[Python-Dev] PyObject_RichCompareBool identity shortcut

[Nick Coghlan]

On Thu, Apr 28, 2011 at 4:20 PM, Glenn Linderman <v+python at g.nevcal.com> wrote:
> In that bug, Nick, you mention that reflexive equality is something that
> container classes rely on in their implementation.  Such reliance seems to
> me to be a bug, or an inappropriate optimization, rather than a necessity.
> I realize that classes that do not define equality use identity as their
> default equality operator, and that is acceptable for items that do not or
> cannot have any better equality operator.  It does lead to the situation
> where two objects that are bit-for-bit clones get separate entries in a
> set... exactly the same as how NaNs of different identity work... the
> situation with a NaN of the same identity not being added to the set
> multiple times seems to simply be a bug because of conflating identity and
> equality, and should not be relied on in container implementations.

No, as Raymond has articulated a number of times over the years, it's
a property of the equivalence relation that is needed in order to
present sane invariants to users of the container. I included in the
bug report the critical invariants I am currently aware of that should
hold, even when the container may hold types with a non-reflexive
definition of equality:

  assert [x] == [x]                     # Generalised to all container types
  assert not [x] != [x]                # Generalised to all container types
  for x in c:
    assert x in c
    assert c.count(x) > 0                   # If applicable
    assert 0 <= c.index(x) < len(c)      # If applicable

The builtin types all already work this way, and that's a deliberate
choice - my proposal is simply to document the behaviour as
intentional, and fix the one case I know of in the standard library
where we don't implement these semantics correctly (i.e.
collections.Sequence).

The question of whether or not float and decimal.Decimal should be
modified to have reflexive definitions of equality (even for NaN
values) is actually orthogonal to the question of clarifying and
documenting the expected semantics of containers in the face of
non-reflexive definitions of equality.

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia