idea why you consider that to be throwing the baby out with the bath water. I think there are two implementation-defined behaviors that are being discussed in this thread. One is that some immutable objects like the empty tuple may be interned such that all instances of them are the same object and have the same identity. This is allowed for efficiency reasons. CPython has used this freedom to good effect. The optimization is not theoretical. The other is that identities may be reused by different objects that do no coexist at the same time. This is to permit implementations where the ID is the address of the object in memory, like CPython. But other implementations with different memory models (including ones where address in memory doesn't make any sense) may forbid reuse of IDs. This allows alternative implementations like Jython and IronPython. There are specific, deliberate, practical consequences of those two implementation-defined behaviors. These are the babies that you would be throwing out. -- Robert Kern I have come to believe that the whole world is an enigma, a harmless enigma that is made terrible by our own mad attempt to interpret it as though it had an underlying truth. -- Umberto Eco -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
On Thu, 26 Apr 2012 11:31:39 -0700, John Nagle wrote: I would suggest that is raise ValueError for the ambiguous cases. If both operands are immutable, is should raise ValueError. That's the case where the internal representation of immutables shows through. You've already made this suggestion before. Unfortunately you failed to think it through: it would break *nearly all Python code*, and not just broken code. It would break code that relies on documented language features. It would break code that applies a standard Python idiom. I count at least 638 places where your suggestion would break the standard library. [steve@ando ~]$ cd /usr/local/lib/python3.2/ [steve@ando python3.2]$ grep if .* is None: *.py | wc -l 638 That's an average of four breakages per module. If this breaks a program, it was broken anyway. Incorrect. Your suggestion breaks working code for no good reason. Astonishingly, your suggestion doesn't break code that actually is broken: def spam(arg=None): if arg == None: ... actually is broken, since it doesn't correctly test for the sentinel. You can break it by passing an object which compares equal to None but isn't actually None. -- Steven -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
On 4/27/2012 13:09, Steven D'Aprano wrote: On Thu, 26 Apr 2012 18:02:31 +0200, Kiuhnm wrote: On 4/26/2012 16:00, Adam Skutt wrote: C# and Python do have a misfeature: '==' is identity comparison only if operator== / __eq__ is not overloaded. Identity comparison and value comparison are disjoint operations, so it's entirely inappropriate to combine them. They're not disjoint, in fact one almost always implies the other (*). Python's idea is that, by default, any object is equal to itself and only itself. The fact that this is equivalent to identity comparison is just a coincidence, from a conceptual point of view. Define your terms: what do you mean by equal? a and b are equal iff a = a a = b = b = a a = b and b = c = a = c If some of this properties are violated, we're talking of something else. The fact that you can define '==' whatever way you want is irrelevant. I call that calling 'equality' something which shouldn't be regarded as such and making Python comply with it anyway. The complication is that equal has many meanings. Does 1/2 equal 2/4? Well, yes, numerically, but numerical equality is not the only useful sense of equality -- not even for mathematicians! Although the convention to write 1/2 = 2/4 is too strong to discard, there are areas of mathematics where 1/2 and 2/4 are not treated as equal regardless of numerical equality. http://en.wikipedia.org/wiki/Mediant_%28mathematics%29 In Python, equal can have any meaning we like, because we can override __eq__. For most meaningful equality comparisons, we expect that X should always equal itself, even if it doesn't equal anything else, and so __eq__ defaulting to an identity comparison if you don't override it makes good sense. Some people (e.g. the creator of Eiffel, Bertrand Meyer) argue that identity should *always* imply equality (reflexivity). I disagree, but regardless, reflexivity is *almost always* the right thing to do. When it comes to equality, Python defaults to sensible behaviour. By default, any object supports equality. By default, a == a is true for any object a. If you want to define a non-reflexive type, you have to do so yourself. If you want to define a type that doesn't support equality at all, you have to do so yourself. But both use-cases are vanishingly rare, and rather troublesome to use. It would be stupid for Python to make them the default behaviour. After all, Python is a tool, not a philosophy. There's no need to force the user to start from a blank slate and define everything from first principles when you can start with the common tools you normally need, and add or subtract from it as needed. (*) nan == nan is false, but, at least conceptually, a 'NotComparable' exception should be raised instead. That wouldn't be very useful, though. NANs are comparable. By definition, NAN != x for every x. They're just not reflexive. As I said, I know they are but I think they shouldn't, at least conceptually. I don't necessarily mind if the two operations have the same symbol, as long as there's some other way in-context to determine which operation is occurring. This is the case in C and C++, for example. Python's way is much much cleaner. Nope. Automatically substituting identity equality for value equality is wrong. While rare, there are legitimate reasons for the former to be True while the latter is False. There shouldn't be, to be fair. I disagree. Violating reflexivity has its uses. NANs are the classic example. Useful... maybe, conceptually sound... no. Conceptually, NaN is the class of all elements which are not numbers, therefore NaN = NaN. The conceptually correct way would be to check for 'NaN' explicitly. Another example is if you redefine == to mean something other than equals. If your class treats == as something other than equality, there is no need for a==a to necessarily return True. Then it wouldn't be equality anymore. I can always call a cat 'dog'. Kiuhnm -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
On Fri, Apr 27, 2012 at 10:07 PM, Kiuhnm kiuhnm03.4t.yahoo...@mail.python.org wrote: Conceptually, NaN is the class of all elements which are not numbers, therefore NaN = NaN. The conceptually correct way would be to check for 'NaN' explicitly. Conceptually, single-digit-numbers is the class of all elements which are integers [0,10). Does that mean that SdN = SdN, and therefore that 2 = 5? ChrisA -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
On 4/27/2012 14:07, Kiuhnm wrote: On 4/27/2012 13:09, Steven D'Aprano wrote: On Thu, 26 Apr 2012 18:02:31 +0200, Kiuhnm wrote: On 4/26/2012 16:00, Adam Skutt wrote: C# and Python do have a misfeature: '==' is identity comparison only if operator== / __eq__ is not overloaded. Identity comparison and value comparison are disjoint operations, so it's entirely inappropriate to combine them. They're not disjoint, in fact one almost always implies the other (*). Python's idea is that, by default, any object is equal to itself and only itself. The fact that this is equivalent to identity comparison is just a coincidence, from a conceptual point of view. Define your terms: what do you mean by equal? a and b are equal iff Nope. What I meant is that we can talk of equality whenever... a = a a = b = b = a a = b and b = c = a = c If some of this properties are violated, we're talking of something else. Kiuhnm -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
On Fri, 27 Apr 2012 14:17:48 +0200, Kiuhnm wrote: Define your terms: what do you mean by equal? a and b are equal iff Nope. What I meant is that we can talk of equality whenever... a = a a = b = b = a a = b and b = c = a = c If some of this properties are violated, we're talking of something else. Sorry, that won't do it. You haven't defined equality, or given any way of deciding whether two entities are equal. What you have listed are three *properties* of equality, namely: - reflexivity (a = a) - symmetry (if a = b then b = a) - transitivity (if a = b and b = c then a = c) But those three properties apply to any equivalence relation, not just equality. Examples: both are odd (of integers) have the same birthday (of people) is congruent to (of triangles) is in the same tax bracket (of tax payers) has the same length (of pieces of string) both contain chocolate (of cakes) For example, if we define the operator ~ to mean has the same genes (to be precise: the same genotype), then if Fred and Barney are identical twins we have: Fred ~ Fred Fred ~ Barney and Barney ~ Fred Identical triplets are rare (at least among human beings), but if we clone Barney to get George, then we also have: Fred ~ Barney and Barney ~ George = Fred ~ George. So have the same genes meets all your conditions for equality, but isn't equality: the three brothers are very different. Fred lost his arm in a car crash, Barney is a hopeless alcoholic, and George is forty years younger than his two brothers. -- Steven -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
On 4/27/2012 1:57, Adam Skutt wrote: On Apr 26, 6:34 pm, Kiuhnmkiuhnm03.4t.yahoo.it wrote: On 4/26/2012 20:54, Adam Skutt wrote: On Apr 26, 12:02 pm, Kiuhnmkiuhnm03.4t.yahoo.itwrote: On 4/26/2012 16:00, Adam Skutt wrote: On Apr 26, 9:37 am, Kiuhnmkiuhnm03.4t.yahoo.it wrote: The fact that you think that that's differing behaviour is what makes it a misfeature. The fact that you think that '==' can take objects as operands confirms that Java *does* confuse programmers. The equality operator can absolutely be used between two objects. Try it if you don't believe me. It always does identity comparison when given two objects. It can also be given two primitives, and in this case, it does value comparison. Despite performing different operations with the same symbol, there's little risk of confusion because I can trivially figure out if a variable is an object or an primitive. No, it can't be used between objects but only between primitives and references (which should be regarded as primitives, by the way). The only way to access an object is through a reference. If you understand that your 'a' is not really an object but a reference to it, everything becomes clear and you see that '==' always do the same thing. Yes, object identity is implemented almost? everywhere by comparing the value of two pointers (references)[1]. I've already said I'm not really sure how else one would go about implementing it. You might tell me that that's just an implementation detail, but when an implementation detail is easier to understand and makes more sense than the whole abstraction which is built upon it, something is seriously wrong. I'm not sure what abstraction is being built here. I think you have me confused for someone else, possibly Steven. The abstraction is this: - There are primitives and objects. - Primitives are not objects. The converse is also true. - Primitives can become objects (boxing). - Two primitives x and y are equal iff x == y. - Two objects x and y are equal iff x.equals(y). - Two objects are the same object iff x == y. - If x is a primitive, then y = x is a deep copy. - If x is an object, then y = x is a shallow copy. - ... The truth: - Primitives can be references. - Two primitives are equal iff x == y. - Operator '.' automatically derefences references. So Java is just C++ plus Garbage colletion plus some automatic dereferencing minus a lot of other things. On the other hand, Python regards everything as an object and that's the way we should go, in my opinion. Some objects are mutable and other are immutable. This distinction doesn't mess with references. Therefore, modifying the behavior of 'is' would be, in my opinion, a huge step back. You're missing the big picture. The two comparisons are asking different questions: Value equality asks if the operands 'have the same state' regardless of how they exist in memory. Identity equality asks if the two operands are the same block of memory. The two are distinct because not all types support both operations. If I write a function that does a value comparison, then it should do value comparison on _every type that can be passed to it_, regardless of whether the type is a primitive or an object, whether it has value or reference semantics, and regardless of how value comparison is actually implemented. If I write some function: f(x : T, y : U) = x == y where T and U are some unknown types, then I want the function to do a value comparison for every type pairing that allows the function to compile. Likewise, if I write a function that does identity comparison, then it logically wants to do identity comparison on _every type that can be passed to it_. Value comparison and identity comparison is a terminology that I don't agree on. Equality or equivalence is a relation which is: - reflexive - symmetric - transitive Everything else... is something else. Call it semi-equality, tricky-equality or whatever, but not equality, please. It's only natural that Python defaults to the Identity relation which is the (set-)intersection of all possible equality relations. To accomplish this, I must have a distinct way of asking each question. In Python we have '==' and 'is'[2]; in Java we have 'Object.equals()' and '=='; in C and C++ we distinguish by the types of the variables being compared (T and T*). Java gives '==' a different meaning for primitive types, but that turns out to be OK because I can't write a function that takes both a primitive type and a reference type at the same position. And as a side effect some integers are primitives and other are objects. Yes, the reason it does this is due to what I said above, but that doesn't have any bearing on why we pick one operation over the other as programmers. The distinction between primitives and objects is unfortunate. It is as if Java tried to get rid of pointers but never completely succeeded in doing that. It's the
Re: why () is () and [] is [] work in other way?
On 4/27/2012 16:07, Steven D'Aprano wrote: On Fri, 27 Apr 2012 14:17:48 +0200, Kiuhnm wrote: Define your terms: what do you mean by equal? a and b are equal iff Nope. What I meant is that we can talk of equality whenever... a = a a = b = b = a a = b and b = c = a = c If some of this properties are violated, we're talking of something else. Sorry, that won't do it. You haven't defined equality, or given any way of deciding whether two entities are equal. What you have listed are three *properties* of equality, namely: - reflexivity (a = a) - symmetry (if a = b then b = a) - transitivity (if a = b and b = c then a = c) But those three properties apply to any equivalence relation, not just equality. Examples: But that's what equality is in programming languages. You choose whatever you want. Just abide to those rules. both are odd (of integers) have the same birthday (of people) is congruent to (of triangles) is in the same tax bracket (of tax payers) has the same length (of pieces of string) both contain chocolate (of cakes) I can very well define a class Z_2 where 1 and 3 are equal (both are odd) and equality is defined as x % 2 == y % 2 This is why I keep saying that asking what 'equal' means lead us nowhere. Equality is whatever you see fit as long as you follows some rules. For example, if we define the operator ~ to mean has the same genes (to be precise: the same genotype), then if Fred and Barney are identical twins we have: Fred ~ Fred Fred ~ Barney and Barney ~ Fred Identical triplets are rare (at least among human beings), but if we clone Barney to get George, then we also have: Fred ~ Barney and Barney ~ George = Fred ~ George. So have the same genes meets all your conditions for equality, but isn't equality: the three brothers are very different. Fred lost his arm in a car crash, Barney is a hopeless alcoholic, and George is forty years younger than his two brothers. /You/ decide if '~' is a good definition for equality in your case. If it isn't, then define it another way. What I'm saying is that equality is in the eye of the beholder. To me, in some situations, all odd numbers are the same. What's wrong with that? Kiuhnm -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
On Apr 27, 8:07 am, Kiuhnm kiuhnm03.4t.yahoo.it wrote: Useful... maybe, conceptually sound... no. Conceptually, NaN is the class of all elements which are not numbers, therefore NaN = NaN. NaN isn't really the class of all elements which aren't numbers. NaN is the result of a few specific IEEE 754 operations that cannot be computed, like 0/0, and for which there's no other reasonable substitute (e.g., infinity) for practical applications . In the real world, if we were doing the math with pen and paper, we'd stop as soon as we hit such an error. Equality is simply not defined for the operations that can produce NaN, because we don't know to perform those computations. So no, it doesn't conceptually follow that NaN = NaN, what conceptually follows is the operation is undefined because NaN causes a halt. This is what programming languages ought to do if NaN is compared to anything other than a (floating-point) number: disallow the operation in the first place or toss an exception. Any code that tries such an operation has a logic error and must be fixed. However, when comparing NaN against floating point numbers, I don't see why NaN == NaN returning false is any less conceptually correct than any other possible result. NaN's very existence implicitly declares that we're now making up the rules as we go along, so we might as well pick the simplest set of functional rules. Plus, floating point numbers violate our expectations of equality anyway, frequently in surprising ways. 0.1 + 0.1 + 0.1 == 0.3 is true with pen and paper, but likely false on your computer. It's even potentially possible to compare two floating point variables twice and get different results each time[1]! As such, we'd have this problem with defining equality even if NaN didn't exist. We must treat floating-point numbers as a special case in order to write useful working programs. This includes defining equality in a way that's different from what works for nearly every other data type. Adam [1] Due to register spilling causing intermediate rounding. This could happen with the x87 FPU since the registers were 80-bits wide but values were stored in RAM as 64-bits. This behavior is less common now, but hardly impossible. -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
On Fri, Apr 27, 2012 at 9:39 AM, Adam Skutt ask...@gmail.com wrote: On Apr 27, 8:07 am, Kiuhnm kiuhnm03.4t.yahoo.it wrote: Useful... maybe, conceptually sound... no. Conceptually, NaN is the class of all elements which are not numbers, therefore NaN = NaN. NaN isn't really the class of all elements which aren't numbers. NaN is the result of a few specific IEEE 754 operations that cannot be computed, like 0/0, and for which there's no other reasonable substitute (e.g., infinity) for practical applications . In the real world, if we were doing the math with pen and paper, we'd stop as soon as we hit such an error. Equality is simply not defined for the operations that can produce NaN, because we don't know to perform those computations. So no, it doesn't conceptually follow that NaN = NaN, what conceptually follows is the operation is undefined because NaN causes a halt. This is what programming languages ought to do if NaN is compared to anything other than a (floating-point) number: disallow the operation in the first place or toss an exception. Any code that tries such an operation has a logic error and must be fixed. NaNs do not signify errors (for instance, a NaN could result simply from subtracting one Inf from another), and they do not necessarily imply that the calculation should halt. They are propagating values indicating the lack of a concrete value, and frequently they can reasonably be ignored. If you need to know about a NaN immediately, then you can trap them with fpectl. If you don't, then you let it propagate and check for it at the end of the calculation. If instead they raised exceptions by default, then we would need to wrap virtually every floating point operation in a try-except, which would quickly become a mess. See also: http://grouper.ieee.org/groups/754/faq.html#exceptions -- http://mail.python.org/mailman/listinfo/python-list
Borg identity [was Re: why () is () and [] is [] work in other way?]
On Thu, 26 Apr 2012 04:42:36 -0700, Adam Skutt wrote: You're going to have to explain the value of an ID that's not 1:1 with an object's identity, for at least the object's lifecycle, for a programmer. If you can't come up with a useful case, then you haven't said anything of merit. I gave an example earlier, but you seem to have misunderstood it, so I'll give more detail. In the Borg design pattern, every Borg instance shares state and are indistinguishable, with only one exception: object identity. We can distinguish two Borg instances by using is. Since the whole point of the pattern is for Borg instances to be indistinguishable, the existence of a way to distinguish Borg instances is a flaw and may be undesirable. At least, it's exposing an implementation detail which some people argue should not be exposed. Why should the caller care whether they are dealing with a singleton object or an unspecified number of Borg objects all sharing state? A clever interpreter could make many Borg instances appear to be a singleton. A really clever one could also make a singleton appear to be many Borg instances. Note that this is virtually the same situation as that which John Nagle objects to, namely that the implementation detail of small ints being singletons is exposed. There is only ever one 0 instance, but potentially many 3579 instances. John's argument is that Python should raise an exception if you compare 2 is 2, or for that matter 3579 is 3579, which is foolish. If you're going to change the semantics of is, why not do something useful and ensure that 3579 is 3579 returns True regardless of whether they actually are the same instance or not? That would be far more useful than raising an exception. It would complicate the definition of is, but perhaps that's a price people are willing to pay for avoiding the (trivial) confusion about object identity. [...] identities. The Borg design pattern, for example, would be an excellent candidate for ID:identity being treated as many-to-one. How would inheritance work if I did that? You don't inherit from Borg instances, and instances inherit from their class the same as any other instance. -- Steven -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
Adam Skutt wrote: [ ... ] In the real world, if we were doing the math with pen and paper, we'd stop as soon as we hit such an error. Equality is simply not defined for the operations that can produce NaN, because we don't know to perform those computations. So no, it doesn't conceptually follow that NaN = NaN, what conceptually follows is the operation is undefined because NaN causes a halt. This is what programming languages ought to do if NaN is compared to anything other than a (floating-point) number: disallow the operation in the first place or toss an exception. Any code that tries such an operation has a logic error and must be fixed. There was a time when subtracting 5 from 3 would have been a logic error. Your phrase if we were doing the math ... lies behind most of the history of math, esp. as it concerns arithmetic. Mathematicians kept extending the definitions so that they wouldn't have to stop. Feynman's _Lectures on Physics_, chapter 22, Algebra gives a stellar account of the whole process. Mel. -- http://mail.python.org/mailman/listinfo/python-list
Direct vs indirect [was Re: why () is () and [] is [] work in other way?]
On Thu, 26 Apr 2012 04:42:36 -0700, Adam Skutt wrote: On Apr 26, 5:10 am, Steven D'Aprano steve +comp.lang.pyt...@pearwood.info wrote: But I was actually referring to something more fundamental than that. The statement a is b is a *direct* statement of identity. John is my father. id(a) == id(b) is *indirect*: The only child of John's grandfather is the parent of the mother-in-law of my sister-in-law sort of thing. (Excuse me if I got the relationships mixed up.) Again, the fact that you somehow think this absurd family tree is relevant only shows you're fundamentally confused about what object oriented identity means. That's rather depressing, seeing as I've given you a link to the definition. Perhaps you failed to notice that this absurd family tree, as you put it, consists of grandparent+parent+sibling+in-law. What sort of families are you familiar with that this seems absurd to you? I think you have inadvertently demonstrated the point I am clumsily trying to make. Even when two expressions are logically equivalent, the form of the expressions make a big difference to the comprehensibility of the text. Which would you rather read? for item in sequence[1:]: ... for item in sequence[sum(ord(c) for c in 'avocado') % 183:]: ... The two are logically equivalent, so logically you should have no preference between the two, yes? In a mathematical sense, you're saying that given f(x) = x+2, using f(x) is somehow more direct (whatever the hell that even means) I thought that the concept of direct and indirect statements would be self-evident. Let me try again. A statement is direct in the sense I mean if it explicitly states the thing you intend it to state. A statement is indirect if it requires one or more logical steps to go from the statement, as given, to the conclusion intended. Queen Elizabeth II is the ruling monarch of the United Kingdom is a direct statement of the fact that Queen Elizabeth II is the ruling monarch of the UK. (Do I really need to explain this?) Queen Elizabeth II is the Commander-in-chief of the Canadian Armed Forces is an *indirect* statement of the fact that Elizabeth is the ruling monarch of the UK. It is indirect because it doesn't explicitly say that she is monarch, but the Commander-in-Chief of the Canadian Armed Forces is always the ruling monarch of Canada, and the ruling monarch of Canada is always the ruling monarch of the UK. Hence, Elizabeth being Commander-in-Chief necessarily implies that she is ruling monarch of the United Kingdom (at least until there is change to Canadian law). a is b is a direct test of whether a is b. (Duh.) id(a) == id(b) is an indirect test of whether a is b, since it requires at least three indirect steps: the knowledge of what the id() function does, the knowledge of what the == operator does, and the knowledge that equal IDs imply identity. -- Steven -- http://mail.python.org/mailman/listinfo/python-list
Python id() does not return an address [was Re: why () is () and [] is [] work in other way?]
On Thu, 26 Apr 2012 04:42:36 -0700, Adam Skutt wrote: On Apr 26, 5:10 am, Steven D'Aprano steve +comp.lang.pyt...@pearwood.info wrote: Solution to *what problem*? This confusion that many people have over what 'is' does, including yourself. I have no confusion over what is does. The is operator returns True if and only if the two operands are the same object, otherwise it returns False. If you think that is does something different, you are wrong. An address is an identifier: a number that I can use to access a value[1]. Then by your own definition, Python's id() does not return an address, since you cannot use it to access a value. The fact Python lacks explicit dereferencing doesn't change the fact that id() returns an address. Replace 'can' with 'could' or 'could potentially' or the whole phrase with 'represents' if you wish. It's a rather pointless thing to quibble over. You can't treat id() as an address. Did you miss my post when I demonstrated that Jython returns IDs generated on demand, starting from 1? In general, there is *no way even in principle* to go from a Python ID to the memory location (address) of the object with that ID, because in general objects *may not even have a fixed address*. Objects in Jython don't, because the Java virtual machine can move them in memory. The fact that CPython happens to use the memory address of objects, suitably converted to an int object, is a red-herring. It leads to nothing but confusion. Would you call the result of casting a C pointer to an int an address? If so, you must call the result of id() an address as well-- you can't dereference either of them. If not, then you need to provide an alternate name for the result of casting a C pointer to an int. I don't need to do anything of the sort. It was *your* definition, not mine. Don't put the responsibility on me for your definition being broken. (And for the record, in C you can cast an integer into a pointer, although the results are implementation-specific. There's no equivalent in Python.) -- Steven -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
On Apr 27, 11:01 am, Kiuhnm kiuhnm03.4t.yahoo.it wrote: On 4/27/2012 1:57, Adam Skutt wrote: On Apr 26, 6:34 pm, Kiuhnmkiuhnm03.4t.yahoo.it wrote: If you understand that your 'a' is not really an object but a reference to it, everything becomes clear and you see that '==' always do the same thing. Yes, object identity is implemented almost? everywhere by comparing the value of two pointers (references)[1]. I've already said I'm not really sure how else one would go about implementing it. You might tell me that that's just an implementation detail, but when an implementation detail is easier to understand and makes more sense than the whole abstraction which is built upon it, something is seriously wrong. I'm not sure what abstraction is being built here. I think you have me confused for someone else, possibly Steven. The abstraction is this: - There are primitives and objects. - Primitives are not objects. The converse is also true. - Primitives can become objects (boxing). - Two primitives x and y are equal iff x == y. - Two objects x and y are equal iff x.equals(y). - Two objects are the same object iff x == y. - If x is a primitive, then y = x is a deep copy. - If x is an object, then y = x is a shallow copy. - ... This is not an abstraction at all, but merely a poor explanation of how things work in Java. Your last statement is totally incorrect, as no copying of the object occurs whatsoever. The reference is merely reseated to refer to the new object. If you're going to chide me for ignoring the difference between the reference and the referent object, then you shouldn't ignore it either, especially in the one case where it actually matters! If we try to extend this to other languages, then it breaks down completely. The truth: - Primitives can be references. - Two primitives are equal iff x == y. - Operator '.' automatically derefences references. You have the first statement backwards. References are a primitive construct, not the other way around. While true, it's still a bad way to think about what's going on. It breaks down once we add C++ / Pascal reference types to the mix, for example. It's better to think about variables (names) and just recognize that not all variables have the same semantics. It avoids details that are irrelevant to writing actual programs and remains consistent. Equality or equivalence is a relation which is: - reflexive - symmetric - transitive Everything else... is something else. Call it semi-equality, tricky-equality or whatever, but not equality, please. Sure, but then it's illegal to allow the usage of '==' with floating point numbers, which will never have these properties in any usable implementation[1]. So we're back to what started this tangent, and we end up needing 'equals()' methods on our classes to distinguish between the different forms of equality. That's precisely what you want to avoid. Or we can just accept that '==' doesn't always possess those properties, which is what essentially every programming language does, and call it (value) equality. As long as we don't cross incompatible meanings, it's hard to believe that this isn't the right thing to do. If anything, you have that backwards. Look at Python: all variables in Python have pointer semantics, not value semantics. When everything is white, the word white becomes redundant. So the fact that everything in Python have reference semantics means that we can't stop thinking about value and reference semantics. Nope. The behavior of variables is absolutely essential to writing correct programs. If I write a program in Python that treats variables as if they were values, it will be incorrect. In imperative languages, pointers have greater utility over value types because not all types can obey the rules for value types. For example, I don't know how to give value semantics to something like a I/O object (e.g, file, C++ fstream, C FILE), since I don't know how to create independent copies. By defining a copy constructor. Then write me a working one. I'll wait. To save yourself some time, you can start with std::fstream. Python is already without pointers (*). A world where everyone is a lawyer is a world without lawyers (really, there isn't any other way we can get rid of them :) ). (*) By the way, some would argue that references are not pointers. They would be completely and utterly wrong, and probably imbuing pointers with properties they don't actually possess. Unless you're talking about C++ / Pascal references, which really aren't pointers and do possess a different set of semantics (alias might be a better term for them). Adam [1] Not in any fashion that's useful to the programmer, at any rate. -- http://mail.python.org/mailman/listinfo/python-list
Re: Python id() does not return an address [was Re: why () is () and [] is [] work in other way?]
Steven, your posts are leaking out of their respective thread(s). Is this intentional? ~Temia -- When on earth, do as the earthlings do. -- http://mail.python.org/mailman/listinfo/python-list
Re: Borg identity [was Re: why () is () and [] is [] work in other way?]
On Apr 27, 12:56 pm, Steven D'Aprano steve +comp.lang.pyt...@pearwood.info wrote: On Thu, 26 Apr 2012 04:42:36 -0700, Adam Skutt wrote: You're going to have to explain the value of an ID that's not 1:1 with an object's identity, for at least the object's lifecycle, for a programmer. If you can't come up with a useful case, then you haven't said anything of merit. I gave an example earlier, but you seem to have misunderstood it, so I'll give more detail. In the Borg design pattern, every Borg instance shares state and are indistinguishable, with only one exception: object identity. We can distinguish two Borg instances by using is. Since the whole point of the pattern is for Borg instances to be indistinguishable, the existence of a way to distinguish Borg instances is a flaw and may be undesirable. At least, it's exposing an implementation detail which some people argue should not be exposed. Then people should stop with such idiocy like the Borg pattern. It's a bad example from an even worse idea. Why should the caller care whether they are dealing with a singleton object or an unspecified number of Borg objects all sharing state? A clever interpreter could make many Borg instances appear to be a singleton. A really clever one could also make a singleton appear to be many Borg instances. Trivial: to break cyclical references in a deep copy operation. John's argument is that Python should raise an exception if you compare 2 is 2, or for that matter 3579 is 3579, which is foolish. identities. The Borg design pattern, for example, would be an excellent candidate for ID:identity being treated as many-to-one. How would inheritance work if I did that? You don't inherit from Borg instances, and instances inherit from their class the same as any other instance. I think you misunderstood me. Define a Borg class where somehow identity is the same for all instances. Inherit from that class and add per-instance members. Now, identity can't be the same for all instances. As a result, you've just violated the Liskov Substituion Principal: code that relies on all Borg class instances having the same identity will fail when passed an instance of the subclass. It's impossible to combine identities and not violate LSP, unless you forbid subclasses. Your idea violates one of the most fundamental tenants of object-oriented programming. This is because object identity is one of the fundamental consequences of object-oriented programming. You can't do away with it, and any attempt to do so really just suggests that you don't understand OOP at all. Adam -- http://mail.python.org/mailman/listinfo/python-list
Re: Borg identity [was Re: why () is () and [] is [] work in other way?]
On Sat, Apr 28, 2012 at 3:33 AM, Adam Skutt ask...@gmail.com wrote: I think you misunderstood me. Define a Borg class where somehow identity is the same for all instances. Inherit from that class and add per-instance members. Now, identity can't be the same for all instances. As a result, you've just violated the Liskov Substituion Principal: code that relies on all Borg class instances having the same identity will fail when passed an instance of the subclass. Why would you subclass a Borg? ChrisA -- http://mail.python.org/mailman/listinfo/python-list
Variables and values [was Re: why () is () and [] is [] work in other way?]
On Fri, 27 Apr 2012 10:15:32 -0700, Adam Skutt wrote: If I write a program in Python that treats variables as if they were values, it will be incorrect. I'm sorry, it is unclear to me what distinction you are making between variables and values. Can you give simple examples of both incorrect and correct code demonstrating what you mean? (I know what distinction *I* would make, but I'm not sure if it is the same one you are making.) -- Steven -- http://mail.python.org/mailman/listinfo/python-list
Re: Python id() does not return an address [was Re: why () is () and [] is [] work in other way?]
On Apr 27, 1:12 pm, Steven D'Aprano steve +comp.lang.pyt...@pearwood.info wrote: On Thu, 26 Apr 2012 04:42:36 -0700, Adam Skutt wrote: On Apr 26, 5:10 am, Steven D'Aprano steve +comp.lang.pyt...@pearwood.info wrote: Solution to *what problem*? This confusion that many people have over what 'is' does, including yourself. I have no confusion over what is does. False. If you did, then you would not have suggested the difference in True/False result between id([1,2]) == id([1, 2]) and [1, 2] is [1, 2] matters. You would understand that the result of an identity test with temporary objects is meaningless, since identity is only meaningful while the objects are alive. That's a fundamental mistake. An address is an identifier: a number that I can use to access a value[1]. Then by your own definition, Python's id() does not return an address, since you cannot use it to access a value. The fact Python lacks explicit dereferencing doesn't change the fact that id() returns an address. Replace 'can' with 'could' or 'could potentially' or the whole phrase with 'represents' if you wish. It's a rather pointless thing to quibble over. You can't treat id() as an address. Did you miss my post when I demonstrated that Jython returns IDs generated on demand, starting from 1? In general, there is *no way even in principle* to go from a Python ID to the memory location (address) of the object with that ID, because in general objects *may not even have a fixed address*. Objects in Jython don't, because the Java virtual machine can move them in memory. Yes, there is a way. You add a function deref() to the language. In CPython, that simply treats the passed value as a memory address and treats it as an object, perhaps with an optional check. In Jython, it'd access a global table of numbers as keys with the corresponding objects as values, and return them. The value of id() is absolutely an address, even in Jython. The fact the values can move about is irrelevant. Again, if this wasn't possible, then you couldn't implement 'is'. Implementing 'is' requires a mechanism for comparing objects that doesn't involve ensuring the contents of the two operands in memory is the same. Would you call the result of casting a C pointer to an int an address? If so, you must call the result of id() an address as well-- you can't dereference either of them. If not, then you need to provide an alternate name for the result of casting a C pointer to an int. I don't need to do anything of the sort. Yes, you do, because you called such a thing an address when talking about CPython. Even if my definition is wrong (it's not), your definition is wrong too. (And for the record, in C you can cast an integer into a pointer, although the results are implementation-specific. There's no equivalent in Python.) Yes, but the lack of that operation doesn't mean that id() doesn't return an address. Adam -- http://mail.python.org/mailman/listinfo/python-list
Re: Borg identity [was Re: why () is () and [] is [] work in other way?]
On Fri, Apr 27, 2012 at 11:33 AM, Adam Skutt ask...@gmail.com wrote: On Apr 27, 12:56 pm, Steven D'Aprano steve +comp.lang.pyt...@pearwood.info wrote: On Thu, 26 Apr 2012 04:42:36 -0700, Adam Skutt wrote: You're going to have to explain the value of an ID that's not 1:1 with an object's identity, for at least the object's lifecycle, for a programmer. If you can't come up with a useful case, then you haven't said anything of merit. I gave an example earlier, but you seem to have misunderstood it, so I'll give more detail. In the Borg design pattern, every Borg instance shares state and are indistinguishable, with only one exception: object identity. We can distinguish two Borg instances by using is. Since the whole point of the pattern is for Borg instances to be indistinguishable, the existence of a way to distinguish Borg instances is a flaw and may be undesirable. At least, it's exposing an implementation detail which some people argue should not be exposed. Then people should stop with such idiocy like the Borg pattern. It's a bad example from an even worse idea. I would argue exactly the opposite. One issue I have with the Singleton pattern is that the very fact that a class is a singleton (or a borg) is an implementation detail. Shared identity exposes that implementation detail, which I consider a flaw. Borg, OTOH, actually gets this right. Why should the caller care whether they are dealing with a singleton object or an unspecified number of Borg objects all sharing state? A clever interpreter could make many Borg instances appear to be a singleton. A really clever one could also make a singleton appear to be many Borg instances. Trivial: to break cyclical references in a deep copy operation. The cycle will break anyway, since there are only a finite number of instances. You just might end up with as many copy instances as there were original instances in the structure. But going back to Steven's point here, that fact should not be important to the caller. I think you misunderstood me. Define a Borg class where somehow identity is the same for all instances. Why? The whole point of Borg is to share state, not identity. If you really want shared identity, use a Singleton. Inherit from that class and add per-instance members. Now, identity can't be the same for all instances. As a result, you've just violated the Liskov Substituion Principal: code that relies on all Borg class instances having the same identity will fail when passed an instance of the subclass. That has nothing to do with Borg. That has to do with having misguidedly defined identity to be the same for all instances. You wouldn't do that for a regular class. Why force it on a Borg class? -- http://mail.python.org/mailman/listinfo/python-list
Re: Direct vs indirect [was Re: why () is () and [] is [] work in other way?]
On Apr 27, 1:06 pm, Steven D'Aprano steve +comp.lang.pyt...@pearwood.info wrote: On Thu, 26 Apr 2012 04:42:36 -0700, Adam Skutt wrote: On Apr 26, 5:10 am, Steven D'Aprano steve +comp.lang.pyt...@pearwood.info wrote: But I was actually referring to something more fundamental than that. The statement a is b is a *direct* statement of identity. John is my father. id(a) == id(b) is *indirect*: The only child of John's grandfather is the parent of the mother-in-law of my sister-in-law sort of thing. (Excuse me if I got the relationships mixed up.) Again, the fact that you somehow think this absurd family tree is relevant only shows you're fundamentally confused about what object oriented identity means. That's rather depressing, seeing as I've given you a link to the definition. Perhaps you failed to notice that this absurd family tree, as you put it, consists of grandparent+parent+sibling+in-law. What sort of families are you familiar with that this seems absurd to you? No, I noticed, but who talks like that? It's not remotely comparable to the sort of difference we're talking about. I think you have inadvertently demonstrated the point I am clumsily trying to make. Even when two expressions are logically equivalent, the form of the expressions make a big difference to the comprehensibility of the text. And if we were talking about 30, 20, 5, maybe even 2 line function versus it's name, you might have a point. We're not talking about such things though, and it's pretty disingenuous to pretend otherwise. Yet, that's precisely what you did with your absurd family relationship. Which would you rather read? for item in sequence[1:]: ... for item in sequence[sum(ord(c) for c in 'avocado') % 183:]: ... The two are logically equivalent, so logically you should have no preference between the two, yes? No, they're not logically equivalent. The first won't even execute, as sequence is undefined. You need two lines in the first case. A statement is direct in the sense I mean if it explicitly states the thing you intend it to state. And in the case of the two ways to compare identity, both statements state exactly what I intend to state. They're synonyms. a is b is a direct test of whether a is b. (Duh.) id(a) == id(b) is an indirect test of whether a is b, since it requires at least three indirect steps: the knowledge of what the id() function does, the knowledge of what the == operator does, and the knowledge that equal IDs imply identity. The problem is that using 'is' correctly requires understanding all of those three things. Adam -- http://mail.python.org/mailman/listinfo/python-list
Re: Borg identity [was Re: why () is () and [] is [] work in other way?]
On Fri, Apr 27, 2012 at 11:38 AM, Chris Angelico ros...@gmail.com wrote: On Sat, Apr 28, 2012 at 3:33 AM, Adam Skutt ask...@gmail.com wrote: I think you misunderstood me. Define a Borg class where somehow identity is the same for all instances. Inherit from that class and add per-instance members. Now, identity can't be the same for all instances. As a result, you've just violated the Liskov Substituion Principal: code that relies on all Borg class instances having the same identity will fail when passed an instance of the subclass. Why would you subclass a Borg? For the same reasons one might want to subclass a Singleton. One of the big advantages of Borg is that it makes this easier, and it also allows the option of either having instances of subclasses share the same state as instances of base classes (usually preferred, because it preserves LSP) or of allowing each individual subclass to have its own unique shared state. -- http://mail.python.org/mailman/listinfo/python-list
Re: Python id() does not return an address [was Re: why () is () and [] is [] work in other way?]
On Sat, Apr 28, 2012 at 3:51 AM, Adam Skutt ask...@gmail.com wrote: Yes, there is a way. You add a function deref() to the language. In CPython, that simply treats the passed value as a memory address and treats it as an object, perhaps with an optional check. In Jython, it'd access a global table of numbers as keys with the corresponding objects as values, and return them. The value of id() is absolutely an address, even in Jython. The fact the values can move about is irrelevant. Python already as dereferenceable addresses. Look. def address(obj,table=[]): for i,o in enumerate(table): if obj is o: return i table.append(obj) return len(table)-1 def deref(addr): return address.__defaults__[0][addr] You can take the address of an object (interning it, effectively), and later dereference it. Proves nothing. ChrisA -- http://mail.python.org/mailman/listinfo/python-list
RE: why () is () and [] is [] work in other way?
I have spent some time searching for a bug in my code, it was due to different work of is with () and []: () is () True [] is [] False (Python 2.7.2+ (default, Oct 4 2011, 20:03:08) [GCC 4.6.1] ) Is this what it should be or maybe yielding unified result is better? D. Congratulations, you've managed to introduce the most useless thread on this ng/ml that I've come across in the ten years that I've been using (c)Python. I've never ever thought about using this weird thing called id in my code, (which comes later in all the responses) so why do some people think it's so important? Somewhere out there (possibly in an alternate universe) Bill Clinton is reading this thread and nodding vigorously about an argument around the [key]word is. Ramit Ramit Prasad | JPMorgan Chase Investment Bank | Currencies Technology 712 Main Street | Houston, TX 77002 work phone: 713 - 216 - 5423 This email is confidential and subject to important disclaimers and conditions including on offers for the purchase or sale of securities, accuracy and completeness of information, viruses, confidentiality, legal privilege, and legal entity disclaimers, available at http://www.jpmorgan.com/pages/disclosures/email. -- http://mail.python.org/mailman/listinfo/python-list
Re: Borg identity [was Re: why () is () and [] is [] work in other way?]
On Fri, 27 Apr 2012 10:33:34 -0700, Adam Skutt wrote: Why should the caller care whether they are dealing with a singleton object or an unspecified number of Borg objects all sharing state? A clever interpreter could make many Borg instances appear to be a singleton. A really clever one could also make a singleton appear to be many Borg instances. Trivial: to break cyclical references in a deep copy operation. I asked why the *caller* should care. If the caller has to break cyclical references manually, the garbage collector is not doing its job. If you're going to propose underpowered or buggy environments as an objection, then I'll simply respond that I'm not talking about any specific (underpowered or buggy) implementation, I'm talking about what is logically possible. [...] How would inheritance work if I did that? You don't inherit from Borg instances, and instances inherit from their class the same as any other instance. I think you misunderstood me. Define a Borg class where somehow identity is the same for all instances. Inherit from that class and add per-instance members. I think that if you're talking about per-instance members of a Borg class, you're confused as to what Borg means. Since all instances share state, you can't have *per-instance* data. Now, identity can't be the same for all instances. As a result, you've just violated the Liskov Substituion Principal: code that relies on all Borg class instances having the same identity will fail when passed an instance of the subclass. Not at all. Obviously each Borg subclass will have it's own fake identity. Code that requires instances of different types to be identical is fundamentally broken, since the mere fact that they are different types means they can't be identical. I'll accept the blame for your confusion as I glossed over something which I thought was obvious, but clearly wasn't. When I said that Borg instances are indistinguishable except for identity, I thought that was obvious that I was talking about instances of a single type. Mea culpa. Clearly if x is an instance of Borg, and y is an instance of BorgSubclass, you can distinguish them by looking at the type. The point is that you shouldn't be able to distinguish instances of a single type. It's impossible to combine identities and not violate LSP, unless you forbid subclasses. Your idea violates one of the most fundamental tenants of object-oriented programming. This is because object identity is one of the fundamental consequences of object-oriented programming. You can't do away with it, and any attempt to do so really just suggests that you don't understand OOP at all. Oh please, enough of the religion of LSP. Barbara Liskov first introduced this idea in 1987, twenty years after Simula 67 first appeared and thirty years after MIT researchers came up with the concept of object oriented programming. That's hardly fundamental to the concept of OOP. People have, and still do, violate LSP all the time. LSP may be best practice but it's hardly essential. OOP was useful before LSP and it will remain useful in the face of violations. Besides: - In real life, subtypes often violate LSP. An electric car is a type of car, but it has no petrol tank. Wolf spiders have eyes, except for the Kauaʻi cave wolf spider, which is is a subtype of wolf spider but is completely eyeless. - Subclasses in Eiffel are not necessarily subtypes and may not be substitutable for superclasses. If it's good enough for Eiffel, it's good enough for my hypothetical Borg subclasses. You can always declare that Bertrand Meyer doesn't understand OOP at all too. -- Steven -- http://mail.python.org/mailman/listinfo/python-list
Re: Borg identity [was Re: why () is () and [] is [] work in other way?]
On Apr 27, 2:40 pm, Steven D'Aprano steve +comp.lang.pyt...@pearwood.info wrote: On Fri, 27 Apr 2012 10:33:34 -0700, Adam Skutt wrote: Why should the caller care whether they are dealing with a singleton object or an unspecified number of Borg objects all sharing state? A clever interpreter could make many Borg instances appear to be a singleton. A really clever one could also make a singleton appear to be many Borg instances. Trivial: to break cyclical references in a deep copy operation. I asked why the *caller* should care. If the caller has to break cyclical references manually, the garbage collector is not doing its job. It's a necessary requirement to serialize any cyclical structure. Garbage collection has nothing to do with it. If I have some structure such that A -- B -- A, I need to be able to determine that I've seen 'A' before in order to serialize the structure to disk, or I will never write it out successfully. There are plenty of situations where we legitimately care whether two pointers are the same and don't give one whit about the state of objects they point to. You cannot conflate the two tests, and that's precisely what your 'give all borg instances the same identity' idea does. I think that if you're talking about per-instance members of a Borg class, you're confused as to what Borg means. I'm not. I'm talking about per-instance members of a subclass of a Borg class. There's nothing about the Borg pattern that forbids such behavior, which is one of the reasons it's such a terrible idea in general. Borg implies promises that it cannot readily keep. Since all instances share state, you can't have *per-instance* data. I most certainly can do so in a subclass. Shared state in a parent doesn't mandate shared state in a child. Not at all. Obviously each Borg subclass will have it's own fake identity. When I said that Borg instances are indistinguishable except for identity, I thought that was obvious that I was talking about instances of a single type. Mea culpa. Clearly if x is an instance of Borg, and y is an instance of BorgSubclass, you can distinguish them by looking at the type. The point is that you shouldn't be able to distinguish instances of a single type. No, that's not the least bit obvious nor apparent, and it still violates LSP. It means every function that takes a Borg as an argument must know about every subclass in order to distinguish between them. The serialization function above would need to do so. Imagine an object x that holds a Borg object and a BorgSubclass object. If the serialization function keeps a list of objects it has seen before and uses that to determine whether to write the object out, it will fail to write out one or the other if we implemented your harebrained 'All Borg objects have the same identity' idea. Your idea means that 'x.borg is x.subborg' must return True. It also means either x.borg isn't going to be written out, or x.subborg isn't going to be written out. The program is broken. If you modify your idea to ignore subtypes, than this function breaks: def write_many(value, channel1, channel2): channel1.write(value) if channel2 is not channel1: channel2.write(value) Calling write_many(foo, x.borg, x.subborg) now gives different behavior than write_many(foo, x.borg, x.borg). That's probably not what the programmer intended! Like it or not, whether you have only one object with shared state or infinite objects with the same shared state is not an implementation detail. Just because you write code that doesn't care about that fact does not make it an implementation detail. I can write code that depends on that fact, and there's not a single thing you can do to stop me. This is why the Borg pattern is a bad idea in general, because it encourages programmers to write code that is subtly wrong. If you have a Borg class, you can't ignore the fact that you have multiple objects even if you want to do so. You will eventually end up writing incorrect code as a result. Yet, many people try to do precisely that, your idea is attempting to do precisely that! Oh please, enough of the religion of LSP. Barbara Liskov first introduced this idea in 1987, twenty years after Simula 67 first appeared and thirty years after MIT researchers came up with the concept of object oriented programming. That's hardly fundamental to the concept of OOP. People have, and still do, violate LSP all the time. People write code with security flaws all of the time too. This doesn't even being to approach being an reasonable argument. It's completely disingenuous. People come up with ideas and fail to properly formalize them all of the time. People come up with useful, revolutionary ideas and get parts of them wrong all of the time. If you violate LSP, then you enable interface users to write buggy code. Correct class hierarchies must follow it; correct interface implementations must follow it.
Re: why () is () and [] is [] work in other way?
On Thu, Apr 26, 2012 at 3:48 PM, John Nagle na...@animats.com wrote: This assumes that everything is, internally, an object. In CPython, that's the case, because Python is a naive interpreter and everything, including numbers, is boxed. That's not true of PyPy or Shed Skin. So does is have to force the creation of a temporary boxed object? Interesting point. Presumably the only types that can be unboxed are those for which identity vs equality is pretty much immaterial, so the question won't really matter. I'd be inclined to either have it return False if either/both is unboxed, or else return True if both are equal unboxed numbers, whichever is the most convenient to implement. My opinion doesn't matter-- My opinion doesn't matter-- My opinion doesn't matter, matter, matter, matter, matter! (WS Gilbert, Ruddigore) ChrisA -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
On Wed, 25 Apr 2012 22:48:33 -0700, John Nagle wrote: On 4/25/2012 5:01 PM, Steven D'Aprano wrote: On Wed, 25 Apr 2012 13:49:24 -0700, Adam Skutt wrote: Though, maybe it's better to use a different keyword than 'is' though, due to the plain English connotations of the term; I like 'sameobj' personally, for whatever little it matters. Really, I think taking away the 'is' operator altogether is better, so the only way to test identity is: id(x) == id(y) Four reasons why that's a bad idea: 1) The is operator is fast, because it can be implemented directly by the interpreter as a simple pointer comparison (or equivalent). This assumes that everything is, internally, an object. No it doesn't. It assumes that everything provides the external interface of an object. Internally, the implementation could be anything you like, so long as it simulates an object when observed from Python. -- Steven -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
On Wed, 25 Apr 2012 20:50:21 -0700, Adam Skutt wrote: On Apr 25, 8:01 pm, Steven D'Aprano steve +comp.lang.pyt...@pearwood.info wrote: On Wed, 25 Apr 2012 13:49:24 -0700, Adam Skutt wrote: Though, maybe it's better to use a different keyword than 'is' though, due to the plain English connotations of the term; I like 'sameobj' personally, for whatever little it matters. Really, I think taking away the 'is' operator altogether is better, so the only way to test identity is: id(x) == id(y) Four reasons why that's a bad idea: 1) The is operator is fast, because it can be implemented directly by the interpreter as a simple pointer comparison (or equivalent). The id() idiom is slow, because it involves two global lookups and an equality comparison. Inside a tight loop, that can make a big difference in speed. The runtime can optimize the two operations to be equivalent, since they are logically equivalent operations. If you removed 'is', there's little reason to believe it would do otherwise. I'm afraid you are mistaken there. *By design*, Python allows shadowing and monkey-patching of built-ins. (Although not quite to the same degree as Ruby, and thank goodness!) Given the language semantics of Python, id(a) == id(b) is NOT equivalent to a is b since the built-in id() function can be shadowed by some other function at runtime, but the is operator cannot be. An extremely clever optimizing implementation like PyPy may be able to recognise at runtime that the built-in id() is being called, but that doesn't change the fact that PyPy MUST support this code in order to claim to be a Python compiler: id = lambda x: 999 id(None) == id(spam) # returns True If your runtime doesn't allow that, it isn't Python. 2) The is operator always has the exact same semantics and cannot be overridden. The id() function can be monkey-patched. I can't see how that's useful at all. Identity is a fundamental property of an object; hence retrieval of it must be a language operation. The fact Python chooses to do otherwise is unfortunate, but also irrelevant to my position. It's useful for the same reason that shadowing any other builtin is useful. id() isn't special enough to complicate the simple, and effective, execution model just to satisfy philosophers. 3) The is idiom semantics is direct: a is b directly tests the thing you want to test, namely whether a is b. The id() idiom is indirect: id(a) == id(b) only indirectly tests whether a is b. The two expressions are logically equivalent, so I don't see how this matters, nor how it is true. They are not *logically* equivalent. First you have to define what you mean by identity, then you have to define what you mean by an ID, and then you have to decide whether or not to enforce the rule that identity and IDs are 1:1 or not, and if so, under what circumstances. My library card ID may, by coincidence, match your drivers licence ID. Doesn't mean we're the same person. Entities may share identities, or may have many identities. The Borg design pattern, for example, would be an excellent candidate for ID:identity being treated as many-to-one. Even if you decide that treating IDs as 1:1 is the only thing that makes sense in your philosophy, in practice that does not hold for Python. IDs may be reused by Python. There are circumstances where different objects get the same ID. Hence, comparing IDs is not equivalent to identity testing. But I was actually referring to something more fundamental than that. The statement a is b is a *direct* statement of identity. John is my father. id(a) == id(b) is *indirect*: The only child of John's grandfather is the parent of the mother-in-law of my sister-in-law sort of thing. (Excuse me if I got the relationships mixed up.) 4) The id() idiom already breaks if you replace names a, b with expressions: id([1,2]) == id([3,4]) True It's not broken at all. It is broken in the sense that id(a) == id(b) is to be treated as equivalent to a is b. The above example demonstrates that you CANNOT treat them as equivalent. [...] The other solution is to do what Java and C# do: banish id() entirely Solution to *what problem*? I do not believe that there is a problem here that needs to be solved. Both id() and is are perfectly fine for what they do. But that's absolutely wrong. id(x) returns an ID, not an address. It just happens that, as an accident of implementation, the CPython interpreter uses the object address as an ID, because objects can't move. That's not the case for all implementations. In Jython, objects can move and the address is not static, and so IDs are assigned on demand starting with 1: steve@runes:~$ jython Jython 2.5.1+ (Release_2_5_1, Aug 4 2010, 07:18:19) [OpenJDK Client VM (Sun Microsystems Inc.)] on java1.6.0_18 Type help, copyright, credits or license for more information. id(42) 1 id(Hello World!) 2 id(None) 3
Re: why () is () and [] is [] work in other way?
On Apr 26, 5:10 am, Steven D'Aprano steve +comp.lang.pyt...@pearwood.info wrote: On Wed, 25 Apr 2012 20:50:21 -0700, Adam Skutt wrote: On Apr 25, 8:01 pm, Steven D'Aprano steve +comp.lang.pyt...@pearwood.info wrote: On Wed, 25 Apr 2012 13:49:24 -0700, Adam Skutt wrote: Though, maybe it's better to use a different keyword than 'is' though, due to the plain English connotations of the term; I like 'sameobj' personally, for whatever little it matters. Really, I think taking away the 'is' operator altogether is better, so the only way to test identity is: id(x) == id(y) Four reasons why that's a bad idea: 1) The is operator is fast, because it can be implemented directly by the interpreter as a simple pointer comparison (or equivalent). The id() idiom is slow, because it involves two global lookups and an equality comparison. Inside a tight loop, that can make a big difference in speed. The runtime can optimize the two operations to be equivalent, since they are logically equivalent operations. If you removed 'is', there's little reason to believe it would do otherwise. I'm afraid you are mistaken there. *By design*, Python allows shadowing and monkey-patching of built-ins. (Although not quite to the same degree as Ruby, and thank goodness!) Yes, I understand that. You still haven't explained why this behavior is correct in this particular situation. Arguing from the position of, What Python does must be correct isn't a valid tactic, I'm afraid. It's useful for the same reason that shadowing any other builtin is useful. id() isn't special enough to complicate the simple, and effective, execution model just to satisfy philosophers. If overriding id() is useful, then overriding 'is' must be useful too. Python is still broken. Unless you can prove the two operations shouldn't be logically equivalent (and you don't), you can't meaningfully argue for different semantics for them. You still end up with a broken language either way. They are not *logically* equivalent. First you have to define what you mean by identity, then you have to define what you mean by an ID, and then you have to decide whether or not to enforce the rule that identity and IDs are 1:1 or not, and if so, under what circumstances. You're going to have to explain the value of an ID that's not 1:1 with an object's identity, for at least the object's lifecycle, for a programmer. If you can't come up with a useful case, then you haven't said anything of merit. Plainly, to show they're not logically equivalent, you need to explain why the guarantee provided by id() is improper. Then, you need to generalize it to all programming languages. Python's concept of identity is not unique nor special. It uses the exact same rules as C+ +, C#, Java, and many other languages. My library card ID may, by coincidence, match your drivers licence ID. Doesn't mean we're the same person. I don't know why you even remotely think this is relevant. All it does is further demonstrate that you don't understand the object- oriented concept of identity at all. Comparing library IDs and drivers license IDs is an improper operation. Languages that can have multiple IDs, possibly overlapping, /do/ disallow such idiocy. identities. The Borg design pattern, for example, would be an excellent candidate for ID:identity being treated as many-to-one. How would inheritance work if I did that? Even if you decide that treating IDs as 1:1 is the only thing that makes sense in your philosophy, in practice that does not hold for Python. IDs may be reused by Python. They may be reused in all languages I can think of. They're only unique for the lifetime of the object, because that's all we need as programmers. There are circumstances where different objects get the same ID. Hence, comparing IDs is not equivalent to identity testing. Two objects only get the same ID if one of the objects is dead. The results of such an comparison are obviously meaningless. Some runtimes even try very hard to prevent you from doing such silly things. But I was actually referring to something more fundamental than that. The statement a is b is a *direct* statement of identity. John is my father. id(a) == id(b) is *indirect*: The only child of John's grandfather is the parent of the mother-in-law of my sister-in-law sort of thing. (Excuse me if I got the relationships mixed up.) Again, the fact that you somehow think this absurd family tree is relevant only shows you're fundamentally confused about what object oriented identity means. That's rather depressing, seeing as I've given you a link to the definition. In a mathematical sense, you're saying that given f(x) = x+2, using f(x) is somehow more direct (whatever the hell that even means) than using 'x+2'. That's just not true. We freely and openly interchange them all the time doing mathematics. Programming is no different. It is broken in the
Re: why () is () and [] is [] work in other way?
On Apr 26, 1:48 am, John Nagle na...@animats.com wrote: On 4/25/2012 5:01 PM, Steven D'Aprano wrote: On Wed, 25 Apr 2012 13:49:24 -0700, Adam Skutt wrote: Though, maybe it's better to use a different keyword than 'is' though, due to the plain English connotations of the term; I like 'sameobj' personally, for whatever little it matters. Really, I think taking away the 'is' operator altogether is better, so the only way to test identity is: id(x) == id(y) Four reasons why that's a bad idea: 1) The is operator is fast, because it can be implemented directly by the interpreter as a simple pointer comparison (or equivalent). This assumes that everything is, internally, an object. In CPython, that's the case, because Python is a naive interpreter and everything, including numbers, is boxed. That's not true of PyPy or Shed Skin. So does is have to force the creation of a temporary boxed object? That's what C# does AFAIK. Java defines '==' as value comparison for primitives and '==' as identity comparison for objects, but I don't exactly know how one would do that in Python. Adam -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
On Thu, Apr 26, 2012 at 9:42 PM, Adam Skutt ask...@gmail.com wrote: Would you call the result of casting a C pointer to an int an address? If so, you must call the result of id() an address as well-- you can't dereference either of them. If not, then you need to provide an alternate name for the result of casting a C pointer to an int. It's an address. You can cast it back to pointer and then dereference it. But you're talking about C, which is portable assembly language. When I write C code, I expect to be able to shoot myself in the foot in numerous ways. With Python, you have to actually work at it a bit, like: len = str Okay, that wasn't very hard, but still, it's not like dereferencing an uninitialized pointer! But all this is predicated on a few things: 1) The CPU addresses memory by numbers of a certain size. 2) The C declaration int *ptr represents an address 3) The C declaration (int)ptr turns that address into an integer that's big enough to store it Assuming #3 to be correct is a major cause of trouble, but let's suppose for the moment that it is. What we have is a direct 1:1 relationship between pointers, integers, and object identities. The fact is, though, that Python does not ever guarantee this. Side point: In Python 2, id() returns an int, not a long. Is it possible to be running Python on a 64-bit machine with a 32-bit int type? And if so, what does CPython do? Return the address modulo 4G? Because that could result in collisions. ChrisA -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
On 4/26/12 12:56 PM, Chris Angelico wrote:
Side point: In Python 2, id() returns an int, not a long. Is it
possible to be running Python on a 64-bit machine with a 32-bit int
type?
Yes. Win64 has 64-bit pointers and 32-bit C longs (and thus 32-bit Python ints).
And if so, what does CPython do? Return the address modulo 4G?
It returns a Python long.
Python 2.7.3 (default, Apr 10 2012, 23:24:47) [MSC v.1500 64 bit (AMD64)] on
win32
Type help, copyright, credits or license for more information.
import sys
sys.maxint
2147483647
id('Hello')
30521584L
--
Robert Kern
I have come to believe that the whole world is an enigma, a harmless enigma
that is made terrible by our own mad attempt to interpret it as though it had
an underlying truth.
-- Umberto Eco
--
http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
On 4/26/2012 2:01, Steven D'Aprano wrote: On Wed, 25 Apr 2012 13:49:24 -0700, Adam Skutt wrote: Though, maybe it's better to use a different keyword than 'is' though, due to the plain English connotations of the term; I like 'sameobj' personally, for whatever little it matters. Really, I think taking away the 'is' operator altogether is better, so the only way to test identity is: id(x) == id(y) Four reasons why that's a bad idea: 1) The is operator is fast, because it can be implemented directly by the interpreter as a simple pointer comparison (or equivalent). The id() idiom is slow, because it involves two global lookups and an equality comparison. Inside a tight loop, that can make a big difference in speed. 2) The is operator always has the exact same semantics and cannot be overridden. The id() function can be monkey-patched. 3) The is idiom semantics is direct: a is b directly tests the thing you want to test, namely whether a is b. The id() idiom is indirect: id(a) == id(b) only indirectly tests whether a is b. 4) The id() idiom already breaks if you replace names a, b with expressions: id([1,2]) == id([3,4]) True You forgot one: 5) It would be a pain to write (and read) if id(my_obj) == id(None) and so anyone would come up with his/her own same_as(), identical_to(), same_inst() and so on... Kiuhnm -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
On Thu, Apr 26, 2012 at 10:12 PM, Robert Kern robert.k...@gmail.com wrote: Yes. Win64 has 64-bit pointers and 32-bit C longs (and thus 32-bit Python ints). It returns a Python long. Ah, that solves that one. Definite improvement in Python 3 with the merging of the two types, though. Machine integers are simply a performance optimization for small values. ChrisA -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
On 26 April 2012 12:42, Adam Skutt ask...@gmail.com wrote: On Apr 26, 5:10 am, Steven D'Aprano steve +comp.lang.pyt...@pearwood.info wrote: On Wed, 25 Apr 2012 20:50:21 -0700, Adam Skutt wrote: On Apr 25, 8:01 pm, Steven D'Aprano steve +comp.lang.pyt...@pearwood.info wrote: On Wed, 25 Apr 2012 13:49:24 -0700, Adam Skutt wrote: [Sterile pedantry] [More sterile pedantry] [Yet more sterile pedantry] [And more] [Record breaking levels of sterile pedantry] Please stop! Steven, I've learnt a lot from your posts on this list over the years, but too often you spoil it with your compulsion to have the last word on every argument you get involved in at any cost. Some arguments aren't worth winning... -- Arnaud -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
On 4/26/2012 13:45, Adam Skutt wrote: On Apr 26, 1:48 am, John Naglena...@animats.com wrote: On 4/25/2012 5:01 PM, Steven D'Aprano wrote: On Wed, 25 Apr 2012 13:49:24 -0700, Adam Skutt wrote: Though, maybe it's better to use a different keyword than 'is' though, due to the plain English connotations of the term; I like 'sameobj' personally, for whatever little it matters. Really, I think taking away the 'is' operator altogether is better, so the only way to test identity is: id(x) == id(y) Four reasons why that's a bad idea: 1) The is operator is fast, because it can be implemented directly by the interpreter as a simple pointer comparison (or equivalent). This assumes that everything is, internally, an object. In CPython, that's the case, because Python is a naive interpreter and everything, including numbers, is boxed. That's not true of PyPy or Shed Skin. So does is have to force the creation of a temporary boxed object? That's what C# does AFAIK. Java defines '==' as value comparison for primitives and '==' as identity comparison for objects, but I don't exactly know how one would do that in Python. Why should we take from Java one of its worst misfeatures and disfigure Python for life? a==b compares references while a.equals(b) compares values. Really??? Come on... Python's way is much much cleaner. Kiuhnm -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
On Apr 26, 4:42 pm, Adam Skutt ask...@gmail.com wrote: In a mathematical sense, you're saying that given f(x) = x+2, using f(x) is somehow more direct (whatever the hell that even means) than using 'x+2'. That's just not true. We freely and openly interchange them all the time doing mathematics. Programming is no different. If f(x) and x+2 are freely interchangeable then you have referential transparency, a property that only purely functional languages have. In python: a = [1,2] m1 = [a, a] m2 = [[1,2],[1,2]] may make m1 and m2 seem like the same until you assign to m1[0][0]. eg One would not be able to distinguish m1 and m2 in Haskell. On the whole I whole-heartedly agree that 'a is b' be replaced by id(a) == id(b), with id itself replaced by something more obviously implementational like addrof. The reasons are fundamental: The word 'is' is arguably the primal existential verb. Whereas 'is' in python is merely a leakage of implementation up to the programmer level: http://www.joelonsoftware.com/articles/LeakyAbstractions.html Such a leakage may be justified just as C allowing inline asm may be justified. However dignifying such a leakage with the primal existential verb causes all the confusions seen on this thread (and the various stackoverflow questions etc). -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
On Apr 26, 9:37 am, Kiuhnm kiuhnm03.4t.yahoo.it wrote: On 4/26/2012 13:45, Adam Skutt wrote: On Apr 26, 1:48 am, John Naglena...@animats.com wrote: On 4/25/2012 5:01 PM, Steven D'Aprano wrote: On Wed, 25 Apr 2012 13:49:24 -0700, Adam Skutt wrote: Though, maybe it's better to use a different keyword than 'is' though, due to the plain English connotations of the term; I like 'sameobj' personally, for whatever little it matters. Really, I think taking away the 'is' operator altogether is better, so the only way to test identity is: id(x) == id(y) Four reasons why that's a bad idea: 1) The is operator is fast, because it can be implemented directly by the interpreter as a simple pointer comparison (or equivalent). This assumes that everything is, internally, an object. In CPython, that's the case, because Python is a naive interpreter and everything, including numbers, is boxed. That's not true of PyPy or Shed Skin. So does is have to force the creation of a temporary boxed object? That's what C# does AFAIK. Java defines '==' as value comparison for primitives and '==' as identity comparison for objects, but I don't exactly know how one would do that in Python. Why should we take from Java one of its worst misfeatures and disfigure Python for life? There are a lot of misfeatures in Java. Lack of operating overloading really isn't one of them. I prefer languages that include operator overloading, but readily understand and accept the arguments against it. Nor is the differing behavior for '==' between primitives and objects a misfeature. C# and Python do have a misfeature: '==' is identity comparison only if operator== / __eq__ is not overloaded. Identity comparison and value comparison are disjoint operations, so it's entirely inappropriate to combine them. I don't necessarily mind if the two operations have the same symbol, as long as there's some other way in-context to determine which operation is occurring. This is the case in C and C++, for example. Python's way is much much cleaner. Nope. Automatically substituting identity equality for value equality is wrong. While rare, there are legitimate reasons for the former to be True while the latter is False. Moreover, it means that class authors must remember to write an __eq__ when appropriate and won't get any sort of error when they forget to do so. That can lead to bugs. Adam -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
On Fri, Apr 27, 2012 at 12:00 AM, Adam Skutt ask...@gmail.com wrote: C# and Python do have a misfeature: '==' is identity comparison only if operator== / __eq__ is not overloaded. Identity comparison and value comparison are disjoint operations, so it's entirely inappropriate to combine them. So what should happen if you don't implement __eq__? Should the == operator throw an exception? This can be done fairly easily: class object(object): def __eq__(self,other): raise NoYouDontException(Naughty programmer, mustn't do that!) (Works only if you always explicitly subclass object, even though that's not necessary in Python 3.) ChrisA -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
On Apr 26, 10:18 am, rusi rustompm...@gmail.com wrote: On Apr 26, 4:42 pm, Adam Skutt ask...@gmail.com wrote: In a mathematical sense, you're saying that given f(x) = x+2, using f(x) is somehow more direct (whatever the hell that even means) than using 'x+2'. That's just not true. We freely and openly interchange them all the time doing mathematics. Programming is no different. If f(x) and x+2 are freely interchangeable then you have referential transparency, a property that only purely functional languages have. In python: I think you misunderstood what I was trying to explain. Steven is trying to claim that there's some sort of meaningful difference between calling an operation/algorithm/function by some name versus handing out its definition. I was merely pointing out that we routinely substitute the two when it is appropriate to do so. My apologies if you somehow took that to mean that I was implying there was referential transparency here. I couldn't think of a better example for what I was trying to say. Adam -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
On 4/26/2012 16:00, Adam Skutt wrote: On Apr 26, 9:37 am, Kiuhnmkiuhnm03.4t.yahoo.it wrote: On 4/26/2012 13:45, Adam Skutt wrote: On Apr 26, 1:48 am, John Naglena...@animats.comwrote: On 4/25/2012 5:01 PM, Steven D'Aprano wrote: On Wed, 25 Apr 2012 13:49:24 -0700, Adam Skutt wrote: Though, maybe it's better to use a different keyword than 'is' though, due to the plain English connotations of the term; I like 'sameobj' personally, for whatever little it matters. Really, I think taking away the 'is' operator altogether is better, so the only way to test identity is: id(x) == id(y) Four reasons why that's a bad idea: 1) The is operator is fast, because it can be implemented directly by the interpreter as a simple pointer comparison (or equivalent). This assumes that everything is, internally, an object. In CPython, that's the case, because Python is a naive interpreter and everything, including numbers, is boxed. That's not true of PyPy or Shed Skin. So does is have to force the creation of a temporary boxed object? That's what C# does AFAIK. Java defines '==' as value comparison for primitives and '==' as identity comparison for objects, but I don't exactly know how one would do that in Python. Why should we take from Java one of its worst misfeatures and disfigure Python for life? There are a lot of misfeatures in Java. Lack of operating overloading really isn't one of them. I prefer languages that include operator overloading, but readily understand and accept the arguments against it. Nor is the differing behavior for '==' between primitives and objects a misfeature. The fact that you think that that's differing behaviour is what makes it a misfeature. The fact that you think that '==' can take objects as operands confirms that Java *does* confuse programmers. C# and Python do have a misfeature: '==' is identity comparison only if operator== / __eq__ is not overloaded. Identity comparison and value comparison are disjoint operations, so it's entirely inappropriate to combine them. They're not disjoint, in fact one almost always implies the other (*). Python's idea is that, by default, any object is equal to itself and only itself. The fact that this is equivalent to identity comparison is just a coincidence, from a conceptual point of view. (*) nan == nan is false, but, at least conceptually, a 'NotComparable' exception should be raised instead. That wouldn't be very useful, though. I don't necessarily mind if the two operations have the same symbol, as long as there's some other way in-context to determine which operation is occurring. This is the case in C and C++, for example. Python's way is much much cleaner. Nope. Automatically substituting identity equality for value equality is wrong. While rare, there are legitimate reasons for the former to be True while the latter is False. There shouldn't be, to be fair. Moreover, it means that class authors must remember to write an __eq__ when appropriate and won't get any sort of error when they forget to do so. That can lead to bugs. I can agree on that, but that's something you can solve with a minor modification to the language. What I was talking about is the core design of Java and Python. Kiuhnm -- http://mail.python.org/mailman/listinfo/python-list
Re: Re: why () is () and [] is [] work in other way?
This thread has already beaten a dead horse enough that the horse came
back as a zombie and was re-killed, but I couldn't help but respond to
this part:
On 01/-10/-28163 01:59 PM, Adam Skutt wrote:
Code that relies on the identity of a temporary object is generally
incorrect. This is why C++ explicitly forbids taking the address
(identity) of temporaries.
Except that C++ *doesn't* really forbid taking the address of a
temporary, at least indirectly:
#include iostream
int const * address_of(int const x) {
return x;
}
int main() {
std::cout address_of(1+2) \n;
}
That complies without warning with GCC 4.6 '-Wall -Wextra', MSVC 2010
'/W4', and Comeau's online front end, and I am pretty confident that the
above code is perfectly legal in terms of provoking undefined behavior
(in the technical C++ sense of your program is now allowed to set your
cat on fire).
Evan
--
http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
On Thu, Apr 26, 2012 at 3:10 AM, Steven D'Aprano steve+comp.lang.pyt...@pearwood.info wrote: But I was actually referring to something more fundamental than that. The statement a is b is a *direct* statement of identity. John is my father. id(a) == id(b) is *indirect*: The only child of John's grandfather is the parent of the mother-in-law of my sister-in-law sort of thing. (Excuse me if I got the relationships mixed up.) I might have used a different example: 1) John is my father. 2) John has the same social security number as my father. The first is concise and clearly expresses the intended statement of identity. The second could be read to imply any number of things: John is my father. Somebody at the SSA messed up and gave John the same SSN as my father. John is an identity thief. If the assertion I'm trying to express is that John is my father, then the direct statement #1 is the best way to convey that, at least in English. Likewise, a is b more clearly expresses the intended comparison than does id(a) == id(b). -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
On Apr 26, 7:44 pm, Adam Skutt ask...@gmail.com wrote: On Apr 26, 10:18 am, rusi rustompm...@gmail.com wrote: On Apr 26, 4:42 pm, Adam Skutt ask...@gmail.com wrote: In a mathematical sense, you're saying that given f(x) = x+2, using f(x) is somehow more direct (whatever the hell that even means) than using 'x+2'. That's just not true. We freely and openly interchange them all the time doing mathematics. Programming is no different. If f(x) and x+2 are freely interchangeable then you have referential transparency, a property that only purely functional languages have. In python: I think you misunderstood what I was trying to explain. Steven is trying to claim that there's some sort of meaningful difference between calling an operation/algorithm/function by some name versus handing out its definition. I was merely pointing out that we routinely substitute the two when it is appropriate to do so. My apologies if you somehow took that to mean that I was implying there was referential transparency here. I couldn't think of a better example for what I was trying to say. Adam And my apologies... I forgot to state my main point: Programmer accessible object identity is the principal impediment to referential transparency. In a functional language one can bind a name to a value -- period. There is nothing more essence-ial -- its platonic id -- to the name than that and so the whole can of worms connected with object identity remains sealed within the language implementation. -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
On 4/26/2012 1:48 AM, John Nagle wrote: This assumes that everything is, internally, an object. In CPython, that's the case, because Python is a naive interpreter and everything, including numbers, is boxed. That's not true of PyPy or Shed Skin. So does is have to force the creation of a temporary boxed object? Python Language Reference 3.1. Objects, values and types Objects are Python’s abstraction for data. All data in a Python program is represented by objects or by relations between objects. ... Every object has an identity, a type and a value. An object’s identity never changes once it has been created; ... The ‘is‘ operator compares the identity of two objects; the id()function* returns an integer representing its identity#. [notes added] * the built-in function bound to 'id' on startup. # and that integer must not change for the lifetime of the object. None of the above is CPython implementation detail. What the spec 'forces' is observable behavior. Internal shortcuts are allowed. If an interpreter 'knows' that 'a' represents anything other than None, than it can evaluate 'a is None' as False without boxing 'a' anymore than it is already. The object model above allows for an object to represent or consist of a collection of raw, non-object internal data fields. A list is a sequence of Python objects. A string is not; it is a sequence of 'characters' or 'bytes'. Structs and arrays, including numpy arrays, are similar in containing non-object values. However, when single characters, bytes, or other binary values are extracted and exposed to Python code by indexing or iteration, they must be objectivized (or at least are in CPython). Worrying about 'is' and 'id' forcing objectness is misplaced. Except for comparing an object to a pre-defined constant or sentinel, 'is' is mainly used for introspection and testing. The main use of 'id()' is to make unambiguous string representations of functions, classes, and modules. The real 'culprits' for (potentially) forcing objectness are everyday indexing and iteration. Numpy avoids boxing internal binary values by providing functions that operate on numpy arrays *without* exposing the internal values at the Python level. I believe that psycho and now pypy can analyze a function to determine whether machine ints and floats can used without being boxed, -- Terry Jan Reedy -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
On 4/26/2012 4:45 AM, Adam Skutt wrote: On Apr 26, 1:48 am, John Naglena...@animats.com wrote: On 4/25/2012 5:01 PM, Steven D'Aprano wrote: On Wed, 25 Apr 2012 13:49:24 -0700, Adam Skutt wrote: Though, maybe it's better to use a different keyword than 'is' though, due to the plain English connotations of the term; I like 'sameobj' personally, for whatever little it matters. Really, I think taking away the 'is' operator altogether is better, so the only way to test identity is: id(x) == id(y) Four reasons why that's a bad idea: 1) The is operator is fast, because it can be implemented directly by the interpreter as a simple pointer comparison (or equivalent). This assumes that everything is, internally, an object. In CPython, that's the case, because Python is a naive interpreter and everything, including numbers, is boxed. That's not true of PyPy or Shed Skin. So does is have to force the creation of a temporary boxed object? That's what C# does AFAIK. Java defines '==' as value comparison for primitives and '==' as identity comparison for objects, but I don't exactly know how one would do that in Python. I would suggest that is raise ValueError for the ambiguous cases. If both operands are immutable, is should raise ValueError. That's the case where the internal representation of immutables shows through. If this breaks a program, it was broken anyway. It will catch bad comparisons like if x is 1000 : ... which is implementation dependent. John Nagle -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
On Apr 26, 12:02 pm, Kiuhnm kiuhnm03.4t.yahoo.it wrote: On 4/26/2012 16:00, Adam Skutt wrote: On Apr 26, 9:37 am, Kiuhnmkiuhnm03.4t.yahoo.it wrote: On 4/26/2012 13:45, Adam Skutt wrote: On Apr 26, 1:48 am, John Naglena...@animats.comwrote: This assumes that everything is, internally, an object. In CPython, that's the case, because Python is a naive interpreter and everything, including numbers, is boxed. That's not true of PyPy or Shed Skin. So does is have to force the creation of a temporary boxed object? That's what C# does AFAIK. Java defines '==' as value comparison for primitives and '==' as identity comparison for objects, but I don't exactly know how one would do that in Python. Why should we take from Java one of its worst misfeatures and disfigure Python for life? There are a lot of misfeatures in Java. Lack of operating overloading really isn't one of them. I prefer languages that include operator overloading, but readily understand and accept the arguments against it. Nor is the differing behavior for '==' between primitives and objects a misfeature. The fact that you think that that's differing behaviour is what makes it a misfeature. The fact that you think that '==' can take objects as operands confirms that Java *does* confuse programmers. The equality operator can absolutely be used between two objects. Try it if you don't believe me. It always does identity comparison when given two objects. It can also be given two primitives, and in this case, it does value comparison. Despite performing different operations with the same symbol, there's little risk of confusion because I can trivially figure out if a variable is an object or an primitive. C# and Python do have a misfeature: '==' is identity comparison only if operator== / __eq__ is not overloaded. Identity comparison and value comparison are disjoint operations, so it's entirely inappropriate to combine them. They're not disjoint, in fact one almost always implies the other (*). Almost always isn't a rebuttal. There's no requirement whatsoever for the results of identity comparison to be related to the results of value comparison, ergo they are disjoint. Changing one doesn't have to influence the other. Please note that I never advocated doing what Java does, I merely noted what it does. Python's idea is that, by default, any object is equal to itself and only itself. Which is just wrong-headed. Many types have no meaningful definition for value equality, ergo any code that attempts to perform the operation is incorrect. (*) nan == nan is false, but, at least conceptually, a 'NotComparable' exception should be raised instead. That wouldn't be very useful, though. Python's way is much much cleaner. Nope. Automatically substituting identity equality for value equality is wrong. While rare, there are legitimate reasons for the former to be True while the latter is False. There shouldn't be, to be fair. Which is the whole problem. It's nice to keep erroneous conditions out of your domain, but it's just not always possible. I don't know how you implement NaN (which you need) without allowing for this. I don't know how you implement SQL NULL without allowing for this. While lots of problems can avoid this issue, I'm not sure all problems can. Moreover, I don't know how to implement a value comparison for many objects, so the operation should just be undefined. I should point out that I was a little hasty in painting Python with the same brush as C# and excluding Java. Python and Java are equally bad: value equality defaults to identity equality but there are distinct operations for telling them apart. People want identity equality in Python write 'is', not '=='. People who explicitly want value equality in Java write 'equals()'. I apologize, and blame skipping breakfast this morning. C# is arguably worse, since '==' on objects is defined as identity equality unless it has been overridden. This means that that the intent of the operation varies with no easy way to figure it out in context, you simply have to know. C# also provides a way to test only for identity, Object.ReferenceEquals(), but it's underused. Ultimately this is really a problem of documentation: the language shouldn't encourage conflation of intent in the manner it does. Moreover, it means that class authors must remember to write an __eq__ when appropriate and won't get any sort of error when they forget to do so. That can lead to bugs. I can agree on that, but that's something you can solve with a minor modification to the language. What I was talking about is the core design of Java and Python. The only difference is I see is which comparison is performed by the == symbol. But I don't see how nor why Python's decisions are superior to Java. Plus, I never suggested that Python should do what Java does, merely noted what it did since it
Re: why () is () and [] is [] work in other way?
On Apr 26, 1:34 pm, rusi rustompm...@gmail.com wrote: On Apr 26, 7:44 pm, Adam Skutt ask...@gmail.com wrote: On Apr 26, 10:18 am, rusi rustompm...@gmail.com wrote: On Apr 26, 4:42 pm, Adam Skutt ask...@gmail.com wrote: In a mathematical sense, you're saying that given f(x) = x+2, using f(x) is somehow more direct (whatever the hell that even means) than using 'x+2'. That's just not true. We freely and openly interchange them all the time doing mathematics. Programming is no different. If f(x) and x+2 are freely interchangeable then you have referential transparency, a property that only purely functional languages have. In python: I think you misunderstood what I was trying to explain. Steven is trying to claim that there's some sort of meaningful difference between calling an operation/algorithm/function by some name versus handing out its definition. I was merely pointing out that we routinely substitute the two when it is appropriate to do so. My apologies if you somehow took that to mean that I was implying there was referential transparency here. I couldn't think of a better example for what I was trying to say. Adam And my apologies... I forgot to state my main point: Programmer accessible object identity is the principal impediment to referential transparency. In a functional language one can bind a name to a value -- period. There is nothing more essence-ial -- its platonic id -- to the name than that and so the whole can of worms connected with object identity remains sealed within the language implementation. Yes, I agree that object identity is a major hold up, but I think side effects are a bigger problem. It's possible in C++ to create types that behave like the primitive types without too much difficulty, hence making object identity unimportant. However, it's considerably more difficult in C++ to write side-effect free code[1]. This is a bit of an apple and orange thing, though. ;) I often wonder what the world would be like if Python, C#, and Java embraced value types more, and had better support for pure functions. Unfortunately, building a language where all types behave like that is rather difficult, as the Haskell guys have shown us ;). Adam [1] Or even just code that only uses side-effects the compiler understands. -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
On Apr 26, 2:31 pm, John Nagle na...@animats.com wrote: On 4/26/2012 4:45 AM, Adam Skutt wrote: On Apr 26, 1:48 am, John Naglena...@animats.com wrote: On 4/25/2012 5:01 PM, Steven D'Aprano wrote: On Wed, 25 Apr 2012 13:49:24 -0700, Adam Skutt wrote: Though, maybe it's better to use a different keyword than 'is' though, due to the plain English connotations of the term; I like 'sameobj' personally, for whatever little it matters. Really, I think taking away the 'is' operator altogether is better, so the only way to test identity is: id(x) == id(y) Four reasons why that's a bad idea: 1) The is operator is fast, because it can be implemented directly by the interpreter as a simple pointer comparison (or equivalent). This assumes that everything is, internally, an object. In CPython, that's the case, because Python is a naive interpreter and everything, including numbers, is boxed. That's not true of PyPy or Shed Skin. So does is have to force the creation of a temporary boxed object? That's what C# does AFAIK. Java defines '==' as value comparison for primitives and '==' as identity comparison for objects, but I don't exactly know how one would do that in Python. I would suggest that is raise ValueError for the ambiguous cases. If both operands are immutable, is should raise ValueError. I don't know how you would easily detect user-defined immutable types, nor do I see why such an operation should be an error. I think it would end up violating the principal of least surprise in a lot of cases, especially when talking about things like immutable sets, maps, or other complicated data structures. What I think you want is what I said above: ValueError raised when either operand is a /temporary/ object. Really, it should probably be a parse-time error, since you could (and should) make the determination at parse time. That's the case where the internal representation of immutables shows through. You still have this problem with mutable temporary objects, as my little snipped showed. You're still going to get a result that's inconsistent and/or surprising sooner or later. The problem is the temporary nature of the object, not mutability. If this breaks a program, it was broken anyway. It will catch bad comparisons like if x is 1000 : ... which is implementation dependent. Yes, I agree that a correct fix shouldn't break anything except already broken programs. Adam -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
On Thu, Apr 26, 2012 at 1:34 PM, Adam Skutt ask...@gmail.com wrote: What I think you want is what I said above: ValueError raised when either operand is a /temporary/ object. Really, it should probably be a parse-time error, since you could (and should) make the determination at parse time. I'm not sure precisely what you mean by temporary object, so I am taking it to mean an object that is referenced only by the VM stack (or something equivalent for other implementations). In that case: no, you can't. Take f() is g(), where the code objects of f and g are supplied at runtime. Are the objects returned by either of those expressions temporary? Without being able to do static analysis of the code of f and g, there is no way to know. -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
On Fri, Apr 27, 2012 at 7:39 AM, Ian Kelly ian.g.ke...@gmail.com wrote: I'm not sure precisely what you mean by temporary object, so I am taking it to mean an object that is referenced only by the VM stack (or something equivalent for other implementations). In that case: no, you can't. Take f() is g(), where the code objects of f and g are supplied at runtime. Are the objects returned by either of those expressions temporary? Without being able to do static analysis of the code of f and g, there is no way to know. The expression itself will have references to all its operands (at least conceptually). If their refcounts are precisely 1, then the objects are temporaries and will be disposable as soon as the expression's fully evaluated. ChrisA -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
On Thu, Apr 26, 2012 at 3:51 PM, Chris Angelico ros...@gmail.com wrote: On Fri, Apr 27, 2012 at 7:39 AM, Ian Kelly ian.g.ke...@gmail.com wrote: I'm not sure precisely what you mean by temporary object, so I am taking it to mean an object that is referenced only by the VM stack (or something equivalent for other implementations). In that case: no, you can't. Take f() is g(), where the code objects of f and g are supplied at runtime. Are the objects returned by either of those expressions temporary? Without being able to do static analysis of the code of f and g, there is no way to know. The expression itself will have references to all its operands (at least conceptually). Yes, the references on the VM stack. If their refcounts are precisely 1, then the objects are temporaries and will be disposable as soon as the expression's fully evaluated. You can't check ref counts at parse time. -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
On Fri, Apr 27, 2012 at 8:04 AM, Ian Kelly ian.g.ke...@gmail.com wrote: You can't check ref counts at parse time. I know, and it'd be impossible to recognize at parse time for any but the most trivial cases (since names can always be rebound). The detection of temporaries can only be done at run time. ChrisA -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
On Thu, 26 Apr 2012 11:31:39 -0700, John Nagle wrote: I would suggest that is raise ValueError for the ambiguous cases. If both operands are immutable, is should raise ValueError. That's the case where the internal representation of immutables shows through. This breaks one of the most common uses of is, i.e. x is None. And it doesn't prevent a programmer from consfusing is and == with mutable types. If this breaks a program, it was broken anyway. It will catch bad comparisons like if x is 1000 : ... which is implementation dependent. The only way to completely eliminate bugs caused by the programmer relying upon implementation-dependent behaviour is to eliminate implementation- dependent behaviour altogether, which is throwing the baby out with the bath water, IMHO. All practical languages have some implementation-defined behaviour, often far more problematic than Python's. -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
On 4/26/2012 20:54, Adam Skutt wrote: On Apr 26, 12:02 pm, Kiuhnmkiuhnm03.4t.yahoo.it wrote: On 4/26/2012 16:00, Adam Skutt wrote: On Apr 26, 9:37 am, Kiuhnmkiuhnm03.4t.yahoo.it wrote: On 4/26/2012 13:45, Adam Skutt wrote: On Apr 26, 1:48 am, John Naglena...@animats.com wrote: This assumes that everything is, internally, an object. In CPython, that's the case, because Python is a naive interpreter and everything, including numbers, is boxed. That's not true of PyPy or Shed Skin. So does is have to force the creation of a temporary boxed object? That's what C# does AFAIK. Java defines '==' as value comparison for primitives and '==' as identity comparison for objects, but I don't exactly know how one would do that in Python. Why should we take from Java one of its worst misfeatures and disfigure Python for life? There are a lot of misfeatures in Java. Lack of operating overloading really isn't one of them. I prefer languages that include operator overloading, but readily understand and accept the arguments against it. Nor is the differing behavior for '==' between primitives and objects a misfeature. The fact that you think that that's differing behaviour is what makes it a misfeature. The fact that you think that '==' can take objects as operands confirms that Java *does* confuse programmers. The equality operator can absolutely be used between two objects. Try it if you don't believe me. It always does identity comparison when given two objects. It can also be given two primitives, and in this case, it does value comparison. Despite performing different operations with the same symbol, there's little risk of confusion because I can trivially figure out if a variable is an object or an primitive. No, it can't be used between objects but only between primitives and references (which should be regarded as primitives, by the way). If you understand that your 'a' is not really an object but a reference to it, everything becomes clear and you see that '==' always do the same thing. You might tell me that that's just an implementation detail, but when an implementation detail is easier to understand and makes more sense than the whole abstraction which is built upon it, something is seriously wrong. The distinction between primitives and objects is unfortunate. It is as if Java tried to get rid of pointers but never completely succeeded in doing that. It's the distinction between primitives and objects that should've been an implementation detail, IMO. Python's lack of this misfeature is what I'm really fond of. Everything else you said, I can agree on. C# and Python do have a misfeature: '==' is identity comparison only if operator== / __eq__ is not overloaded. Identity comparison and value comparison are disjoint operations, so it's entirely inappropriate to combine them. They're not disjoint, in fact one almost always implies the other (*). Almost always isn't a rebuttal. There's no requirement whatsoever for the results of identity comparison to be related to the results of value comparison, ergo they are disjoint. Changing one doesn't have to influence the other. Please note that I never advocated doing what Java does, I merely noted what it does. Python's idea is that, by default, any object is equal to itself and only itself. Which is just wrong-headed. Many types have no meaningful definition for value equality, ergo any code that attempts to perform the operation is incorrect. (*) nan == nan is false, but, at least conceptually, a 'NotComparable' exception should be raised instead. That wouldn't be very useful, though. Python's way is much much cleaner. Nope. Automatically substituting identity equality for value equality is wrong. While rare, there are legitimate reasons for the former to be True while the latter is False. There shouldn't be, to be fair. Which is the whole problem. It's nice to keep erroneous conditions out of your domain, but it's just not always possible. I don't know how you implement NaN (which you need) without allowing for this. I don't know how you implement SQL NULL without allowing for this. While lots of problems can avoid this issue, I'm not sure all problems can. Moreover, I don't know how to implement a value comparison for many objects, so the operation should just be undefined. I should point out that I was a little hasty in painting Python with the same brush as C# and excluding Java. Python and Java are equally bad: value equality defaults to identity equality but there are distinct operations for telling them apart. People want identity equality in Python write 'is', not '=='. People who explicitly want value equality in Java write 'equals()'. I apologize, and blame skipping breakfast this morning. C# is arguably worse, since '==' on objects is defined as identity equality unless it has been overridden. This means that that the intent of the operation varies with no easy
Re: why () is () and [] is [] work in other way?
Nobody nob...@nowhere.com writes: All practical languages have some implementation-defined behaviour, often far more problematic than Python's. The usual reason for accepting implementation-defined behavior is to enable low-level efficiency hacks written for specific machines. C and C++ are used for that sort of purpose, so they leave many things implementation-defined. Python doesn't have the same goals and should leave less up to the implementation. Java, Ada, Standard ML, etc. all try to eliminate implementation-defined behavior in the language much more than Python does. I don't have any idea why you consider that to be throwing the baby out with the bath water. -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
On Thu, 26 Apr 2012 12:22:55 -0700, Adam Skutt wrote: I often wonder what the world would be like if Python, C#, and Java embraced value types more, and had better support for pure functions. They would be slower, require more memory, harder to use, and far, far less popular. Some other languages just like Python, C# and Java would be invented to fill those niches, and the functional-obsessed crowd would then complain that they wished those languages would be more like Python, C# and Java. -- Steven -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
On 20/04/2012 20:10, dmitrey wrote: I have spent some time searching for a bug in my code, it was due to different work of is with () and []: () is () True [] is [] False (Python 2.7.2+ (default, Oct 4 2011, 20:03:08) [GCC 4.6.1] ) Is this what it should be or maybe yielding unified result is better? D. Congratulations, you've managed to introduce the most useless thread on this ng/ml that I've come across in the ten years that I've been using (c)Python. I've never ever thought about using this weird thing called id in my code, (which comes later in all the responses) so why do some people think it's so important? -- Cheers. Mark Lawrence. -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
On Apr 26, 6:34 pm, Kiuhnm kiuhnm03.4t.yahoo.it wrote: On 4/26/2012 20:54, Adam Skutt wrote: On Apr 26, 12:02 pm, Kiuhnmkiuhnm03.4t.yahoo.it wrote: On 4/26/2012 16:00, Adam Skutt wrote: On Apr 26, 9:37 am, Kiuhnmkiuhnm03.4t.yahoo.it wrote: The fact that you think that that's differing behaviour is what makes it a misfeature. The fact that you think that '==' can take objects as operands confirms that Java *does* confuse programmers. The equality operator can absolutely be used between two objects. Try it if you don't believe me. It always does identity comparison when given two objects. It can also be given two primitives, and in this case, it does value comparison. Despite performing different operations with the same symbol, there's little risk of confusion because I can trivially figure out if a variable is an object or an primitive. No, it can't be used between objects but only between primitives and references (which should be regarded as primitives, by the way). The only way to access an object is through a reference. If you understand that your 'a' is not really an object but a reference to it, everything becomes clear and you see that '==' always do the same thing. Yes, object identity is implemented almost? everywhere by comparing the value of two pointers (references)[1]. I've already said I'm not really sure how else one would go about implementing it. You might tell me that that's just an implementation detail, but when an implementation detail is easier to understand and makes more sense than the whole abstraction which is built upon it, something is seriously wrong. I'm not sure what abstraction is being built here. I think you have me confused for someone else, possibly Steven. You're missing the big picture. The two comparisons are asking different questions: Value equality asks if the operands 'have the same state' regardless of how they exist in memory. Identity equality asks if the two operands are the same block of memory. The two are distinct because not all types support both operations. If I write a function that does a value comparison, then it should do value comparison on _every type that can be passed to it_, regardless of whether the type is a primitive or an object, whether it has value or reference semantics, and regardless of how value comparison is actually implemented. If I write some function: f(x : T, y : U) = x == y where T and U are some unknown types, then I want the function to do a value comparison for every type pairing that allows the function to compile. Likewise, if I write a function that does identity comparison, then it logically wants to do identity comparison on _every type that can be passed to it_. To accomplish this, I must have a distinct way of asking each question. In Python we have '==' and 'is'[2]; in Java we have 'Object.equals()' and '=='; in C and C++ we distinguish by the types of the variables being compared (T and T*). Java gives '==' a different meaning for primitive types, but that turns out to be OK because I can't write a function that takes both a primitive type and a reference type at the same position. Yes, the reason it does this is due to what I said above, but that doesn't have any bearing on why we pick one operation over the other as programmers. The distinction between primitives and objects is unfortunate. It is as if Java tried to get rid of pointers but never completely succeeded in doing that. It's the distinction between primitives and objects that should've been an implementation detail, IMO. Python's lack of this misfeature is what I'm really fond of. If anything, you have that backwards. Look at Python: all variables in Python have pointer semantics, not value semantics. In imperative languages, pointers have greater utility over value types because not all types can obey the rules for value types. For example, I don't know how to give value semantics to something like a I/O object (e.g, file, C++ fstream, C FILE), since I don't know how to create independent copies. One can obviously create an imperative language without pointers, but I/O gets rather tricky. Adam [1] Though it need not be (and often isn't) as simple as comparing two integers. [2] Well, I suspect 'is' gets used mostly for comparisons against None, True, and False in Python. -- http://mail.python.org/mailman/listinfo/python-list
Re: Re: why () is () and [] is [] work in other way?
On Thu, Apr 26, 2012 at 12:05 PM, Evan Driscoll drisc...@cs.wisc.edu wrote:
This thread has already beaten a dead horse enough that the horse came back
as a zombie and was re-killed, but I couldn't help but respond to this part:
On 01/-10/-28163 01:59 PM, Adam Skutt wrote:
Code that relies on the identity of a temporary object is generally
incorrect. This is why C++ explicitly forbids taking the address
(identity) of temporaries.
Except that C++ *doesn't* really forbid taking the address of a temporary,
at least indirectly:
#include iostream
int const * address_of(int const x) {
return x;
}
int main() {
std::cout address_of(1+2) \n;
}
That complies without warning with GCC 4.6 '-Wall -Wextra', MSVC 2010 '/W4',
and Comeau's online front end, and I am pretty confident that the above code
is perfectly legal in terms of provoking undefined behavior (in the
technical C++ sense of your program is now allowed to set your cat on
fire).
Yes, you can get a const reference to a temporary object, but that's
the only thing you can do. This is intentional, so you can use
temporaries (e.g., std::string(Hello World) ) in the same contexts
where one would use a literal (e.g., 3 or 4.2). Note that it's
impossible to mutate the temporary and impossible for the reference to
outlive the temporary.
What the standard says is: The result of the unary operator is a
pointer to its operand. The operand shall be an lvalue or a
qualified-id. The unary operator is known as the address-of
operator. The C++ standard is actually going further than forbidding
temporaries, it forbids rvalues, which are things one expects to see
on the Right hand side of an assignment, or =.
One of Scott Meyer's Effective C++ books covers all of this in great
detail, including how you can get a temporary that's an lvalue as
opposed to an rvalue.
Adam
--
http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
On Thu, Apr 26, 2012 at 5:39 PM, Ian Kelly ian.g.ke...@gmail.com wrote: On Thu, Apr 26, 2012 at 1:34 PM, Adam Skutt ask...@gmail.com wrote: What I think you want is what I said above: ValueError raised when either operand is a /temporary/ object. Really, it should probably be a parse-time error, since you could (and should) make the determination at parse time. I'm not sure precisely what you mean by temporary object, so I am taking it to mean an object that is referenced only by the VM stack (or something equivalent for other implementations). In that case: no, you can't. Take f() is g(), where the code objects of f and g are supplied at runtime. Are the objects returned by either of those expressions temporary? Without being able to do static analysis of the code of f and g, there is no way to know. A temporary object would be anything that need not be alive longer than the duration of the 'is' operation. I am not a Python language expert so that definition may not be exactly correct or workable for Python. In the example: [1, 2] is [3, 4] [1,2] and [3,4] don't need to exist before the 'is' operation, nor after it, so they are temporaries. Adam -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
On Apr 26, 7:33 pm, Steven D'Aprano steve +comp.lang.pyt...@pearwood.info wrote: On Thu, 26 Apr 2012 12:22:55 -0700, Adam Skutt wrote: I often wonder what the world would be like if Python, C#, and Java embraced value types more, and had better support for pure functions. They would be slower, require more memory, Funny, Haskell frequently beats C in both categories. MATLAB is faster and more memory efficient than naive C matrix code, since it has a very efficient copy-on-write implementation. As the various C++ matrix libraries will show you, efficient COW is much harder when you have to deal with C++ aliasing rules. harder to use, and far, far less popular. Alas, these two are probably true. Adam -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
Adam Skutt ask...@gmail.com writes: harder to use, and far, far less popular. Alas, these two are probably true. Haskell is kind of abstruse and has a notoriously steep learning curve, as it's mostly meant as a research testbed and as a playground for language geeks. ML/OCaml is by all accounts much easier, and I know of a couple of former Python projects that successfully migrated to OCaml once Python's warts and low performance got too annoying. Erlang (which is functional but untyped) has also been displacing Python in some settings. -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
On 27/04/2012 00:57, Adam Skutt wrote: ...And Saint Adam Skutt raised the hand grenade up on high, saying, O LORD, bless this Thy hand grenade that with it Thou mayest blow Thine Id to tiny bits, in Thy mercy. And the LORD did grin and the people did feast upon the lambs and sloths and carp and anchovies and orangutans and breakfast cereals, and fruit bats and large chu... [At this point, the friar is urged by Brother Maynard to skip a bit, brother]... And the LORD spake, saying, First shalt thou take out the Holy Pin, then shalt thou count to three, no more, no less. Three shall be the number thou shalt count, and the number of the counting shall be three. Four shalt thou not count, neither count thou two, excepting that thou then proceed to three. Five is right out. Once the number three, being the third number, be reached, then lobbest thou thy Holy Hand Grenade of Antioch towards thy foe, who being naughty in My sight, shall snuff it. -- Cheers. Mark Lawrence. -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
On Thu, 26 Apr 2012 17:16:10 -0700, Adam Skutt wrote: On Apr 26, 7:33 pm, Steven D'Aprano steve +comp.lang.pyt...@pearwood.info wrote: On Thu, 26 Apr 2012 12:22:55 -0700, Adam Skutt wrote: I often wonder what the world would be like if Python, C#, and Java embraced value types more, and had better support for pure functions. They would be slower, require more memory, Funny, Haskell frequently beats C in both categories. We've both been guilty of this, but don't confuse a language implementation with a language. Haskell and C are languages, which in a sense are like Platonic ideals: languages specify behaviour and semantics, but have no costs. When talking about resource usage, you need to talk about concrete implementations of concrete tests, not hand-wavy language X is faster. And I'm afraid that your claim of Haskell frequently beating C doesn't stand up to scrutiny. http://shootout.alioth.debian.org/u64q/benchmark.php?test=alllang=ghclang2=gcc I'm seeing code generated by the Haskell GHC compiler being 2-4 times slower than code from the C gcc compiler, and on average using 2-3 times as much memory (and as much as 7 times). Feel free to find your own set of benchmarks that show the opposite. I'd be interested to see under what conditions Haskell might be faster than C. -- Steven -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
Adam Skutt wrote: If I write a function that does a value comparison, then it should do value comparison on _every type that can be passed to it_, regardless of whether the type is a primitive or an object, whether it has value or reference semantics, and regardless of how value comparison is actually implemented. If I write some function: f(x : T, y : U) = x == y where T and U are some unknown types, then I want the function to do a value comparison for every type pairing that allows the function to compile. Likewise, if I write a function that does identity comparison, then it logically wants to do identity comparison on _every type that can be passed to it_. What you say here makes perfect sense, but also shows that you really shouldn't be using Python if you want stuff to work this way. In Python any value of any type can be passed to any function. The claims you are making about object identity and object equality are reasonable, but as you show here, to really handle them requires dragging in a huge amount of type-system baggage. Python's behavior is perfectly well- defined. You might think it's not the best way to do it based on abstract conceptual frameworks for how programming languages should work, but it works just fine. -- --OKB (not okblacke) Brendan Barnwell Do not follow where the path may lead. Go, instead, where there is no path, and leave a trail. --author unknown -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
On Thu, Apr 26, 2012 at 7:33 PM, Steven D'Aprano steve+comp.lang.pyt...@pearwood.info wrote: On Thu, 26 Apr 2012 12:22:55 -0700, Adam Skutt wrote: I often wonder what the world would be like if Python, C#, and Java embraced value types more, and had better support for pure functions. They would be slower, require more memory, harder to use, and far, far less popular. That's odd. PyPy supports value types and pure functions as a performance optimization. For reference, see e.g. http://morepypy.blogspot.ca/2011/03/controlling-tracing-of-interpreter-with_15.html http://morepypy.blogspot.ca/2011/08/visualization-of-jitted-code.html PyPy can detect some pure functions (or purity annotations) and can run them at (JIT-)compile-time to speed up the runtime. Also, JIT'd code uses unboxed values, rather than objects. (So it can unbox something once, then do all the arithmetic, then box again). (Of course, this is only a special case of a value type, as far as I understand the term. But hey!) Also somebody else was talking about Haskell being crazy fast, but if you want a really fast functional language, take a look at ATS (an ML variant). (Also pay close attention to the bytes of code comparison -- it's rare one sees a language more verbose than C) http://shootout.alioth.debian.org/u64q/benchmark.php?test=alllang=atslang2=gcc -- Devin -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
On Fri, Apr 27, 2012 at 12:47 PM, Steven D'Aprano steve+comp.lang.pyt...@pearwood.info wrote: On Thu, 26 Apr 2012 17:16:10 -0700, Adam Skutt wrote: On Apr 26, 7:33 pm, Steven D'Aprano steve +comp.lang.pyt...@pearwood.info wrote: On Thu, 26 Apr 2012 12:22:55 -0700, Adam Skutt wrote: I often wonder what the world would be like if Python, C#, and Java embraced value types more, and had better support for pure functions. They would be slower, require more memory, Funny, Haskell frequently beats C in both categories. I'm seeing code generated by the Haskell GHC compiler being 2-4 times slower than code from the C gcc compiler, and on average using 2-3 times as much memory (and as much as 7 times). Also, I don't see a Python interpreter written in Haskell that's outperforming CPython. ChrisA -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
Am 24.04.2012 15:25 schrieb rusi: Identity, sameness, equality and the verb to be are all about the same concept(s) and their definitions are *intrinsically* circular; see http://plato.stanford.edu/entries/identity/#2 Mybe in real life language. In programming and mathematics there are several forms of equality, where identity (≡) is stronger than equality (=). Two objects can be equal (=) without being identical (≡), but not the other way. As the ≡ is quite hard to type, programming languages tend to use other operators for this. E.g., in C, you can have int a; int b; a = 4; b = 4; Here a and b are equal, but not identical. One can be changed without changing the other. With int x; int *a=x, *b=x; *a and *b are identical, as they point to the same location. *a = 4 results in *b becoming 4 as well. In Python, you can have the situations described here as well. You can have a list and bind it to 2 names, or you can take 2 lists and bind them to that name. a = [3] b = [3] Here a == b is True, while a is b results in False. Thomas And the seeming simplicity of the circular definitions hide the actual complexity of 'to be' for python: http://docs.python.org/reference/expressions.html#id26 (footnote 7) for math/philosophy: http://www.math.harvard.edu/~mazur/preprints/when_is_one.pdf -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
On Mon, 23 Apr 2012 10:01:24 -0700, Paul Rubin wrote: I can't think of a single case where 'is' is ill-defined. If I can't predict the output of print (20+30 is 30+20) # check whether addition is commutative print (20*30 is 30*20) # check whether multiplication is commutative by just reading the language definition and the code, I'd have to say is is ill-defined. If anything is ill-defined, then it's + and *, i.e. it's unspecified whether the value which they return is a unique object or a reference to some other object. More accurately, the existence of is, is not and id cause many other constructs to have ill-defined behaviour. a is b is true iff 'a' and 'b' are the same object. Why should 'is' lie to the user? Whether a and b are the same object is implementation-dependent. And what's wrong with that? If you want a language which precisely specifies all observable behaviour, you're going to end up with a rather useless language. For a start, it can't have a time() function. For similar reasons, you can't have networking or any form of preemptive concurrency (which includes any form of inter-process communication on an OS which uses preemptive multi-tasking). -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
On Wed, 25 Apr 2012 13:42:31 +0200, Thomas Rachel wrote:
Two objects can be equal (=) without being identical (≡), but not the
other way.
x = float('nan')
y = x
x is y
True
x == y
False
By the way, in mathematics, ≡ normally means is equivalent to, which is
not quite the same as identical to.
http://mathworld.wolfram.com/Equivalent.html
--
Steven
--
http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
On Thu, Apr 26, 2012 at 3:27 AM, Steven D'Aprano steve+comp.lang.pyt...@pearwood.info wrote: By the way, in mathematics, ≡ normally means is equivalent to, which is not quite the same as identical to. That's perhaps because, in mathematics, nobody would even think of asking if this 4 is the same as that 4. What sort of question is it? Four is four! How could you tell one four from another? Mathematics is not the same as programming, and the whole concept of objects in memory simply isn't a mathematical one at all. ChrisA -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
On Apr 25, 10:38 am, Nobody nob...@nowhere.com wrote: On Mon, 23 Apr 2012 10:01:24 -0700, Paul Rubin wrote: I can't think of a single case where 'is' is ill-defined. If I can't predict the output of print (20+30 is 30+20) # check whether addition is commutative print (20*30 is 30*20) # check whether multiplication is commutative by just reading the language definition and the code, I'd have to say is is ill-defined. If anything is ill-defined, then it's + and *, i.e. it's unspecified whether the value which they return is a unique object or a reference to some other object. Such a definition precludes meaningful operator overloading and is highly problematic for floating-point numbers. There's also no way to enforce it, but I think you know that too. :) Identity and equality are distinct concepts in programming languages. There's nothing that can be done about that, and no particularly good reason to force certain language behaviors because some programmers have difficulty with the distinction. Though, maybe it's better to use a different keyword than 'is' though, due to the plain English connotations of the term; I like 'sameobj' personally, for whatever little it matters. Really, I think taking away the 'is' operator altogether is better, so the only way to test identity is: id(x) == id(y) Though I would prefer: addr(x) == addr(y) myself, again, for what little it matters. The right thing to do when confronted with this problem is teach the difference and move on. As an aside, the whole problem with 'is' and literals is perhaps the only really good argument for a 'new' keyword/operator like C++ and Java have. Then it's more explicit to the programmer that they've created two objects (in this case, anyway). More accurately, the existence of is, is not and id cause many other constructs to have ill-defined behaviour. a is b is true iff 'a' and 'b' are the same object. Why should 'is' lie to the user? Whether a and b are the same object is implementation-dependent. And what's wrong with that? If you want a language which precisely specifies all observable behaviour, you're going to end up with a rather useless language. For a start, it can't have a time() function. For similar reasons, you can't have networking or any form of preemptive concurrency (which includes any form of inter-process communication on an OS which uses preemptive multi-tasking). Fully specified does not mean fully deterministic. What makes a specification of Any value in the range 0 through N less 'full' than a specification of X or a constant? Adam -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
On 4/25/2012 4:49 PM, Adam Skutt wrote: Identity and equality are distinct concepts in programming languages. There's nothing that can be done about that, and no particularly good reason to force certain language behaviors because some programmers have difficulty with the distinction. Though, maybe it's better to use a different keyword than 'is' though, due to the plain English connotations of the term; I like 'sameobj' personally, for whatever little it matters. Really, I think taking away the 'is' operator altogether is better, so the only way to test identity is: id(x) == id(y) Though I would prefer: addr(x) == addr(y) myself, again, for what little it matters. The fact that id(x) is machine_addr(x) in CPython is specific to CPython, not required by the language spec, and not true in implementations that move objects around when garbage collecting. -- Terry Jan Reedy -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
On Wed, 25 Apr 2012 13:49:24 -0700, Adam Skutt wrote: Though, maybe it's better to use a different keyword than 'is' though, due to the plain English connotations of the term; I like 'sameobj' personally, for whatever little it matters. Really, I think taking away the 'is' operator altogether is better, so the only way to test identity is: id(x) == id(y) Four reasons why that's a bad idea: 1) The is operator is fast, because it can be implemented directly by the interpreter as a simple pointer comparison (or equivalent). The id() idiom is slow, because it involves two global lookups and an equality comparison. Inside a tight loop, that can make a big difference in speed. 2) The is operator always has the exact same semantics and cannot be overridden. The id() function can be monkey-patched. 3) The is idiom semantics is direct: a is b directly tests the thing you want to test, namely whether a is b. The id() idiom is indirect: id(a) == id(b) only indirectly tests whether a is b. 4) The id() idiom already breaks if you replace names a, b with expressions: id([1,2]) == id([3,4]) True Though I would prefer: addr(x) == addr(y) But that's absolutely wrong. id(x) returns an ID, not an address. It just happens that, as an accident of implementation, the CPython interpreter uses the object address as an ID, because objects can't move. That's not the case for all implementations. In Jython, objects can move and the address is not static, and so IDs are assigned on demand starting with 1: steve@runes:~$ jython Jython 2.5.1+ (Release_2_5_1, Aug 4 2010, 07:18:19) [OpenJDK Client VM (Sun Microsystems Inc.)] on java1.6.0_18 Type help, copyright, credits or license for more information. id(42) 1 id(Hello World!) 2 id(None) 3 Other implementations may make other choices. I don't believe that the language even defines the id as a number, although I could be wrong about that. Personally, I prefer the Jython approach, because it avoids those annoying questions like How do I dereference the address of an object? (answer: Python is not C, you can't do that), and IDs are globally unique and never reused for the lifetime of the process. -- Steven -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
In article 4f9833ff$0$29965$c3e8da3$54964...@news.astraweb.com,
Steven D'Aprano steve+comp.lang.pyt...@pearwood.info wrote:
On Wed, 25 Apr 2012 13:42:31 +0200, Thomas Rachel wrote:
Two objects can be equal (=) without being identical (â¡), but not the
other way.
x = float('nan')
y = x
x is y
True
x == y
False
I love it. Thanks for posting that.
--
http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
On Apr 25, 8:01 pm, Steven D'Aprano steve +comp.lang.pyt...@pearwood.info wrote: On Wed, 25 Apr 2012 13:49:24 -0700, Adam Skutt wrote: Though, maybe it's better to use a different keyword than 'is' though, due to the plain English connotations of the term; I like 'sameobj' personally, for whatever little it matters. Really, I think taking away the 'is' operator altogether is better, so the only way to test identity is: id(x) == id(y) Four reasons why that's a bad idea: 1) The is operator is fast, because it can be implemented directly by the interpreter as a simple pointer comparison (or equivalent). The id() idiom is slow, because it involves two global lookups and an equality comparison. Inside a tight loop, that can make a big difference in speed. The runtime can optimize the two operations to be equivalent, since they are logically equivalent operations. If you removed 'is', there's little reason to believe it would do otherwise. 2) The is operator always has the exact same semantics and cannot be overridden. The id() function can be monkey-patched. I can't see how that's useful at all. Identity is a fundamental property of an object; hence retrieval of it must be a language operation. The fact Python chooses to do otherwise is unfortunate, but also irrelevant to my position. 3) The is idiom semantics is direct: a is b directly tests the thing you want to test, namely whether a is b. The id() idiom is indirect: id(a) == id(b) only indirectly tests whether a is b. The two expressions are logically equivalent, so I don't see how this matters, nor how it is true. 4) The id() idiom already breaks if you replace names a, b with expressions: id([1,2]) == id([3,4]) True It's not broken at all. The lifetime of temporary objects is intentionally undefined, and that's a /good/ thing. What's unfortunate is that CPython optimizes temporaries differently between the two logically equivalent expressions. As long as this holds: class A(object): ... def __del__(self): ...print Farewell to: %d % id(self) ... A() is A() Farewell to: 4146953292 Farewell to: 4146953260 False id(A()) == id(A()) Farewell to: 4146953420 Farewell to: 4146953420 True then there's nothing broken about the behavior of either expression. I personally think logically equivalent expressions should give the same results, but since both operations follow the rules of object identity correctly, it's not the end of the world. It's only surprising to the programmer if: 1) They don't understand identity. 2) They don't understand what objects are and are not temporaries. Code that relies on the identity of a temporary object is generally incorrect. This is why C++ explicitly forbids taking the address (identity) of temporaries. As such, the language behavior in your case is inconsequential. Making demons fly out of the programmer's nose would be equally appropriate. The other solution is to do what Java and C# do: banish id() entirely and only provide 'is' (== in Java, Object.ReferenceEquals() in C#). That seems just as fine, really, Practically, it's also probably the better solution for CPython, which is fine by me. My preference for keeping id() and removing 'is' probably comes from my background as a C ++ programmer, and I already said it matters very little. But that's absolutely wrong. id(x) returns an ID, not an address. It just happens that, as an accident of implementation, the CPython interpreter uses the object address as an ID, because objects can't move. That's not the case for all implementations. In Jython, objects can move and the address is not static, and so IDs are assigned on demand starting with 1: steve@runes:~$ jython Jython 2.5.1+ (Release_2_5_1, Aug 4 2010, 07:18:19) [OpenJDK Client VM (Sun Microsystems Inc.)] on java1.6.0_18 Type help, copyright, credits or license for more information. id(42) 1 id(Hello World!) 2 id(None) 3 An address is an identifier: a number that I can use to access a value[1]. I never said that id() must return an address the host CPU understands (virtual, physical, or otherwise). Most languages use addresses that the host CPU cannot understand without assistance at least sometimes, including C on some platforms. Other implementations may make other choices. I don't believe that the language even defines the id as a number, although I could be wrong about that. http://docs.python.org/library/functions.html#id says it must be an integer of some sort. Even if it didn't say that, it hardly seems as a practical imposition. Personally, I prefer the Jython approach, because it avoids those annoying questions like How do I dereference the address of an object? (answer: Python is not C, you can't do that), The right way to solve that question isn't to fix the runtime, but to teach people what pointer semantics actually mean, much like the identity problem we're discussing now. Adam [1] I'd be
Re: why () is () and [] is [] work in other way?
On Thu, Apr 26, 2012 at 1:50 PM, Adam Skutt ask...@gmail.com wrote: On Apr 25, 8:01 pm, Steven D'Aprano steve +comp.lang.pyt...@pearwood.info wrote: 2) The is operator always has the exact same semantics and cannot be overridden. The id() function can be monkey-patched. I can't see how that's useful at all. Identity is a fundamental property of an object; hence retrieval of it must be a language operation. ... The other solution is to do what Java and C# do: banish id() entirely and only provide 'is' (== in Java, Object.ReferenceEquals() in C#). The 'is' operator is a language feature. The id() function is not. ChrisA -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
On 4/25/2012 5:01 PM, Steven D'Aprano wrote: On Wed, 25 Apr 2012 13:49:24 -0700, Adam Skutt wrote: Though, maybe it's better to use a different keyword than 'is' though, due to the plain English connotations of the term; I like 'sameobj' personally, for whatever little it matters. Really, I think taking away the 'is' operator altogether is better, so the only way to test identity is: id(x) == id(y) Four reasons why that's a bad idea: 1) The is operator is fast, because it can be implemented directly by the interpreter as a simple pointer comparison (or equivalent). This assumes that everything is, internally, an object. In CPython, that's the case, because Python is a naive interpreter and everything, including numbers, is boxed. That's not true of PyPy or Shed Skin. So does is have to force the creation of a temporary boxed object? The concept of object vs. the implementation of objects is one reason you don't necessarily want to expose the implementation. John Nagle -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
On Apr 23, 9:34 am, Steven D'Aprano steve +comp.lang.pyt...@pearwood.info wrote: is is never ill-defined. is always, without exception, returns True if the two operands are the same object, and False if they are not. This is literally the simplest operator in Python. Circular definition: In case you did not notice, 'is' and 'are' are (or is it is?) the same verb. -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
Am 24.04.2012 08:02 schrieb rusi: On Apr 23, 9:34 am, Steven D'Apranosteve +comp.lang.pyt...@pearwood.info wrote: is is never ill-defined. is always, without exception, returns True if the two operands are the same object, and False if they are not. This is literally the simplest operator in Python. Circular definition: In case you did not notice, 'is' and 'are' are (or is it is?) the same verb. Steven's definition tries not to define the verb is, but it defines the meanung of the *operator* 'is'. He says that 'a is b' iff a and be are *the same objects*. We don't need to define the verb to be, but the target of the definition is the entity object and its identity. Thomas -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
On 4/24/2012 8:02, rusi wrote: On Apr 23, 9:34 am, Steven D'Apranosteve +comp.lang.pyt...@pearwood.info wrote: is is never ill-defined. is always, without exception, returns True if the two operands are the same object, and False if they are not. This is literally the simplest operator in Python. Circular definition: In case you did not notice, 'is' and 'are' are (or is it is?) the same verb. Python is not English. Double-quoted 'is' is a Python operator, while non-quoted 'is' and 'are' are forms of the English verb 'to be'. If you change the name of the operator or the language in which you define the operator, you'll realize that there's no real circularity in that definition. Kiuhnm -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
On Apr 24, 4:06 pm, Thomas Rachel nutznetz-0c1b6768-bfa9-48d5- a470-7603bd3aa...@spamschutz.glglgl.de wrote: Am 24.04.2012 08:02 schrieb rusi: On Apr 23, 9:34 am, Steven D'Apranosteve +comp.lang.pyt...@pearwood.info wrote: is is never ill-defined. is always, without exception, returns True if the two operands are the same object, and False if they are not. This is literally the simplest operator in Python. Circular definition: In case you did not notice, 'is' and 'are' are (or is it is?) the same verb. Steven's definition tries not to define the verb is, but it defines the meanung of the *operator* 'is'. He says that 'a is b' iff a and be are *the same objects*. We don't need to define the verb to be, but the target of the definition is the entity object and its identity. Identity, sameness, equality and the verb to be are all about the same concept(s) and their definitions are *intrinsically* circular; see http://plato.stanford.edu/entries/identity/#2 And the seeming simplicity of the circular definitions hide the actual complexity of 'to be' for python: http://docs.python.org/reference/expressions.html#id26 (footnote 7) for math/philosophy: http://www.math.harvard.edu/~mazur/preprints/when_is_one.pdf -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
On 4/24/2012 15:25, rusi wrote:
On Apr 24, 4:06 pm, Thomas Rachelnutznetz-0c1b6768-bfa9-48d5-
a470-7603bd3aa...@spamschutz.glglgl.de wrote:
Am 24.04.2012 08:02 schrieb rusi:
On Apr 23, 9:34 am, Steven D'Apranosteve
+comp.lang.pyt...@pearwood.infowrote:
is is never ill-defined. is always, without exception, returns True
if the two operands are the same object, and False if they are not. This
is literally the simplest operator in Python.
Circular definition: In case you did not notice, 'is' and 'are' are
(or is it is?) the same verb.
Steven's definition tries not to define the verb is, but it defines
the meanung of the *operator* 'is'.
He says that 'a is b' iff a and be are *the same objects*. We don't need
to define the verb to be, but the target of the definition is the
entity object and its identity.
Identity, sameness, equality and the verb to be are all about the same
concept(s) and their definitions are *intrinsically* circular; see
http://plato.stanford.edu/entries/identity/#2
And the seeming simplicity of the circular definitions hide the actual
complexity of 'to be'
for python: http://docs.python.org/reference/expressions.html#id26
(footnote 7)
for math/philosophy:
http://www.math.harvard.edu/~mazur/preprints/when_is_one.pdf
What you say is true in general, but not from an operational point of
view, especially if we restrict the set of objects whose sameness or
identity we want to check:
Let O be a set of tuples (id, data) where
{(id, data1), (id, data2)} subset O = data1 = data2
Def. (id1, data1) and (id2, data2) in O are /the same/ iff id1 = id2.
Now, it's easy to find a bijection between O and the set of Python's
objects which are in memory at any single point in time.
Anyway, you're being unnecessarily pedantic.
Kiuhnm
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Re: why () is () and [] is [] work in other way?
Am 21.04.2012 14:51, schrieb gst: Hi, I played (python3.2) a bit on that and : case 1) Ok to me (right hand side is a tuple, whose elements are evaluated one per one and have same effect as your explanation (first [] is destroyed right before the second one is created) : x, y = id([]), id([]) x == y True case 2) also ok to me: x = id([]) ; y = id([]) x == y True case 3) NOT ok to me : x = id([]) y = id([]) x == y False case 4) ok to me : def f(): x = id([]) y = id([]) return x == y f() True I'd have thought that cases 23 are totally, while 34 nearly, syntactically equal and that case 3 is the incorrect result.. how would you explain case 3 vs cases 2 and 4 ?? It is simply not defined if, after creation and destruction of an empty list with some id, a newly created empty list should carry the same id or not. In cases 1,2 and 4 it happens, but in case 3 it doesn't. This is simply an implementation detail and neither behaviour is right or wrong. Alex -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
On 4/22/2012 9:34 PM, Steven D'Aprano wrote: On Sun, 22 Apr 2012 12:43:36 -0700, John Nagle wrote: On 4/20/2012 9:34 PM, john.tant...@gmail.com wrote: On Friday, April 20, 2012 12:34:46 PM UTC-7, Rotwang wrote: I believe it says somewhere in the Python docs that it's undefined and implementation-dependent whether two identical expressions have the same identity when the result of each is immutable Bad design. Where is is ill-defined, it should raise ValueError. is is never ill-defined. is always, without exception, returns True if the two operands are the same object, and False if they are not. This is literally the simplest operator in Python. John, you've been using Python for long enough that you should know this. I can only guess that you are trolling, although I can't imagine why. Because the language definition should not be what CPython does. As PyPy advances, we need to move beyond that. John Nagle -- http://mail.python.org/mailman/listinfo/python-list
Re: why () is () and [] is [] work in other way?
On 4/22/2012 21:43, John Nagle wrote: On 4/20/2012 9:34 PM, john.tant...@gmail.com wrote: On Friday, April 20, 2012 12:34:46 PM UTC-7, Rotwang wrote: I believe it says somewhere in the Python docs that it's undefined and implementation-dependent whether two identical expressions have the same identity when the result of each is immutable Bad design. Where is is ill-defined, it should raise ValueError. A worse example, one which is very implementation-dependent: http://stackoverflow.com/questions/306313/python-is-operator-behaves-unexpectedly-with-integers a = 256 b = 256 a is b True # this is an expected result a = 257 b = 257 a is b False Operator is should be be an error between immutables unless one is a built-in constant. [...] I can't think of a single case where 'is' is ill-defined. You're blaming 'is' for revealing what's really going on. 'is' is /not/ implementation-dependent. It's /what's going on/ that's implementation-dependent. a is b is true iff 'a' and 'b' are the same object. Why should 'is' lie to the user? It does exactly what it says it does: it tells the user whether two objects are the same. In C++, for greater efficiency, two objects are compared for equality by first checking their addresses and then their contents. 'is' would be perfect for that. What I don't like is 'isinstance'. I'd prefer an infix operator 'isa' or 'is_a'. Kiuhnm -- http://mail.python.org/mailman/listinfo/python-list
