[Python-Dev] PyObject_RichCompareBool identity shortcut

[Guido van Rossum]

On Wed, Apr 27, 2011 at 7:39 AM, Raymond Hettinger
<raymond.hettinger at gmail.com> wrote:
>
> 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.

Maybe we should just call off the odd NaN comparison behavior?

-- 
--Guido van Rossum (python.org/~guido)