On Fri, 24 Jan 2020 at 02:36, Guido van Rossum <gu...@python.org> wrote:
> I'm tempted to declare this implementation-defined behavior -- *implicit* > calls to __eq__ and __ne__ *may* be skipped if both sides are the same object > depending on the whim of the implementation. This sounds reasonable to me. It could be added to https://docs.python.org/3/reference/expressions.html#value-comparisons """ The default behavior for equality comparison (== and !=) is based on the identity of the objects. Hence, equality comparison of instances with the same identity results in equality, and equality comparison of instances with different identities results in inequality. A motivation for this default behavior is the desire that all objects should be reflexive (i.e. x is y implies x == y). """ We could add an extra clause here: """As an optimisation, when the implementation *implicitly* compares two values for equality (for example, in list comparison or `list.count`) it is allowed (but not required) to omit the equality check if the objects being compared are the same. This can result in different behaviour for objects with user-defined equality that is not reflexive - that is acceptable.""" > We should probably also strongly recommend that __eq__ and __ne__ not do what > math.nan does. >From https://docs.python.org/3/reference/expressions.html#comparisons """ User-defined classes that customize their comparison behavior should follow some consistency rules, if possible: Equality comparison should be reflexive. In other words, identical objects should compare equal: x is y implies x == y """ and also """ Python does not enforce these consistency rules. In fact, the not-a-number values are an example for not following these rules. """ So that's covered already :-) Paul _______________________________________________ Python-Dev mailing list -- python-dev@python.org To unsubscribe send an email to python-dev-le...@python.org https://mail.python.org/mailman3/lists/python-dev.python.org/ Message archived at https://mail.python.org/archives/list/python-dev@python.org/message/H4KSOTEOZNXIGJT52PZD64O2ASWPZG3S/ Code of Conduct: http://python.org/psf/codeofconduct/
