On Tue, Sep 22, 2009 at 3:37 PM, Chris Withers <ch...@simplistix.co.uk> wrote: > Where are the specifications on what happens if two objects are compared and > both have implementations of __eq__? Which __eq__ is called? > What happens if the first one called returns False? Is the second one called? > What is one implements __eq__ and the other __ne__?
I (still :-) think this is covered, for Python 2.x at least, by: http://docs.python.org/reference/datamodel.html#coercion-rules Specifically, the bits that say: - For objects x and y, first x.__op__(y) is tried. If this is not implemented or returns NotImplemented, y.__rop__(x) is tried. If this is also not implemented or returns NotImplemented, a TypeError exception is raised. But see the following exception: - Exception to the previous item: if the left operand is an instance of a built-in type or a new-style class, and the right operand is an instance of a proper subclass of that type or class and overrides the base’s __rop__() method, the right operand’s __rop__() method is tried before the left operand’s __op__() method. I agree that having these rules in a section called 'Coercion rules' is a bit confusing. > Python 2 > http://pastebin.com/f8f19ab3 > > Python 3 > http://pastebin.com/f55e44630 The duplicate __eq__ calls in these pastes are disturbing: in short, if A() and B() are new-style classes defining __eq__, it seems that A() == B() ends up calling A.__eq__ twice *and* B.__eq__ twice, in the order A.__eq__, B.__eq__, B.__eq__, A.__eq__. In 3.x, slot_tp_richcompare (in typeobject.c) makes two calls to half_richcompare; I think the second is redundant. The coercion rules are already taken care of in do_richcompare (in object.c). I tried removing the second call to half_richcompare, and the entire test-suite still runs without errors. In 2.x, it's possible that this call is necessary for some bizarre combinations of __cmp__ and __eq__; I haven't tried to get my head around this yet. I'll open an issue for the duplicate __eq__ calls. Mark _______________________________________________ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
