Re: [Python-Dev] python and super
Ricardo Kirkner wrote: I'll give you the example I came upon: I have a TestCase class, which inherits from both Django's TestCase and from some custom TestCases that act as mixin classes. So I have something like class MyTestCase(TestCase, Mixin1, Mixin2): ... now django's TestCase class inherits from unittest2.TestCase, which we found was not calling super. Even if this is a bug and should be fixed in unittest2, this is an example where I, as a consumer of django, shouldn't have to be worried about how django's TestCase class is implemented. I have to disagree -- anytime you are using somebody else's code you need to be aware of what it's supposed to do -- especially when playing with multiple inheritance. This response to the decorator I wrote for this situation may be helpful: Carl Banks wrote (on Python-List): The problem is that he was doing mixins wrong. Way wrong. Here is my advice on mixins: Mixins should almost always be listed first in the bases. (The only exception is to work around a technicality. Otherwise mixins go first.) If a mixin defines __init__, it should always accept self, *args and **kwargs (and no other arguments), and pass those on to super().__init__. Same deal with any other function that different sister classes might define in varied ways (such as __call__). A mixin should not accept arguments in __init__. Instead, it should burden the derived class to accept arguments on its behalf, and set attributes before calling super().__init__, which the mixin can access. If you insist on a mixin that accepts arguments in __init__, then it should should pop them off kwargs. Avoid using positional arguments, and never use named arguments. Always go through args and kwargs. If mixins follow these rules, they'll be reasonably safe to use on a variety of classes. (Maybe even safe enough to use in Django classes.) ~Ethan~ ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] python and super
On 17 April 2011 02:48, Steven D'Aprano st...@pearwood.info wrote: Michael Foord wrote: On 15/04/2011 02:23, Steven D'Aprano wrote: [...] If we treat django's failure to use super as a bug, you want the Python language to work-around that bug so that: What you say (that this particular circumstance could be treated as a bug in django) is true, however consider the recently introduced problem caused by object.__init__ not taking arguments. This makes it impossible to use super correctly in various circumstances. [...] It is impossible to inherit from both C and A and have all parent __init__ methods called correctly. Changing the semantics of super as described would fix this problem. So you say. I don't have an an opinion on whether or not you are technically correct, but adding DWIM black-magic to super scares me. Well, super is already pretty magic and what I'm suggesting is no more magic than currently exists. I'm suggesting (but it won't happen - no-one else is in favour :-) *extending* the existing algorithm in a predictable and understandable way. The main advantage is that it allows methods to express don't call my parent class methods but don't halt the chain of calling, which is currently not possible (so in that context I don't really know what you mean by DWIM black-magic). I'm *not* suggesting full auto calling. All the best, Michael It scares me even if it were guaranteed to *only* apply to __init__, but if it applied to arbitrary methods, it frankly terrifies me. If it were limited to only apply to __init__, there would be a constant stream of requests that we loosen the restriction and make super just work for all methods, despite the dangers of DWIM code. -- Steven ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/fuzzyman%40voidspace.org.uk -- http://www.voidspace.org.uk/ May you do good and not evil May you find forgiveness for yourself and forgive others May you share freely, never taking more than you give. -- the sqlite blessing http://www.sqlite.org/different.html ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] python and super
16.04.2011 03:38, Greg Ewing пишет: Michael Foord wrote: consider the recently introduced problem caused by object.__init__ not taking arguments. This makes it impossible to use super correctly in various circumstances. ... It is impossible to inherit from both C and A and have all parent __init__ methods called correctly. Changing the semantics of super as described would fix this problem. I don't see how, because auto-super-calling would eventually end up trying to call object.__init__ with arguments and fail. You might think to fix this by making a special case of object.__init__ and refraining from calling it. But the same problem arises in a more general way whenever some class in the mix has a method with the right name but the wrong signature, which is likely to happen if you try to mix classes that weren't designed to be mixed together. Michael's words are not about *auto-calling* but about *stopping prevention* of parent's method call by a class that unrelated to such parent. In the example above A is such a stopper that prevents calling of B.__init__ and B is a stopper for calling A.__init__ but A and B are completely unrelated to each other. object.__init__ would not be called anyway (in this example) but the point is that nobody (at least among Michael and myself) going to *auto-call* object.__init__ with some automagically picked arguments. -- Nikolay Zakharov ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] python and super
Mark Janssen wrote: I have to say it is quite strange to me that there is no distinction made between IS-A relationship and HAS-A relationships with regard to the issue of Inheritence. I'm not sure what you mean by that. Inheritance is (or should be) used only for is-a relationships. Misusing it for has-a relationships leads to problems. Python, confusingly makes no syntactic distinction, Yes, it does, as long as you use composition instead of inheritance for has-a relationships. -- Greg ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] python and super
Michael Foord wrote: On 15/04/2011 02:23, Steven D'Aprano wrote: [...] If we treat django's failure to use super as a bug, you want the Python language to work-around that bug so that: What you say (that this particular circumstance could be treated as a bug in django) is true, however consider the recently introduced problem caused by object.__init__ not taking arguments. This makes it impossible to use super correctly in various circumstances. [...] It is impossible to inherit from both C and A and have all parent __init__ methods called correctly. Changing the semantics of super as described would fix this problem. So you say. I don't have an an opinion on whether or not you are technically correct, but adding DWIM black-magic to super scares me. It scares me even if it were guaranteed to *only* apply to __init__, but if it applied to arbitrary methods, it frankly terrifies me. If it were limited to only apply to __init__, there would be a constant stream of requests that we loosen the restriction and make super just work for all methods, despite the dangers of DWIM code. -- Steven ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] python and super
On Thu, Apr 14, 2011 at 7:09 AM, Ricardo Kirkner ricardokirk...@gmail.com wrote: I recently stumbled upon an issue with a class in the mro chain not calling super, therefore breaking the chain (ie, further base classes along the chain didn't get called). I understand it is currently a requirement that all classes that are part of the mro chain behave and always call super. My question is, shouldn't/wouldn't it be better, if python took ownership of that part, and ensured all classes get called, even if some class misbehaved? I get annoyed by this issue as well, in various forms. It seems like such a discussion would have been resolved by now in the multitude of OOP languages, but I have to say it is quite strange to me that there is no distinction made between IS-A relationship and HAS-A relationships with regard to the issue of Inheritence. Python, confusingly makes no syntactic distinction, and its semantic distinction (through MRO and programmer conventions) seems quite suboptimal and special-cased. --No fault of anyone's, perhaps it is indeed an unresolved issue within Computer Science. It should be clear that IS-A inheritance is really trying to say (or should be) that the following set/class (of methods and attributes) is a *super-set* of its parent (--See how the OO lexicon is already confused and mixing metaphors?). In this case, manually calling super() is not only completely redundant but adds various confusions. With regard to inheritence, I too would like to see automatic calls to super classes in every case were there is a complete sClearly there is utility in the notion of a set-theoretic containment DISCARDING:: the points are moot and need finer granularity that only the pangaia model can fix. ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] python and super
Argh! Sorry list. I meant to discard the post that was just sent. Please accept my humblest apologies... Mark ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] python and super
R. David Murray wrote: Why not? It seems more useful than using it for chaining, especially given the compiler hack in Python3. Because it's prone to doing the wrong thing if the class using it is ever involved in multiple inheritance. If you're expecting the call to go to a particular class, it's safer to explicitly name that class. -- Greg ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] python and super
On 15/04/2011 02:02, Greg Ewing wrote: Michael Foord wrote: What I was suggesting is that a method not calling super shouldn't stop a *sibling* method being called, but could still prevent the *parent* method being called. There isn't necessarily a clear distinction between parents and siblings. class A: ... class B(A): ... class C(A, B): ... In C, is A a parent of B or a sibling of B? For a super call in C, B is a sibling to A. For a super call in B, A is a parent. With the semantics I was suggesting if C calls super, but A doesn't then B would still get called. All the best, Michael -- http://www.voidspace.org.uk/ May you do good and not evil May you find forgiveness for yourself and forgive others May you share freely, never taking more than you give. -- the sqlite blessing http://www.sqlite.org/different.html ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] python and super
On 15/04/2011 02:23, Steven D'Aprano wrote:
Ricardo Kirkner wrote:
I have a TestCase class, which inherits from both Django's TestCase
and from some custom TestCases that act as mixin classes. So I have
something like
class MyTestCase(TestCase, Mixin1, Mixin2):
...
now django's TestCase class inherits from unittest2.TestCase, which we
found was not calling super. Even if this is a bug and should be fixed
in unittest2, this is an example where I, as a consumer of django,
shouldn't have to be worried about how django's TestCase class is
implemented. Since I explicitely base off 3 classes, I expected all 3
classes to be initialized, and I expect the setUp method to be called
on all of them.
If I'm assuming/expecting unreasonable things, please enlighten me.
If we treat django's failure to use super as a bug, you want the
Python language to work-around that bug so that:
What you say (that this particular circumstance could be treated as a
bug in django) is true, however consider the recently introduced
problem caused by object.__init__ not taking arguments. This makes it
impossible to use super correctly in various circumstances.
http://freshfoo.com/blog/object__init__takes_no_parameters
Given the following classes (Python 3):
class A:
def __init__(self, a):
print ('A')
class B:
def __init__(self, a):
print ('B')
class C(B):
def __init__(self, a):
print ('C')
super().__init__(a)
It is impossible to inherit from both C and A and have all parent
__init__ methods called correctly. Changing the semantics of super as
described would fix this problem.
For:
class D(C, A):
def __init__(self, a):
super().__init__(a)
D(1)
This is printed:
C
B
(A __init__ is not called).
For this:
class D(A, C):
def __init__(self, a):
super().__init__(a)
D(1)
The following is printed:
A
(B and C __init__ methods are not called.)
All the best,
Michael Foord
I, as a consumer of django, shouldn't have to be worried about bugs
in django. (For at least one class of bug.)
If we *don't* treat django's failure to use super as a bug, but as a
deliberate design choice, then you are trying to do something which
django doesn't support. Possibly *deliberately* doesn't support. You
want the Python language to add that support so that:
I, as a consumer of django, shouldn't have to be worried about
whether django supports what I want to do or not.
Either way you look at it, I think it's extremely unreasonable to
expect the language to work-around bugs in third-party applications,
or to add features to them that the third-party developers either
didn't consider or don't want.
Multiple inheritance is tricky enough to get right without adding Do
What I Mean black magic to it. I'd rather work around bugs in
third-party classes than try to deal with Python actively subverting
the code I read and write by mysteriously calling superclass methods
where there is no call to a superclass method.
--
http://www.voidspace.org.uk/
May you do good and not evil
May you find forgiveness for yourself and forgive others
May you share freely, never taking more than you give.
-- the sqlite blessing http://www.sqlite.org/different.html
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Re: [Python-Dev] python and super
Michael Foord wrote: On 15/04/2011 02:02, Greg Ewing wrote: Michael Foord wrote: What I was suggesting is that a method not calling super shouldn't stop a *sibling* method being called, but could still prevent the *parent* method being called. There isn't necessarily a clear distinction between parents and siblings. class A: ... class B(A): ... class C(A, B): ... In C, is A a parent of B or a sibling of B? Its neither, as C can't exist: class A: pass ... class B(A): pass ... class C(A,B):pass ... Traceback (most recent call last): File stdin, line 1, in module TypeError: Cannot create a consistent method resolution order (MRO) for bases B, A For a super call in C, B is a sibling to A. For a super call in B, A is a parent. With the semantics I was suggesting if C calls super, but A doesn't then B would still get called. A class cannot precede any of its sub-classes in an MRO, see http://en.wikipedia.org/wiki/C3_linearization If A is a parent (super-class) of B, then B must precede A in any MRO that contains them both. Siblings, in the context of a single MRO are thus classes between which there is no sub-class/super-class relation. Mark. ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] python and super
On 04/15/2011 08:53 AM, Michael Foord wrote: If we treat django's failure to use super as a bug, you want the Python language to work-around that bug so that: What you say (that this particular circumstance could be treated as a bug in django) is true, Just as a side note: if there is a bug demonstrated here, it is in unittest2, not Django. Django's TestCase subclasses don't even override __init__ or setUp, so there is no opportunity for them to call or fail to call super() in either case. If you re-read Ricardo's original presentation of the case, he correctly noted that it is unittest2's TestCase which does not call super() and thus prevents cooperative multiple inheritance. I'm not sure who in this thread first mis-read his post and called it a possible bug in Django, but it was a mis-reading which now appears to be self-propagating ;-) Carl ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] python and super
On 15/04/2011 16:18, Carl Meyer wrote: On 04/15/2011 08:53 AM, Michael Foord wrote: If we treat django's failure to use super as a bug, you want the Python language to work-around that bug so that: What you say (that this particular circumstance could be treated as a bug in django) is true, Just as a side note: if there is a bug demonstrated here, it is in unittest2, not Django. Django's TestCase subclasses don't even override __init__ or setUp, so there is no opportunity for them to call or fail to call super() in either case. If you re-read Ricardo's original presentation of the case, he correctly noted that it is unittest2's TestCase which does not call super() and thus prevents cooperative multiple inheritance. I'm not sure who in this thread first mis-read his post and called it a possible bug in Django, but it was a mis-reading which now appears to be self-propagating ;-) Well yes, but it is also a bug in the copy of unittest2 embedded in django - so whilst it can be fixed in unittest2 (simply deleting the setUp and tearDown methods which do nothing but override unittest.TestCase.setUp and tearDown) it *also* needs to be fixed in django. This particular issue does illustrate the problem well though - the methods in unittest2 don't call up to their parent class (which is fine because those methods are empty), but in not calling up also they prevent sibling methods being called in a multiple inheritance situation. So for those who have been saying that not wanting to call up to parents is a valid use case, yes I quite agree. But you have to be aware that because of the semantics of super, not calling up to your parents basically prevents those methods being used in the presence of multiple inheritance. All the best, Michael Foord Carl ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/fuzzyman%40voidspace.org.uk -- http://www.voidspace.org.uk/ May you do good and not evil May you find forgiveness for yourself and forgive others May you share freely, never taking more than you give. -- the sqlite blessing http://www.sqlite.org/different.html ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] python and super
On Fri, Apr 15, 2011 at 11:30 PM, Michael Foord fuzzy...@voidspace.org.uk wrote: On 15/04/2011 02:02, Greg Ewing wrote: There isn't necessarily a clear distinction between parents and siblings. class A: ... class B(A): ... class C(A, B): ... In C, is A a parent of B or a sibling of B? As has been pointed out elsewhere in the thread, that definition of C isn't allowed :) class C(A, B): pass ... Traceback (most recent call last): File stdin, line 1, in module TypeError: Cannot create a consistent method resolution order (MRO) for bases A, B Once you turn the order of definition around (class C(B, A)) it becomes clear that A remains B's parent regardless of the existence of C: C.__mro__ (class '__main__.C', class '__main__.B', class '__main__.A', class 'object') The whole discussion of trying to distinguish parents from siblings when invoking super() *doesn't make sense*. The entire *point* of the multiple inheritance handling is to linearise the type hierarchy into a method resolution order that consists of a single chain of classes that are called in sequence (with any class in the chain allowed to terminate the sequence at any time). Cooperative super() calls are exactly that: cooperative. Just as cooperative threading breaks down if one task doesn't play by the rules, such is also the case with cooperative super calls. There are two ways to handle this: - Option 1 is to tailor your inheritance hierarchy such that any non-cooperative classes always appear on the right-most end of the MRO (e.g. as A and object do in the example above). This can be tricky, but is doable if there is just the one recalcitrant class causing problems (e.g. I wouldn't be surprised to hear that a simple rearrangement to class MyTestCase(Mixin1, Mixin2, TestCase) sufficiently rearranged the MyTestCase MRO to make this problem go away). - Option 2 is to do as Raymond suggests: noncooperative classes are incorporated via has-a composition (potentially as a proxy object) rather than is-a inheritance. For any methods which require cooperative calls, the cooperative wrapper provides that behaviour, while delegating the heavy lifting to the underlying object. Essentially, any cooperative hierarchy requires a base class that defines the rules of cooperation and provides no-op termination methods for any cooperative calls. Non-cooperative classes must either be parents of that base class, or else they must be wrapped as described in Option 2. Cheers, Nick. -- Nick Coghlan | ncogh...@gmail.com | Brisbane, Australia ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] python and super
Michael Foord wrote: consider the recently introduced problem caused by object.__init__ not taking arguments. This makes it impossible to use super correctly in various circumstances. ... It is impossible to inherit from both C and A and have all parent __init__ methods called correctly. Changing the semantics of super as described would fix this problem. I don't see how, because auto-super-calling would eventually end up trying to call object.__init__ with arguments and fail. You might think to fix this by making a special case of object.__init__ and refraining from calling it. But the same problem arises in a more general way whenever some class in the mix has a method with the right name but the wrong signature, which is likely to happen if you try to mix classes that weren't designed to be mixed together. -- Greg ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] python and super
Mark Shannon wrote: class A: pass class B(A): pass class C(A,B):pass Traceback (most recent call last): File stdin, line 1, in module TypeError: Cannot create a consistent method resolution order (MRO) for bases B, A All right, but this is okay: class C(B, A): pass Michael Foord wrote: For a super call in C, B is a sibling to A. For a super call in B, A is a parent. With the semantics I was suggesting if C calls super, but A doesn't then B would still get called. which is contradicted by: Siblings, in the context of a single MRO are thus classes between which there is no sub-class/super-class relation. So I maintain that the situation is far from clear. :-) -- Greg ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] python and super
Michael Foord wrote: But you have to be aware that because of the semantics of super, not calling up to your parents basically prevents those methods being used in the presence of multiple inheritance. No, it prevents them being used in the presence of super(). Multiple inheritance is still possible the old-fashioned way using explicit upcalls as long as the classes are sufficiently independent. If they're *not* sufficiently independent, and haven't been specifically designed to cooperate with each other, attempting to make them cooperate automatically is as likely to do harm as good. -- Greg ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] python and super
2011/4/14 Ricardo Kirkner ricardokirk...@gmail.com: Hi all, I recently stumbled upon an issue with a class in the mro chain not calling super, therefore breaking the chain (ie, further base classes along the chain didn't get called). I understand it is currently a requirement that all classes that are part of the mro chain behave and always call super. My question is, shouldn't/wouldn't it be better, if python took ownership of that part, and ensured all classes get called, even if some class misbehaved? For example, if using a stack-like structure, pushing super calls and popping until the stack was empty, couldn't this restriction be removed? No. See line 2 of the Zen of Python. -- Regards, Benjamin ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] python and super
On Thu, 14 Apr 2011 08:15:10 -0500 Benjamin Peterson benja...@python.org wrote: 2011/4/14 Ricardo Kirkner ricardokirk...@gmail.com: Hi all, I recently stumbled upon an issue with a class in the mro chain not calling super, therefore breaking the chain (ie, further base classes along the chain didn't get called). I understand it is currently a requirement that all classes that are part of the mro chain behave and always call super. My question is, shouldn't/wouldn't it be better, if python took ownership of that part, and ensured all classes get called, even if some class misbehaved? For example, if using a stack-like structure, pushing super calls and popping until the stack was empty, couldn't this restriction be removed? No. See line 2 of the Zen of Python. You could have quoted it explicitly :) FWIW, line 2 is: Explicit is better than implicit. Regards Antoine. ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] python and super
:-) 2011/4/14 Antoine Pitrou solip...@pitrou.net On Thu, 14 Apr 2011 08:15:10 -0500 Benjamin Peterson benja...@python.org wrote: 2011/4/14 Ricardo Kirkner ricardokirk...@gmail.com: Hi all, I recently stumbled upon an issue with a class in the mro chain not calling super, therefore breaking the chain (ie, further base classes along the chain didn't get called). I understand it is currently a requirement that all classes that are part of the mro chain behave and always call super. My question is, shouldn't/wouldn't it be better, if python took ownership of that part, and ensured all classes get called, even if some class misbehaved? For example, if using a stack-like structure, pushing super calls and popping until the stack was empty, couldn't this restriction be removed? No. See line 2 of the Zen of Python. You could have quoted it explicitly :) FWIW, line 2 is: Explicit is better than implicit. Regards Antoine. ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/g.rodola%40gmail.com ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] python and super
Ricardo Kirkner wrote: Hi all, I recently stumbled upon an issue with a class in the mro chain not calling super, therefore breaking the chain (ie, further base classes along the chain didn't get called). I understand it is currently a requirement that all classes that are part of the mro chain behave and always call super. My question is, shouldn't/wouldn't it be better, if python took ownership of that part, and ensured all classes get called, even if some class misbehaved? Consider the difference between extending the method and replacing it. (I've always known that as overloading and overriding, but the terminology varies.) If Python automagically always called super(), how would you replace a method? For that matter, at which point would you automagically call super()? At the start of the overloaded method, before the subclass code runs? At the end, after the subclass code? Somewhere in the middle? class Spam(Ham): def method(self): # Overload method. super().method() # at the start of the method? do_stuff() super().method() # in the middle of the method? do_more_stuff() super().method() # or at the end of the overloaded method? What arguments should be passed? What do you do with the result? If you can think of a way for Python to automagically tell when to call super(), what arguments to pass to it, and what to do with the result, your crystal ball is better than mine. -- Steven ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] python and super
On 14 Apr, 2011, at 15:09, Ricardo Kirkner wrote: Hi all, I recently stumbled upon an issue with a class in the mro chain not calling super, therefore breaking the chain (ie, further base classes along the chain didn't get called). I understand it is currently a requirement that all classes that are part of the mro chain behave and always call super. My question is, shouldn't/wouldn't it be better, if python took ownership of that part, and ensured all classes get called, even if some class misbehaved? Not calling a method on super isn't necessarily misbehavior. It would be odd to not call super in __init__, but for other methods not calling the superclass implementation is fairly common. Ronald ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] python and super
On 14/04/2011 15:18, Ronald Oussoren wrote: On 14 Apr, 2011, at 15:09, Ricardo Kirkner wrote: Hi all, I recently stumbled upon an issue with a class in the mro chain not calling super, therefore breaking the chain (ie, further base classes along the chain didn't get called). I understand it is currently a requirement that all classes that are part of the mro chain behave and always call super. My question is, shouldn't/wouldn't it be better, if python took ownership of that part, and ensured all classes get called, even if some class misbehaved? Not calling a method on super isn't necessarily misbehavior. It would be odd to not call super in __init__, but for other methods not calling the superclass implementation is fairly common. Right, but where you have an inheritance chain where all the classes do call super but one doesn't then you can get breakage. This is a problem where you want to use multiple inheritance but a parent class of *one* of the classes doesn't call super. Not only does the super of its parents not get called - but the chain stops and other methods (in another branch of the inheritance tree) also don't get called. And if the base classes are not all under your control there maybe no fix - except possibly monkey patching. Ricardo isn't suggesting that Python should always call super for you, but when you *start* the chain by calling super then Python could ensure that all the methods are called for you. If an individual method doesn't call super then a theoretical implementation could skip the parents methods (unless another child calls super). All the best, Michael Foord Ronald ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/fuzzyman%40voidspace.org.uk -- http://www.voidspace.org.uk/ May you do good and not evil May you find forgiveness for yourself and forgive others May you share freely, never taking more than you give. -- the sqlite blessing http://www.sqlite.org/different.html ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] python and super
I think that if you add this, people will start relying on it. Laura ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] python and super
On 14/04/2011 16:02, Laura Creighton wrote: I think that if you add this, people will start relying on it. And the specific problem with that would be? Michael Laura -- http://www.voidspace.org.uk/ May you do good and not evil May you find forgiveness for yourself and forgive others May you share freely, never taking more than you give. -- the sqlite blessing http://www.sqlite.org/different.html ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] python and super
At 03:55 PM 4/14/2011 +0100, Michael Foord wrote: Ricardo isn't suggesting that Python should always call super for you, but when you *start* the chain by calling super then Python could ensure that all the methods are called for you. If an individual method doesn't call super then a theoretical implementation could skip the parents methods (unless another child calls super). That would break classes that deliberately don't call super. I can think of examples in my own code that would break, especially in __init__() cases. It's perfectly sensible and useful for there to be classes that intentionally fail to call super(), and yet have a subclass that wants to use super(). So, this change would expose an internal implementation detail of a class to its subclasses, and make fragile base class problems worse. (i.e., where an internal change to a base class breaks a previously-working subclass). ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] python and super
On 14/04/2011 16:34, P.J. Eby wrote: At 03:55 PM 4/14/2011 +0100, Michael Foord wrote: Ricardo isn't suggesting that Python should always call super for you, but when you *start* the chain by calling super then Python could ensure that all the methods are called for you. If an individual method doesn't call super then a theoretical implementation could skip the parents methods (unless another child calls super). That would break classes that deliberately don't call super. I can think of examples in my own code that would break, especially in __init__() cases. It's perfectly sensible and useful for there to be classes that intentionally fail to call super(), and yet have a subclass that wants to use super(). So, this change would expose an internal implementation detail of a class to its subclasses, and make fragile base class problems worse. (i.e., where an internal change to a base class breaks a previously-working subclass). It shouldn't do. What I was suggesting is that a method not calling super shouldn't stop a *sibling* method being called, but could still prevent the *parent* method being called. Michael -- http://www.voidspace.org.uk/ May you do good and not evil May you find forgiveness for yourself and forgive others May you share freely, never taking more than you give. -- the sqlite blessing http://www.sqlite.org/different.html ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] python and super
Exactly what Michael said. Stopping the chain going upwards is one thing. Stopping it going sideways is another. On Thu, Apr 14, 2011 at 12:37 PM, Michael Foord fuzzy...@voidspace.org.uk wrote: On 14/04/2011 16:34, P.J. Eby wrote: At 03:55 PM 4/14/2011 +0100, Michael Foord wrote: Ricardo isn't suggesting that Python should always call super for you, but when you *start* the chain by calling super then Python could ensure that all the methods are called for you. If an individual method doesn't call super then a theoretical implementation could skip the parents methods (unless another child calls super). That would break classes that deliberately don't call super. I can think of examples in my own code that would break, especially in __init__() cases. It's perfectly sensible and useful for there to be classes that intentionally fail to call super(), and yet have a subclass that wants to use super(). So, this change would expose an internal implementation detail of a class to its subclasses, and make fragile base class problems worse. (i.e., where an internal change to a base class breaks a previously-working subclass). It shouldn't do. What I was suggesting is that a method not calling super shouldn't stop a *sibling* method being called, but could still prevent the *parent* method being called. Michael -- http://www.voidspace.org.uk/ May you do good and not evil May you find forgiveness for yourself and forgive others May you share freely, never taking more than you give. -- the sqlite blessing http://www.sqlite.org/different.html ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] python and super
On Thu, Apr 14, 2011 at 16:18, Ronald Oussoren ronaldousso...@mac.com wrote: It would be odd to not call super in __init__, but for other methods not calling the superclass implementation is fairly common. Yes it is odd, that for example list.__init__ doesn't call super :-) (http://bugs.python.org/issue8733) Daniel ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] python and super
On Apr 14, 2011, at 8:34 AM, P.J. Eby wrote: At 03:55 PM 4/14/2011 +0100, Michael Foord wrote: Ricardo isn't suggesting that Python should always call super for you, but when you *start* the chain by calling super then Python could ensure that all the methods are called for you. If an individual method doesn't call super then a theoretical implementation could skip the parents methods (unless another child calls super). That would break classes that deliberately don't call super. I can think of examples in my own code that would break, especially in __init__() cases. It's perfectly sensible and useful for there to be classes that intentionally fail to call super(), and yet have a subclass that wants to use super(). So, this change would expose an internal implementation detail of a class to its subclasses, and make fragile base class problems worse. (i.e., where an internal change to a base class breaks a previously-working subclass). I agree. Better for someone to submit a recipe for a variant of super and see if there is any uptake. Raymond ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] python and super
On 14/04/2011 17:02, Raymond Hettinger wrote: On Apr 14, 2011, at 8:34 AM, P.J. Eby wrote: At 03:55 PM 4/14/2011 +0100, Michael Foord wrote: Ricardo isn't suggesting that Python should always call super for you, but when you *start* the chain by calling super then Python could ensure that all the methods are called for you. If an individual method doesn't call super then a theoretical implementation could skip the parents methods (unless another child calls super). That would break classes that deliberately don't call super. I can think of examples in my own code that would break, especially in __init__() cases. It's perfectly sensible and useful for there to be classes that intentionally fail to call super(), and yet have a subclass that wants to use super(). So, this change would expose an internal implementation detail of a class to its subclasses, and make fragile base class problems worse. (i.e., where an internal change to a base class breaks a previously-working subclass). I agree. Better for someone to submit a recipe for a variant of super and see if there is any uptake. In Python 3 super is treated specially by the compiler, so an alternative implementation that behaves similarly to the built-in one modulo this change is not possible. Two use cases for the suggested alternative behaviour have been presented. What is the use case for a method not wanting to prevent its *sibling* methods in a multiple inheritance situation being called? I believe the use case Phillip (and others) have presented is for methods preventing their *parent* methods being called. All the best, Michael Foord Raymond -- http://www.voidspace.org.uk/ May you do good and not evil May you find forgiveness for yourself and forgive others May you share freely, never taking more than you give. -- the sqlite blessing http://www.sqlite.org/different.html ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] python and super
On 14 Apr, 2011, at 18:10, Michael Foord wrote: On 14/04/2011 17:02, Raymond Hettinger wrote: On Apr 14, 2011, at 8:34 AM, P.J. Eby wrote: At 03:55 PM 4/14/2011 +0100, Michael Foord wrote: Ricardo isn't suggesting that Python should always call super for you, but when you *start* the chain by calling super then Python could ensure that all the methods are called for you. If an individual method doesn't call super then a theoretical implementation could skip the parents methods (unless another child calls super). That would break classes that deliberately don't call super. I can think of examples in my own code that would break, especially in __init__() cases. It's perfectly sensible and useful for there to be classes that intentionally fail to call super(), and yet have a subclass that wants to use super(). So, this change would expose an internal implementation detail of a class to its subclasses, and make fragile base class problems worse. (i.e., where an internal change to a base class breaks a previously-working subclass). I agree. Better for someone to submit a recipe for a variant of super and see if there is any uptake. In Python 3 super is treated specially by the compiler, so an alternative implementation that behaves similarly to the built-in one modulo this change is not possible. Two use cases for the suggested alternative behaviour have been presented. What is the use case for a method not wanting to prevent its *sibling* methods in a multiple inheritance situation being called? I believe the use case Phillip (and others) have presented is for methods preventing their *parent* methods being called. What would the semantics be of a super that intentially calls all siblings? In particular what is the return value of such a call? The implementation can't know how to combine the implementations in the inheritance chain and should refuse the tempation to guess. Ronald ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] python and super
On Apr 14, 2011, at 12:59 PM, Ronald Oussoren wrote: What would the semantics be of a super that (...) I think it's long past time that this move to python-ideas, if you don't mind. ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] python and super
On Thu, 2011-04-14 at 17:10 +0100, Michael Foord wrote: On 14/04/2011 17:02, Raymond Hettinger wrote: On Apr 14, 2011, at 8:34 AM, P.J. Eby wrote: At 03:55 PM 4/14/2011 +0100, Michael Foord wrote: Ricardo isn't suggesting that Python should always call super for you, but when you *start* the chain by calling super then Python could ensure that all the methods are called for you. If an individual method doesn't call super then a theoretical implementation could skip the parents methods (unless another child calls super). That would break classes that deliberately don't call super. I can think of examples in my own code that would break, especially in __init__() cases. It's perfectly sensible and useful for there to be classes that intentionally fail to call super(), and yet have a subclass that wants to use super(). So, this change would expose an internal implementation detail of a class to its subclasses, and make fragile base class problems worse. (i.e., where an internal change to a base class breaks a previously-working subclass). I agree. Better for someone to submit a recipe for a variant of super and see if there is any uptake. In Python 3 super is treated specially by the compiler, so an alternative implementation that behaves similarly to the built-in one modulo this change is not possible. I know that super does some astonishing *runtime* hackery with co_code when you don't pass arguments, but I thought that was all that was needed. What does the compiler have to do specially for super that would prevent somebody from implementing something like it? Two use cases for the suggested alternative behaviour have been presented. What is the use case for a method not wanting to prevent its *sibling* methods in a multiple inheritance situation being called? I believe the use case Phillip (and others) have presented is for methods preventing their *parent* methods being called. All the best, Michael Foord Raymond ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] python and super
What would the semantics be of a super that intentially calls all siblings? In particular what is the return value of such a call? The implementation can't know how to combine the implementations in the inheritance chain and should refuse the tempation to guess. I'll give you the example I came upon: I have a TestCase class, which inherits from both Django's TestCase and from some custom TestCases that act as mixin classes. So I have something like class MyTestCase(TestCase, Mixin1, Mixin2): ... now django's TestCase class inherits from unittest2.TestCase, which we found was not calling super. Even if this is a bug and should be fixed in unittest2, this is an example where I, as a consumer of django, shouldn't have to be worried about how django's TestCase class is implemented. Since I explicitely base off 3 classes, I expected all 3 classes to be initialized, and I expect the setUp method to be called on all of them. If I'm assuming/expecting unreasonable things, please enlighten me. Otherwise, there you have a real-world use case for when you'd want the sibling classes to be called even if one class breaks the mro chain (in this case TestCase). Thanks, Ricardo ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] python and super
Ricardo Kirkner wrote: What would the semantics be of a super that intentially calls all siblings? In particular what is the return value of such a call? The implementation can't know how to combine the implementations in the inheritance chain and should refuse the tempation to guess. I'll give you the example I came upon: I have a TestCase class, which inherits from both Django's TestCase and from some custom TestCases that act as mixin classes. So I have something like class MyTestCase(TestCase, Mixin1, Mixin2): ... now django's TestCase class inherits from unittest2.TestCase, which we found was not calling super. Even if this is a bug and should be fixed in unittest2, this is an example where I, as a consumer of django, shouldn't have to be worried about how django's TestCase class is implemented. Since I explicitely base off 3 classes, I expected all 3 classes to be initialized, and I expect the setUp method to be called on all of them. If I'm assuming/expecting unreasonable things, please enlighten me. Otherwise, there you have a real-world use case for when you'd want the sibling classes to be called even if one class breaks the mro chain (in this case TestCase). How does python tell your use-case from, say, this: class Mixin3(unittest2.TestCase): stuff happens class MyTestCase(TestCase, Mixin1, Mixin2, Mixin3): ... Here we have django's TestCase that does *not* want to call unittest2.TestCase (assuming that's not a bug), but it gets called anyway because the Mixin3 sibling has it as a base class. So does this mean that TestCase and Mixin3 just don't play well together? Maybe composition instead of inheritance is the answer (in this case, anyway ;). ~Ethan~ ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] python and super
On Apr 14, 2011, at 3:32 PM, Ricardo Kirkner wrote: What would the semantics be of a super that intentially calls all siblings? In particular what is the return value of such a call? The implementation can't know how to combine the implementations in the inheritance chain and should refuse the tempation to guess. I'll give you the example I came upon: I have a TestCase class, which inherits from both Django's TestCase and from some custom TestCases that act as mixin classes. So I have something like class MyTestCase(TestCase, Mixin1, Mixin2): ... now django's TestCase class inherits from unittest2.TestCase, which we found was not calling super. Even if this is a bug and should be fixed in unittest2, this is an example where I, as a consumer of django, shouldn't have to be worried about how django's TestCase class is implemented. Since I explicitely base off 3 classes, I expected all 3 classes to be initialized, and I expect the setUp method to be called on all of them. If I'm assuming/expecting unreasonable things, please enlighten me. For cooperative-multiple-inheritance to work, the classes need to cooperate by having been designed to work together in a series of cooperative super calls. If an external non-cooperative class needs to be used, then it should be wrapped in a class that makes an explicit __init__ call to the external class and then calls super().__init__() to continue the forwarding. Raymond ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] python and super
Ricardo Kirkner wrote: My question is, shouldn't/wouldn't it be better, if python took ownership of that part, and ensured all classes get called, even if some class misbehaved? I don't think so. If a class isn't designed to be part of a super chain, there are likely to be other issues that can't be fixed as simply as this. -- Greg ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] python and super
P.J. Eby wrote: It's perfectly sensible and useful for there to be classes that intentionally fail to call super(), and yet have a subclass that wants to use super(). One such case is where someone is using super() in a single-inheritance environment as a way of not having to write the base class name explicitly into calls to base methods. (I wouldn't recommend using super() that way myself, but some people do.) In that situation, any failure to call super() is almost certainly deliberate. -- Greg ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] python and super
Michael Foord wrote: What I was suggesting is that a method not calling super shouldn't stop a *sibling* method being called, but could still prevent the *parent* method being called. There isn't necessarily a clear distinction between parents and siblings. class A: ... class B(A): ... class C(A, B): ... In C, is A a parent of B or a sibling of B? -- Greg Michael ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] python and super
Ricardo Kirkner wrote: I have a TestCase class, which inherits from both Django's TestCase and from some custom TestCases that act as mixin classes. So I have something like class MyTestCase(TestCase, Mixin1, Mixin2): ... now django's TestCase class inherits from unittest2.TestCase, which we found was not calling super. Even if this is a bug and should be fixed in unittest2, this is an example where I, as a consumer of django, shouldn't have to be worried about how django's TestCase class is implemented. Since I explicitely base off 3 classes, I expected all 3 classes to be initialized, and I expect the setUp method to be called on all of them. If I'm assuming/expecting unreasonable things, please enlighten me. If we treat django's failure to use super as a bug, you want the Python language to work-around that bug so that: I, as a consumer of django, shouldn't have to be worried about bugs in django. (For at least one class of bug.) If we *don't* treat django's failure to use super as a bug, but as a deliberate design choice, then you are trying to do something which django doesn't support. Possibly *deliberately* doesn't support. You want the Python language to add that support so that: I, as a consumer of django, shouldn't have to be worried about whether django supports what I want to do or not. Either way you look at it, I think it's extremely unreasonable to expect the language to work-around bugs in third-party applications, or to add features to them that the third-party developers either didn't consider or don't want. Multiple inheritance is tricky enough to get right without adding Do What I Mean black magic to it. I'd rather work around bugs in third-party classes than try to deal with Python actively subverting the code I read and write by mysteriously calling superclass methods where there is no call to a superclass method. -- Steven ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] python and super
Raymond Hettinger wrote: If an external non-cooperative class needs to be used, then it should be wrapped in a class that makes an explicit __init__ call to the external class and then calls super().__init__() to continue the forwarding. I don't think it's as simple as that. Isn't that super() call going to call the __init__() method that you just explicitly called *again*? Seems like you would at least need to use super(BaseClass)... to skip the one you just called. But it's not immediately obvious to me that this won't ever skip other classes that you *do* want to call. -- Greg ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] python and super
On Fri, 15 Apr 2011 12:58:14 +1200, Greg Ewing greg.ew...@canterbury.ac.nz wrote: P.J. Eby wrote: It's perfectly sensible and useful for there to be classes that intentionally fail to call super(), and yet have a subclass that wants to use super(). One such case is where someone is using super() in a single-inheritance environment as a way of not having to write the base class name explicitly into calls to base methods. (I wouldn't recommend using super() that way myself, but some people do.) In that situation, any failure to call super() is almost certainly deliberate. Why not? It seems more useful than using it for chaining, especially given the compiler hack in Python3. -- R. David Murray http://www.bitdance.com ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
