On Apr 27, 2011, at 2:37 AM, Hrvoje Niksic wrote:
> The other day I was surprised to learn this:
>
> >>> nan = float('nan')
> >>> nan == nan
> False
> >>> [nan] == [nan]
> True # also True in tuples, dicts, etc.
Would also be surprised if you put an object in a dictionary but couldn't get
it out? Or added it to a list but its count was zero?
Identity-implies-equality is necessary so that classes can maintain their
invariants and so that programmers can reason about their code. It is not just
in PyObject_RichCompareBool, it is deeply embedded in the language (the logic
inside dicts for example). It is not a short-cut, it is a way of making sure
that internally we can count on equality relations reflexive, symmetric, and
transitive. A programmer needs to be able to make basic deductions such as the
relationship between the two forms of the in-operator: for elem in somelist:
assert elem in somelist # this should never fail.
What surprises me is that anyone gets surprised by anything when experimenting
with an object that isn't equal to itself. It is roughly in the same category
as creating a __hash__ that has no relationship to __eq__ or making
self-referencing sets or setting False,True=1,0 in python 2. See
http://bertrandmeyer.com/2010/02/06/reflexivity-and-other-pillars-of-civilization/
for a nice blog post on the subject.
Raymond
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