[Python-Dev] PyObject_RichCompareBool identity shortcut

[Mark Dickinson]

On Wed, Apr 27, 2011 at 10:37 AM, Hrvoje Niksic <hrvoje.niksic at avl.com> 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.

That one surprises me a bit too:  I knew we were using
identity-then-equality checks for containment (nan in [nan]), but I
hadn't realised identity-then-equality was also used for the
item-by-item comparisons when comparing two lists.  It's defensible,
though: [nan] == [nan] should presumably produce the same result as
{nan} == {nan}, and the latter is a test that's arguably based on
containment (for sets s and t, s == t if each element of s is in t,
and vice versa).

I don't think any of this should change.  It seems to me that we've
currently got something approaching the best approximation to
consistency and sanity achievable, given the fundamental
incompatibility of (1) nan breaking reflexivity of equality and (2)
containment being based on equality.  That incompatibility is bound to
create inconsistencies somewhere along the line.

Declaring that 'nan == nan' should be True seems attractive in theory,
but I agree that it doesn't really seem like a realistic option in
terms of backwards compatibility and compatibility with other
mainstream languages.

Mark