date:20140510

Re: The � debate

2014-05-10 Thread Gregory Ewing


Steven D'Aprano wrote:

some_function(x, y+1)[key].attribute[num](arg)[spam or eggs] = 42

I'm pretty sure that it isn't common to call the LHS of that assignment a 
variable.


A better way of putting it might be something in the data
model that can be assigned to.

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Greg
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Re: Values and objects

2014-05-10 Thread Steven D'Aprano

On Sat, 10 May 2014 12:33:28 +1000, Chris Angelico wrote:

 On Sat, May 10, 2014 at 12:19 PM, Rustom Mody rustompm...@gmail.com
 wrote:
 For me, Marko's comment that variables in python are not first-class
 whereas in C they are is for me the most important distinction Ive seen
 (in a long time of seeing these discussions).


 https://en.wikipedia.org/wiki/First-class_citizen
 
 For variables in C to be considered first-class, they must, by most
 definitions, be able to be passed around as parameters and return
 values. Some definitions also require that they be able to be
 constructed at run-time. How do C variables shape up?
 
 1) Passing them as parameters. You can pass a pointer to a variable,
 which is effectively the same as passing a variable to a function. 

No it is not. It is nothing like passing a variable to a function. You 
are passing a pointer, which is itself a value. True, it is a value which 
you, the author, gives meaning as a pointer to a variable, but that need 
not be the case. It might be a pointer to a part of an array, or a 
record, or to some random address in memory, or a dangling pointer. C 
allows you to perform arithmetic on pointers, which means you can 
construct pointers to nothing in particular.

In C, pointers are first-class values, since you can:

- construct pointers at runtime;
- pass them to functions;
- return them from functions;
- assign them to variables;
- stuff them in structs or arrays.

But variables are not. You cannot create new variables, nor treat 
variables themselves as function arguments, or return them from 
functions. The closest you can do is *manually* use a pointer to a 
variable to simulate the same effect, but this is both too much and too 
little:

- too much, because you aren't limited to passing pointers 
  to a variable, you can pass pointers to anything you 
  like, or nothing at all;

- too little, because you are responsible for managing the 
  pointers, instead of having the compiler do so.

In Pascal, like C, you can't create new variables, or return them from 
functions, or embed them in records. But unlike C, you can pass them to 
functions, using a var parameter to use pass-by-reference. Algol is 
similar, except pass-by-name instead. I guess that makes Pascal variables 
second-and-a-half class values. Given:

procedure foo(var inside: integer);
  begin
{...}
  end;

begin
  foo(outside);
end.

the global variable outside is passed to foo where it effectively 
becomes the local variable inside. Notice that we don't manually have 
to concern ourselves with pointers, in fact whether Pascal uses pointers 
to implement this or not is entirely invisible to the coder using the 
feature. It could be using magic fairy dust for all we know. We simply 
write the most direct, natural code we possibly can:

foo(outside);

and the compiler does whatever magic is needed to ensure the variable 
outside, rather than the value of outside, is passed to the procedure foo.

There is nothing like that in C. You can only manually simulate it by 
passing a pointer to a variable, not by passing the variable itself. To 
argue that C pointers are pass by reference is like arguing that C has 
a garbage collector, because you can write one yourself.


 The
 callee can mutate your variable through the pointer. 2) Returning them.
 This is a lot more dodgy, owing to the dangling-pointer issue, but as
 long as you accept that the reference to a variable doesn't ensure its
 continued life, I suppose this might be acceptable. Maybe. But it's
 pushing it. 3) Constructing at run-time. Really REALLY pushing it. You
 can malloc and call that a variable, but it's not a variable any more,
 it's just a patch of memory. In fact, all this proves is that variables
 represent patches of memory, and patches of memory are first-class.

It proves that pointers are first class (well, duh!). The C compiler 
makes very little effort to ensure that pointers actually point to 
anything. Nor can you create patches of memory (I'll like an extra 
couple of terrabytes please), or return them from functions, or insert 
them in arrays.

Rather than *creating* patches of memory, malloc merely allocates it from 
pre-existing memory.


 Not liking the results here. You might just as well define that all
 Python variables must be backed by a dictionary (it's just as true as
 all C variables must be backed by memory) and then define the
 first-class-variable as a combination of a dict and a key.

Python variables aren't first-class either, but in fact we can get a bit 
closer to first-class than either C or Pascal.

Creating new variables is trivial. Since they don't need to be declared, 
you create a new variable just by assigning to it:

try:
spam
except NameError:
spam = 23


There are at least two other ways:

globals()['spam'] = 23
exec('spam = 23')


You can also delete variables:

del spam


Now this really shows first-class-ness. We

Re: How to implement key of key in python?

2014-05-10 Thread Andrea D'Amore


On 2014-05-10 03:28:29 +, eckhle...@gmail.com said:


While it is fine for a small dataset, I need a more generic way to do so.


I don't get how the dataset size affects the generality of the solution here.


From your first message:



attr = {}
with open('test.txt','rb') as tsvin:
tsvin = csv.reader(tsvin, delimiter='\t')
for row in tsvin:
ID = row[1]


so your is solved by adding a simple
 attr[ID] = {}

after the ID assignment. It seems simple to implement and generic enough to me.


unfortunately none of them illustrates how to store the values and 
access them later.


You access the stored value by using the variable name that holds it, 
but here you should probabily make more clear what your actual issue is.




Moreover, they bring some new terms, e.g. combined, [], etc.


The [] syntax is used in Python for lists.

The term combined hasn't a specific pythonic meaning there and is 
just used as a meaningful variable name as the author is combining, 
i.e. adding, numerical values.



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Andrea

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Re: The � debate

2014-05-10 Thread Chris Angelico

On Sat, May 10, 2014 at 3:58 PM, Gregory Ewing
greg.ew...@canterbury.ac.nz wrote:
 Steven D'Aprano wrote:

 some_function(x, y+1)[key].attribute[num](arg)[spam or eggs] = 42

 I'm pretty sure that it isn't common to call the LHS of that assignment a
 variable.


 A better way of putting it might be something in the data
 model that can be assigned to.

https://en.wikipedia.org/wiki/Assignment_(computer_science)

Go ahead, start an edit war at that page over its use of variable.
:) Right there it talks about copying values into variables. So if
Python has no variables, then either that article is inappropriate, or
Python has no assignment either.

ChrisA
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Re: Why isn't my re.sub replacing the contents of my MS Word file?

2014-05-10 Thread wxjmfauth

Le samedi 10 mai 2014 06:22:00 UTC+2, Rustom Mody a écrit :
 On Saturday, May 10, 2014 1:21:04 AM UTC+5:30, scott...@gmail.com wrote:
 
  Hi,
 
  
 
  
 
  
 
   here is a snippet of code that opens a file (fn contains the path\name) 
  and first tried to replace all endash, emdash etc characters with simple 
  dash characters, before doing a search.
 
  
 
But the replaces are not having any effect. Obviously a syntax 
  problemwwhat silly thing am I doing wrong?
 
 
 
 If you are using MS-Word use that, not python.
 
 
 
 Yeah it is possible to script MS with something like this
 
 http://timgolden.me.uk/pywin32-docs/
 
 [no experience myself!]
 
 but its probably not worth the headache for such a simple job.
 
 
 
 The VBA (or whatever is the modern equivalent) will be about as short and 
 simple
 
 as your attempted python and making it work will be far easier.
 
 
 
 I way I used to do it with Windows-98 Word. 
 
 Start a macro
 
 Do a simple single search and replace by hand
 
 Close the macro
 
 Edit the macro (VBA version)
 
 Replace the single search-n-replace with all the many you require

=

That's a wise reommendation.

Anyway, as Python may fail as soon as one uses an
EM DASH or an EM DASH, I think it's not worth the
effort to spend to much time with it.

LibreOffice could be a solution.

jmf


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Re: Values and objects

2014-05-10 Thread Chris Angelico

On Sat, May 10, 2014 at 4:15 PM, Steven D'Aprano
steve+comp.lang.pyt...@pearwood.info wrote:
 On Sat, 10 May 2014 12:33:28 +1000, Chris Angelico wrote:
 1) Passing them as parameters. You can pass a pointer to a variable,
 which is effectively the same as passing a variable to a function.

 No it is not. It is nothing like passing a variable to a function. You
 are passing a pointer, which is itself a value. True, it is a value which
 you, the author, gives meaning as a pointer to a variable, but that need
 not be the case. It might be a pointer to a part of an array, or a
 record, or to some random address in memory, or a dangling pointer. C
 allows you to perform arithmetic on pointers, which means you can
 construct pointers to nothing in particular.

I think at this point it's arguable, in that you can get so close to
passing a variable to a function that it doesn't really matter about
the distinction. But as I explained further down, it really just shows
that patch of memory can be passed around, and that a variable can
be backed by such a patch of memory.

 Rather than *creating* patches of memory, malloc merely allocates it from
 pre-existing memory.

I disagree. On a modern system with memory management, malloc can grab
memory from the system, thus making it available to your process.
Sure, physical memory will normally have to have been installed in the
system, but conceptually you could have a malloc function that
actually freezes the program, asks the user to build a new computer
and turn it on, connects to a new service on that computer, and
allocates memory from there. As far as your program's concerned,
malloc actually does (attempt to) give you more room than you had.

 Python variables aren't first-class either, but in fact we can get a bit
 closer to first-class than either C or Pascal.

 Creating new variables is trivial. Since they don't need to be declared,
 you create a new variable just by assigning to it:

 try:
 spam
 except NameError:
 spam = 23

No no no, this is really creating them at compile time. If you do this
inside a function, the name has to be created as a local name before
the function begins execution.

 There are at least two other ways:

 globals()['spam'] = 23
 exec('spam = 23')

With exec, you can do anything at run time. Does that mean that, in
languages with an exec action, absolutely everything is first-class?
I'm not sure that that counts. Maybe I'm wrong.

Subscript-assigning to globals() is actually creating new
(module-level) variables, though. And if Python allowed you to assign
to locals() inside a function, then I would accept that local function
variables can be created and destroyed at run time, but you can't, so
local variables aren't first-class.

ChrisA
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Re: The � debate

2014-05-10 Thread Steven D'Aprano

On Sat, 10 May 2014 17:10:29 +1000, Chris Angelico wrote:

 On Sat, May 10, 2014 at 3:58 PM, Gregory Ewing
 greg.ew...@canterbury.ac.nz wrote:
 Steven D'Aprano wrote:

 some_function(x, y+1)[key].attribute[num](arg)[spam or eggs] = 42

 I'm pretty sure that it isn't common to call the LHS of that
 assignment a variable.


 A better way of putting it might be something in the data model that
 can be assigned to.
 
 https://en.wikipedia.org/wiki/Assignment_(computer_science)
 
 Go ahead, start an edit war at that page over its use of variable. :)
 Right there it talks about copying values into variables. So if Python
 has no variables, then either that article is inappropriate, or Python
 has no assignment either.

Python assignment doesn't copy values.




-- 
Steven D'Aprano
http://import-that.dreamwidth.org/
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Re: The “does Python have variables?” debate

2014-05-10 Thread Larry Hudson

On 05/09/2014 06:11 PM, Steven D'Aprano wrote:

On Fri, 09 May 2014 17:34:17 -0500, Mark H Harris wrote:

On 5/7/14 8:27 PM, Steven D'Aprano wrote:

snip

Why are new Python coders 'always' confused by this question of
variable (name value) vs. {name: object} model of Python?

Always? I don't think anyone, not even Ben, claims that new Python
coders are always confused about Python's variable semantics. That
would be a straw man, easy to disprove by just finding a single person
who wasn't confused. Perhaps someone who had never learned C and didn't
know C variable semantics?

Well, here is that one person. ;-)

I'm an entirely self-taught amateur/hobbyist programmer, coming to Python from a C background.
Very early in my reading of books/tutorials on Python I learned that Python variables are
different, and how they are different from C variables. Essentially I said, Okay, they're
different. What's next? There was no confusion at all. All I had to do was accept that they
_are_ different. (BTW, I'm now quite hooked on Python -- and still learning it.)

The reason I suggest is that the person has a preconceived idea of
what 'variable' means, and they then attempt to apply their conception
of variable on to Python in some way ending in a surprise.

That's the problem as some of us see it.

As I said above, not a problem. I already accepted the fact that all programming languages are
different -- often with similarities, but more often quite different, especially in the details.
So when programming in C I think in C-style variables, in Python I think in Python-style
variables. And uv course, I nefer make mmistakes... Yeah, right!;-)

We need a way to speak about Pythons name object model to avoid
this confusion.

And that would be the name binding part.

This is something I absolutely disagree with. I still think it's only necessary to teach that
variables in different languages are handled differently. Insisting on a different name is NOT
helpful. And insisting that Python does not have variables is ludicrous!

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