[issue22052] Comparison operators called in reverse order for subclasses with no override.
New submission from Mark Dickinson:
As reported in a StackOverflow question [1]: the order in which the special comparison methods are called seems to be contradictory to the docs [2]. In the following snippet, __eq__ is called with reversed operands first:
class A:
... def __eq__(self, other): ... print(type(self), type(other)) ... return True ...
class B(A):
... pass ...
A() == B()
<class '__main__.B'> <class '__main__.A'> True
However, the docs note that:
"""If the right operand’s type is a subclass of the left operand’s type and that subclass provides the reflected method for the operation, this method will be called before the left operand’s non-reflected method. This behavior allows subclasses to override their ancestors’ operations."""
... which suggests that this reversal should only happen when the subclass B *overrides* A's definition of __eq__ (and indeed that's the usual behaviour for arithmetic operations like __add__).
Looking more closely, that statement in the docs is in the 'numeric-types' section, so it's not clear that its rules should apply to the comparison operators. But either way, some doc clarification could be useful.
[1] http://stackoverflow.com/q/24919375/270986 [2] https://docs.python.org/3.5/reference/datamodel.html#emulating-numeric-types
---------- assignee: docs@python components: Documentation messages: 223778 nosy: docs@python, mark.dickinson priority: normal severity: normal status: open title: Comparison operators called in reverse order for subclasses with no override. versions: Python 2.7, Python 3.4, Python 3.5
_______________________________________ Python tracker report@bugs.python.org http://bugs.python.org/issue22052 _______________________________________
R. David Murray added the comment:
"the subclass provides" doesn't actually imply anything about overriding, I think. For __eq__, though, that means that *every* class "provides" it. Indeed, I've always thought of the rule as "the subclass goes first" with no qualification, but that's only true for __eq__.
It looks like the "subclass goes first" note is missing from the __eq__ section.
But see issue 21408. I find it hard to reason about this algorithm, so I could be completely wrong :)
---------- nosy: +r.david.murray
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Mark Dickinson added the comment:
"the subclass provides" doesn't actually imply anything about overriding, I think.
Yes, that was the thrust of one of the SO answers. Unfortunately, that explanation doesn't work for arithmetic operators, though: there an explicit override is necessary. Here's another example, partly to get away from the extra complication of __eq__ being its own inverse. After:
class A(object): def __lt__(self, other): return True def __gt__(self, other): return False def __add__(self, other): return 1729 def __radd__(self, other): return 42
class B(A): pass
we get:
>>> A() + B() 1729 >>> A() < B() False
So the addition is calling the usual __add__ method first (the special exception in the docs doesn't apply: while B *is* a subclass of A, it doesn't *override* A's __radd__ method). But the comparison is (surprisingly) calling the __gt__ method first.
So we've got two different rules being followed: one for arithmetic operators, and a different one for comparisons.
This isn't a big deal behaviour-wise: I'm certainly not advocating a behaviour change here. But it would be nice to document it.
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R. David Murray added the comment:
Ah yes. I remember there being a discussion somewhere about the differences between comparison operator inverses and the arithmetic 'r' methods, but I can't find it at the moment. I *thought* there was a full discussion of the logic involved in these cases, but I can't find that either. We need one somewhere that we can crosslink to if it doesn't already exist.
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Martin Panter added the comment:
I have included some rules about the priority for calling reflected operator methods in my patch to Issue 4395
---------- nosy: +vadmium
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Martin Panter added the comment:
My patch was committed for Python 3.4+. The priority of the comparator methods is now documented at the end of https://docs.python.org/dev/reference/datamodel.html#richcmpfuncs. Perhaps all that is left to do here is to apply similar changes to the Python 2 documentation.
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Martin Panter added the comment:
Does anyone know enough about Python 2 to propose a fix? I don’t know enough about object classes versus “instance” classes, and potential interference of the __cmp__() method. In Python 2 the order seems to depend on the class type:
(<__main__.A instance at 0x7f730d37f5f0>, <__main__.B instance at 0x7f730d37f518>) (<__main__.B object at 0x7f730d37dc10>, <__main__.A object at 0x7f730d37d110>)
Or perhaps we should just close this now and forget about Python 2 ;)
---------- stage: -> needs patch versions: -Python 3.4, Python 3.5
_______________________________________ Python tracker report@bugs.python.org http://bugs.python.org/issue22052 _______________________________________
Mark Dickinson added the comment:
For Python 2, I think the most we should do is document the behaviour somewhere; changing it in a bugfix release seems both unnecessary and potentially risky.
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Mark Dickinson added the comment:
the most we should do is document the behaviour somewhere
And indeed, perhaps this issue counts as sufficient documentation...
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Serhiy Storchaka storchaka+cpython@gmail.com added the comment:
Python 2.7 is no longer supported.
---------- nosy: +serhiy.storchaka resolution: -> fixed stage: needs patch -> resolved status: open -> closed
_______________________________________ Python tracker report@bugs.python.org https://bugs.python.org/issue22052 _______________________________________
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Mark Dickinson -
Martin Panter
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R. David Murray
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Serhiy Storchaka