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
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