PyObject_RichCompareBool(x, y, op) has a (valuable!) shortcut: if x and y are the same object, then equality comparison returns True and inequality False. No attempt is made to execute __eq__ or __ne__ methods in those cases.
This has visible consequences all over the place, but they don't appear to be documented. For example, >>> import math >>> ([math.nan] * 5).count(math.nan) 5 despite that `math.nan == math.nan` is False. It's usually clear which methods will be called, and when, but not really here. Any _context_ that calls PyObject_RichCompareBool() under the covers, for an equality or inequality test, may or may not invoke __eq__ or __ne__, depending on whether the comparands are the same object. Also any context that inlines these special cases to avoid the overhead of calling PyObject_RichCompareBool() at all. If it's intended that Python-the-language requires this, that needs to be documented. Or if it's implementation-defined, then _that_ needs to be documented. Which isn't straightforward in either case, in part because PyObject_RichCompareBool isn't a language-level concept. This came up recently when someone _noticed_ the list.count(NaN) behavior, and Victor made a PR to document it: https://github.com/python/cpython/pull/18130 I'm pushing back, because documenting it _only_ for .count() makes .count() seem unique in a way it isn't, and doesn't resolve the fundamental issue: is this language behavior, or implementation behavior? Which I don't want to argue about. But you certainly should ;-) _______________________________________________ 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/3ZAMS473HGHSI64XB3UV4XBICTG2DKVF/ Code of Conduct: http://python.org/psf/codeofconduct/
