Re: 'isimmutable' and 'ImmutableNester'
(1) hash()-ability != immutability (!) Proof: class X: def __hash__(self): return 0 def pseudo_isimmutable(this): try: hash(this) return True except TypeError: return False shapeshifter = (1, 2, X()) print pseudo_isimmutable(shapeshifter) shapeshifter[2].changed = 4711 (2) The intended scenario is not described by a fragment such as: if isimmutable(obj): x = obj else: x = copy.copy(obj) function_that_might_modify(x) But instead, a more characteristic scenario is assert isimmutable(obj) # What happens behind the curtain may rely on referencing things_behind_the_curtain(obj) Or, def let_me_know(): obj = get_what_is_wanted() assert isimmutable(obj) # The caller may do with it what he wants without risking consistency return obj where lots of copying 2013/11/11 random...@fastmail.us: A built-in function 'isimmutable()' shall tell efficiently whether the object of concern is mutable or not. What's the benefit over attempting to hash() the object? copy.deepcopy already has special case for int, string, and tuples (including tuples that do and do not have mutable members) - could what you need be accomplished by overriding __copy__ and __deepcopy__ in your custom class to return itself if it is immutable? 2013/11/11 random...@fastmail.us: A built-in function 'isimmutable()' shall tell efficiently whether the object of concern is mutable or not. What's the benefit over attempting to hash() the object? copy.deepcopy already has special case for int, string, and tuples (including tuples that do and do not have mutable members) - could what you need be accomplished by overriding __copy__ and __deepcopy__ in your custom class to return itself if it is immutable? -- https://mail.python.org/mailman/listinfo/python-list
Re: 'isimmutable' and 'ImmutableNester'
On Tue, Nov 12, 2013 at 8:12 PM, Frank-Rene Schäfer fsch...@gmail.com wrote: (1) hash()-ability != immutability (!) Proof: class X: def __hash__(self): return 0 x == y != y == x Proof: class X: def __eq__(self,other): return True class Y: def __eq__(self,other): return False All you've done is proven that you can subvert things. By fiddling with __hash__, __eq__, and so on, you can make sets and dicts behave very oddly. Means nothing. Fundamentally, your mutability check is going to need some form of assistance from user-defined classes. That means a class can break your rules. ChrisA -- https://mail.python.org/mailman/listinfo/python-list
Re: 'isimmutable' and 'ImmutableNester'
All you've done is proven that you can subvert things. By fiddling with __hash__, __eq__, and so on, you can make sets and dicts behave very oddly. Means nothing. To the contrary, it means everything about what 'isimmutable' could contribute: security against advert or inadvert insertion of mutable objects. 2013/11/11 random...@fastmail.us: A built-in function 'isimmutable()' shall tell efficiently whether the object of concern is mutable or not. What's the benefit over attempting to hash() the object? copy.deepcopy already has special case for int, string, and tuples (including tuples that do and do not have mutable members) - could what you need be accomplished by overriding __copy__ and __deepcopy__ in your custom class to return itself if it is immutable? -- https://mail.python.org/mailman/listinfo/python-list
Re: 'isimmutable' and 'ImmutableNester'
On Tue, Nov 12, 2013 at 8:39 PM, Frank-Rene Schäfer fsch...@gmail.com wrote: All you've done is proven that you can subvert things. By fiddling with __hash__, __eq__, and so on, you can make sets and dicts behave very oddly. Means nothing. To the contrary, it means everything about what 'isimmutable' could contribute: security against advert or inadvert insertion of mutable objects. So how do you figure out whether something's immutable or not? Are you going to ask the object itself? If so, stick with __hash__, and just follow the rule that mutable objects aren't hashable - which is, if I'm not mistaken, how things already are. And if not, then how? How will you know if something has mutator methods? ChrisA -- https://mail.python.org/mailman/listinfo/python-list
Re: 'isimmutable' and 'ImmutableNester'
So how do you figure out whether something's immutable or not? Are you going to ask the object itself? If so, stick with __hash__, and just follow the rule that mutable objects aren't hashable - which is, if I'm not mistaken, how things already are. And if not, then how? How will you know if something has mutator methods? Admittedly, I have no knowledge about the python implementation. A possible way would be to say: def isimmutable(this): if isinstance(this, tuple): for x in this: if not isimmutable(x): return False return True return isisintance(this, (int, str, ImmutableNester)) The ImmutableNester special class type would be a feature to help checks to avoid recursion. Objects of classes derived from ImmutableNester have no mutable access functions and allow insertion of members only at construction time. At construction time it checks whether all entered elements are immutable in the above sense. As said, I have no idea how much this fits into the general python implementation. 2013/11/11 random...@fastmail.us: A built-in function 'isimmutable()' shall tell efficiently whether the object of concern is mutable or not. What's the benefit over attempting to hash() the object? copy.deepcopy already has special case for int, string, and tuples (including tuples that do and do not have mutable members) - could what you need be accomplished by overriding __copy__ and __deepcopy__ in your custom class to return itself if it is immutable? -- https://mail.python.org/mailman/listinfo/python-list
Re: 'isimmutable' and 'ImmutableNester'
On Tue, 12 Nov 2013 18:12:43 +1100, Chris Angelico wrote: def isimmutable(x): try: hash(x) return True except TypeError: return False I'm afraid that doesn't test for immutability. It tests for hashability, which is different. No well-behaved mutable object can be hashable, but that's not to say that badly-behaved mutable objects won't be hashable. And every immutable object should be hashable, but that's not to say that some immutable objects might choose, for their own reasons, not to be hashable. So your function is subject to both false negatives and false positives. -- Steven -- https://mail.python.org/mailman/listinfo/python-list
Re: 'isimmutable' and 'ImmutableNester'
On Tue, 12 Nov 2013 08:01:19 +0100, Frank-Rene Schäfer wrote: the existence of a built-in function 'isimmutable' puts the concept of immutability some more into the spotlight. That is an argument against the proposal, not in favour. The concept of immutability doesn't need to be in the spotlight. It is rather unimportant. I've been using Python for over 15 years, and have never missed an isimmutable function. Once, when I was just starting with Python, I thought I'd try writing one. I found it harder than I expected, and less useful, and soon gave up. And have never missed it yet. You might indeed implement some personal 'policy for copy/deepcopy'. But, how can you prevent the insertion of an object into the data tree which does not follow your copy/deepcopy convention? I don't understand what this policy is supposed to be. As soon as you allow members of type 'tuple' you must either check recursively or only allow ints and strings as tuple members. Why do you think you need to check at all? I think this is where we are talking past each other -- you seem to believe that testing for immutability is a critical piece of functionality which is missing from Python, as if lists had no way to query their length, or floats had no way to do multiplication. But that is not the case. Python has no isimmutable built-in function because, for the 20+ years that Python has existed, nobody who wanted it was willing to do the work to write it, and nobody willing to do the work thought it was important. I believe that if you wish this PEP to go anywhere, you need to concentrate on two things: 1) demonstrating that checking for immutability is *necessary* 2) demonstrating that it is *possible* -- Steven -- https://mail.python.org/mailman/listinfo/python-list
Re: 'isimmutable' and 'ImmutableNester'
On 2013-11-12 11:14, Steven D'Aprano wrote: On Tue, 12 Nov 2013 18:12:43 +1100, Chris Angelico wrote: def isimmutable(x): try: hash(x) return True except TypeError: return False I'm afraid that doesn't test for immutability. It tests for hashability, which is different. I am going to nitpick below for nitpicking's sake, but I agree with this. No well-behaved mutable object can be hashable, but that's not to say that badly-behaved mutable objects won't be hashable. That's not quite true. A well-behaved mutable may be (well-behaved) hashable as long as the allowed mutations do not affect the equality comparison. For example, in Python 2, all new classes are mutable by default, but they are also well-behaved hashable by default because their equality comparison is identity comparison. None of the mutations affect object identity, so the hash based on identity remains well-behaved. And every immutable object should be hashable, but that's not to say that some immutable objects might choose, for their own reasons, not to be hashable. I would also dispute this. A tuple itself is immutable, but it may not be hashable because one of its contained objects is unhashable (whether due to mutability or something else). So your function is subject to both false negatives and false positives. Agreed. -- 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 -- https://mail.python.org/mailman/listinfo/python-list
Re: 'isimmutable' and 'ImmutableNester'
=?UTF-8?Q?Frank=2DRene_Sch=C3=A4fer?= fsch...@gmail.com wrote: The ImmutableNester special class type would be a feature to help checks to avoid recursion. Objects of classes derived from ImmutableNester have no mutable access functions and allow insertion of members only at construction time. At construction time it checks whether all entered elements are immutable in the above sense. How does this help anything? If the objects are all immutable the object cannot contain any recursive references. If you cannot see this think about tuples: a tuple containing immutable objects including other tuples can never contain a reference to itself because by definition the tuple did not exist at the point where the elements it contains were constructed. Python already relies on the non-recursive nature of nested tuples when handling exceptions: The expression in the 'except' clause is compatible with an exception if it is the class or a base class of the exception object or a tuple containing an item compatible with the exception. If you try using something like a list in the exception specification you get a TypeError; only tuples and exception classes (subclasses of BaseException) are permitted. This means the structure can be as deeply nested as you wish, but can never be recursive and no checks against recursion need to be implemented. -- Duncan Booth http://kupuguy.blogspot.com -- https://mail.python.org/mailman/listinfo/python-list
Re: 'isimmutable' and 'ImmutableNester'
On Tue, Nov 12, 2013, at 4:39, Frank-Rene Schäfer wrote: All you've done is proven that you can subvert things. By fiddling with __hash__, __eq__, and so on, you can make sets and dicts behave very oddly. Means nothing. To the contrary, it means everything about what 'isimmutable' could contribute: security against advert or inadvert insertion of mutable objects. If an object can lie about its hashability, it can lie to your function too... unless you don't intend to provide a way for a _genuinely_ immutable class to say so. -- https://mail.python.org/mailman/listinfo/python-list
Re: 'isimmutable' and 'ImmutableNester'
On 12/11/2013 11:10, Frank-Rene Schäfer wrote: Admittedly, I have no knowledge about the python implementation. There is no the regarding Python implementations. Cpython alone is at either 2.7.6 or 3.3.3 with 3.4 at alpha, then there's IronPython, Jython, PyPy and lots more that I'm sure Steven D'Aprano can probably list from the top of his head :) -- Python is the second best programming language in the world. But the best has yet to be invented. Christian Tismer Mark Lawrence -- https://mail.python.org/mailman/listinfo/python-list
'isimmutable' and 'ImmutableNester'
I prepared a PEP and was wondering what your thoughts are about it: PEP:pep number Title: ``isimmutable(Obj)`` and/or ``ImmutableNester`` Version:version string Last-Modified: date string Author: Frank-Rene Schaefer, fsch...@users.sourceforge.net * BDFL-Delegate: PEP czar's real name * Discussions-To: fsch...@users.sourceforge.net Status: Draft Type: Standards Track | Informational | Process * Content-Type: text/x-rst * Requires: pep numbers Created:11-nov-2013 * Python-Version: 2.7.1 Post-History: dates of postings to python-list and python-dev * Replaces: pep number * Superseded-By: pep number * Resolution: url General Idea A built-in function 'isimmutable()' shall tell efficiently whether the object of concern is mutable or not. That is it must reflect on the whole object tree whether it contains mutable elements or not. For example, in the code fragment :: verdict_0 = isimmutable(3.14) verdict_1 = isimmutable((1,2,3)) verdict_2 = isimmutable(((1,2),(2,3),(3,4))) all verdicts are 'True' because the tested objects consist of purely immutable components. However, the ``x`` in :: x = (1,(2,abc, [1,2,3])) verdict = isimmutable(x) triggers the verdict to be 'False' because ``x[1][2]`` is a list and therefore mutable. It may be conceivable to have a special class-type called ``ImmutableNester`` which has no write-access member functions and does not allow its derived classes to have write-access member functions. Instead, any derived class aggregates members at the time of construction. At this point in time, when members are nested in the class, it is checked if the members are of subclasses of ``ImmutableNester``. The advantage of the ``isimmutable()`` function is that no extra class functionality needs to be implemented. The disadvantage is that the immutability must be checked manually and at each time the object is used. The ``ImmutableNester`` class type checks for immutability once, at construction time and no further manual checks are necessary. Rationale = If an object is immutable then copying of it can be safely be replaced by a setting of a reference. The principal scenario is when an instance A gives an instance B access to some data D under the provision that B does not change it. Therefore, B must either clone the data or it must be safe to assume that the data cannot change, i.e. is immutable. If the objects are large and/or many there a significant performance impact may raise from a deepcopy or manual cloning of objects. Therefore, the ``isimmutable()`` built-in function is key for a safe implementation of reference-instead-of-copying. Ensuring immutability is also key for the so called 'Flyweight Design Pattern'. -- https://mail.python.org/mailman/listinfo/python-list
Re: 'isimmutable' and 'ImmutableNester'
On Monday, November 11, 2013 3:47:45 PM UTC-5, Frank-Rene Schäfer wrote: I prepared a PEP and was wondering what your thoughts are about it: The best place to discuss proposals for changes to the Python language and library is the Python-Ideas mailing list: https://mail.python.org/mailman/listinfo/python-ideas There you will get in-depth discussion about the details of your proposal. Fair warning: it's very unlikely that your proposal will be adopted (most are not), but you will learn a lot about how Python works in the process. :) --Ned. -- https://mail.python.org/mailman/listinfo/python-list
Re: 'isimmutable' and 'ImmutableNester'
A built-in function 'isimmutable()' shall tell efficiently whether the object of concern is mutable or not. What's the benefit over attempting to hash() the object? copy.deepcopy already has special case for int, string, and tuples (including tuples that do and do not have mutable members) - could what you need be accomplished by overriding __copy__ and __deepcopy__ in your custom class to return itself if it is immutable? -- https://mail.python.org/mailman/listinfo/python-list
Re: 'isimmutable' and 'ImmutableNester'
On Mon, 11 Nov 2013 12:55:56 -0800, Ned Batchelder wrote: On Monday, November 11, 2013 3:47:45 PM UTC-5, Frank-Rene Schäfer wrote: I prepared a PEP and was wondering what your thoughts are about it: The best place to discuss proposals for changes to the Python language and library is the Python-Ideas mailing list: https://mail.python.org/mailman/listinfo/python-ideas Actually it is recommended to get at least initial feedback here first, to weed out proposals like: Python ought to allow function currying, like in Haskell. You mean like functools.partial? Oh, never mind then. -- Steven -- https://mail.python.org/mailman/listinfo/python-list
Re: 'isimmutable' and 'ImmutableNester'
On 12/11/2013 00:17, Steven D'Aprano wrote: On Mon, 11 Nov 2013 12:55:56 -0800, Ned Batchelder wrote: On Monday, November 11, 2013 3:47:45 PM UTC-5, Frank-Rene Schäfer wrote: I prepared a PEP and was wondering what your thoughts are about it: The best place to discuss proposals for changes to the Python language and library is the Python-Ideas mailing list: https://mail.python.org/mailman/listinfo/python-ideas Actually it is recommended to get at least initial feedback here first, to weed out proposals like: Python ought to allow function currying, like in Haskell. You mean like functools.partial? But I don't want to do it like that, I want... -- Python is the second best programming language in the world. But the best has yet to be invented. Christian Tismer Mark Lawrence -- https://mail.python.org/mailman/listinfo/python-list
Re: 'isimmutable' and 'ImmutableNester'
On Tue, Nov 12, 2013 at 11:17 AM, Steven D'Aprano steve+comp.lang.pyt...@pearwood.info wrote: On Mon, 11 Nov 2013 12:55:56 -0800, Ned Batchelder wrote: On Monday, November 11, 2013 3:47:45 PM UTC-5, Frank-Rene Schäfer wrote: I prepared a PEP and was wondering what your thoughts are about it: The best place to discuss proposals for changes to the Python language and library is the Python-Ideas mailing list: https://mail.python.org/mailman/listinfo/python-ideas Actually it is recommended to get at least initial feedback here first, to weed out proposals like: Python ought to allow function currying, like in Haskell. You mean like functools.partial? Oh, never mind then. aka Guido's Time Machine situations. I think this might be one of them - I read the proposal and thought Hashability should answer that, and random832 also posted the same. Love that time machine! ChrisA -- https://mail.python.org/mailman/listinfo/python-list
Re: 'isimmutable' and 'ImmutableNester'
Hi Frank-Rene, and welcome. Comments below. On Mon, 11 Nov 2013 21:47:45 +0100, Frank-Rene Schäfer wrote: I prepared a PEP and was wondering what your thoughts are about it: PEP:pep number Title: ``isimmutable(Obj)`` and/or ``ImmutableNester`` [...] * Python-Version: 2.7.1 That won't do. Python 2.7 is in maintenance mode, it will not gain any new functionality. There won't be a Python 2.8 either. If you want to propose new functionality, it will have to go into 3.5. (You've missed the opportunity for 3.4, since feature-freeze is only weeks away.) General Idea A built-in function 'isimmutable()' shall tell efficiently whether the object of concern is mutable or not. That is it must reflect on the whole object tree whether it contains mutable elements or not. For example, in the code fragment This has been proposed before. It has failed because there is no way to tell in general whether an arbitrary object is immutable or not. If you only look at the common built-in types, it is quite trivial, e.g.: - int, str, bytes, float, frozenset, bool, None are immutable; - list, dict, set are not immutable; - tuple is immutable if all of its elements are immutable. But as soon as you allow arbitrary objects, you're in trouble. How do you tell whether an object is immutable? I recommend that you start by writing a reference implementation: def isimmutable(obj): ... # ? Some obvious thoughts: - It is not enough to include a big list of immutable classes: # don't do this if isinstance(obj, (float, int, frozenset, ...)): return True because that list will never be complete and can become out-of-date. - You could try writing to the object (how?), and if it succeeds, you know it is mutable. But if it fails, that might just mean that you haven't tried writing to it in the correct manner. - But if the obj is mutable, you've just mutated it. That's bad. - You can try hashing the object: hash(obj) If that fails, then the object *might as well* be mutable, since you can't use it in sets or as dict keys. But if that's all isimmutable() does, why not just call hash(obj) directly? Anyway, I recommend you spend some time on this exercise. The PEP will not be accepted without a reference implementation, so you're going to have to do it at some point. Another thing which your proto-PEP fails to cover in sufficient detail is why you think such a function and/all class would be useful. You do say this: If an object is immutable then copying of it can be safely be replaced by a setting of a reference. The principal scenario is when an instance A gives an instance B access to some data D under the provision that B does not change it. Therefore, B must either clone the data or it must be safe to assume that the data cannot change, i.e. is immutable. but I really don't think much of this as the principle scenario. I don't think I've ever written code that matches this scenario. If possible, you should give a real example. If not real, at least a toy example. Either way, using code rather than just a vague description is better. If the objects are large and/or many there a significant performance impact may raise from a deepcopy or manual cloning of objects. Therefore, the ``isimmutable()`` built-in function is key for a safe implementation of reference-instead-of-copying. I don't think this is true. deepcopy (at least sometimes) will avoid making a copy if the object is immutable: py import copy py x = (10001, 20002, 30003, (40004, 50005, (60006, 70007)), 80008) py copy.copy(x) is x True py copy.deepcopy(x) is x True so what advantage does isimmutable give you? Ensuring immutability is also key for the so called 'Flyweight Design Pattern'. More details please. Ultimately, nothing knows whether an object is immutable or not better than the object itself. copy.copy and copy.deepcopy know this, and ask the object to copy itself rather than copy it from the outside. Since the object knows whether it is immutable, it knows whether or not to make a copy or just return itself. It seems to me that isimmutable() *appears* to be a useful function to have, but if you look into it in detail the apparently uses for it don't hold up. In practice, it would be almost impossible to implement (except as below) and even if you could you would never need it. Really, it seems to me that the only way to implement isimmutable would be to delegate to the object: def isimmutable(obj): return obj.__isimmutable__() This gives you: if isimmutable(obj): x = obj else: x = copy.copy(obj) function_that_might_modify(x) but that gives you no advantage at all that the simpler: function_that_might_modify(copy.copy(obj)) doesn't give. So what's the point? -- Steven -- https://mail.python.org/mailman/listinfo/python-list
Re: 'isimmutable' and 'ImmutableNester'
A tuple is immutable but it may contain mutable objects. In larger hierarchies of objects it may become less obvious whether down the lines, there is some mutable object somewhere in the data tree. One can define a recursive function to check for immutability manually. However first, it may not be as efficient as if it was built-in. Second, the existence of a built-in function 'isimmutable' puts the concept of immutability some more into the spotlight. You might indeed implement some personal 'policy for copy/deepcopy'. But, how can you prevent the insertion of an object into the data tree which does not follow your copy/deepcopy convention? As soon as you allow members of type 'tuple' you must either check recursively or only allow ints and strings as tuple members. -- https://mail.python.org/mailman/listinfo/python-list
Re: 'isimmutable' and 'ImmutableNester'
On Tue, Nov 12, 2013 at 6:01 PM, Frank-Rene Schäfer fsch...@gmail.com wrote: A tuple is immutable but it may contain mutable objects. In larger hierarchies of objects it may become less obvious whether down the lines, there is some mutable object somewhere in the data tree. One can define a recursive function to check for immutability manually. However first, it may not be as efficient as if it was built-in. Second, the existence of a built-in function 'isimmutable' puts the concept of immutability some more into the spotlight. You might indeed implement some personal 'policy for copy/deepcopy'. But, how can you prevent the insertion of an object into the data tree which does not follow your copy/deepcopy convention? As soon as you allow members of type 'tuple' you must either check recursively or only allow ints and strings as tuple members. x=1,2,3 hash(x) -378539185 x=1,2,[3] hash(x) Traceback (most recent call last): File pyshell#424, line 1, in module hash(x) TypeError: unhashable type: 'list' There's your recursive function! def isimmutable(x): try: hash(x) return True except TypeError: return False -- https://mail.python.org/mailman/listinfo/python-list
