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Author mark.dickinson
Recipients docs@python, mark.dickinson
Date 2014-07-23.21:03:29
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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'>

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.

Date User Action Args
2014-07-23 21:03:30mark.dickinsonsetrecipients: + mark.dickinson, docs@python
2014-07-23 21:03:30mark.dickinsonsetmessageid: <>
2014-07-23 21:03:30mark.dickinsonlinkissue22052 messages
2014-07-23 21:03:29mark.dickinsoncreate