On 28/04/2011 15:58, Steven D'Aprano wrote:
Fundamentally, the problem is that some containers bypass equality tests for identity tests. There may be good reasons for that shortcut, but it leads to problems with *any* object that does not define equality to be reflexive, not just NANs. I say you have that backwards. It is a legitimate shortcut, and any object that (perversely) doesn't define equality to be reflexive leads (unsurprisingly) to problems with it (and with *anything else* that - very reasonably - assumes that identity implies equality).
Mark Shannon wrote:
Although both NaN == NaN and NaN != NaN could arguably be a "maybe" value, the all important reflexivity (x == x is True) is effectively part of the language. All collections rely on it and Python wouldn't be much use without dicts, tuples and lists.
Perhaps they shouldn't rely on it. Identity tests are an implementation detail. But in any case, reflexivity is *not* a guarantee of Python. With rich comparisons, you can define __eq__ to do anything you like.
And you can write True = False (at least in older versions of Python you could). No language stops you from writing stupid programs. In fact I would propose that the language should DEFINE the meaning of "==" to be True if its operands are identical, and only if they are not would it use the comparison operators, thus enforcing reflexivity. (Nothing stops you from writing your own non-reflexive __eq__ and calling it explicitly, and I think it is right that you should have to work harder and be more explicit if you want that behaviour.) Please, please, can we have a bit of common sense and perspective here. No-one (not even a mathematician) except someone from Wonderland would seriously want an object not equal to itself. Regards Rob Cliffe