New submission from Vasiliy Faronov: Consider the attached example program. I expect it to run successfully, because the Python 3 language reference says [1]:
> For container types such as list, tuple, set, frozenset, dict, or > collections.deque, the expression `x in y` is equivalent to `any(x is e or x > == e for e in y)`. and [2]: > `x==y` calls `x.__eq__(y)` Instead, under Python 3.5.2, the program crashes with an assertion error, because `dict.__contains__` calls `Bar.__eq__` instead of `Foo.__eq__`. The same happens if you replace the dict with a set or a frozenset. But if you replace it with a list or a tuple, the behavior is as documented. This seems to me like a bug in either the implementation or the documentation. The language reference clearly says [3] that equality should be symmetric "if possible", but is not required to be, and indeed that is hard to guarantee when different classes are involved. [1] https://docs.python.org/3/reference/expressions.html#membership-test-details [2] https://docs.python.org/3/reference/datamodel.html#object.__eq__ [3] https://docs.python.org/3/reference/expressions.html#value-comparisons ---------- files: contains_eq.py messages: 271145 nosy: vfaronov priority: normal severity: normal status: open title: Inconsistent calls to __eq__ from built-in __contains__ type: behavior Added file: http://bugs.python.org/file43857/contains_eq.py _______________________________________ Python tracker <rep...@bugs.python.org> <http://bugs.python.org/issue27605> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
