[Python-Dev] PyObject_RichCompareBool identity shortcut

[Guido van Rossum]

On Thu, Apr 28, 2011 at 4:04 PM, Nick Coghlan <ncoghlan at gmail.com> wrote:
> On Fri, Apr 29, 2011 at 8:55 AM, Guido van Rossum <guido at python.org> wrote:
>> Sorry, we'll have to make an exception for those of course. This will
>> somewhat complicate the interpretation of well-behaved, because those
>> are *not* well-behaved as far as containers go (both dict key lookup
>> and list membership are affected) but it is not a bug -- however it is
>> a bug to put these in containers and then use container comparisons on
>> the container.
>
> That's a point in favour of the status quo, though - with the burden
> of enforcing reflexivity placed on the containers, types are free to
> make use of rich comparisons to return more than just simple
> True/False results.

Possibly. Though for types that *do* return True/False, NaN's behavior
can still be infuriating.

> I hadn't really thought about it that way before this discussion - it
> is the identity checking behaviour of the builtin containers that lets
> us sensibly handle cases like sets of NumPy arrays.

But do they? For non-empty arrays, __eq__ will always return something
that is considered true, so any hash collisions will cause false
positives. And look at this simple example:

>>> class C(list):
...   def __eq__(self, other):
...     if isinstance(other, C):
...       return [x == y for x, y in zip(self, other)]
...
>>> a = C([1,2,3])
>>> b = C([2,1,3])
>>> a == b
[False, False, True]
>>> x = [a, a]
>>> b in x
True

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