[Python-Dev] redefining is

[Walter Dörwald]

Michael Chermside wrote:

> Andrew Koenig writes:
> 
>>The reason for that, I think, is that there are really three kinds of
>>"equality" that make sense, and only two of them are reasonably available:
>>
>>	Value equality (==, which might be user-defined, because
>>		the concept of "value" can be user-defined);

And there might be more than one user.

>>	Object identity ("is")
>>
>>	Object equivalence (no easy way of expressing it today)
>>
>>By "object equivalence", I mean mutual substitutability--which is the same
>>as identity for mutable objects, but not for immutable ones.

But this mutual substitutability is context dependant.
"is" guarantees that both objects are mutual substitutable in
*any* context. "==" defines the mutual substitutability for the
"default" context. Any other context is application dependant,
so the comparison function should be defined by the application.

Bye,
    Walter Dörwald