[Python-Dev] redefining is

[Andrew Koenig]

> > By "object equivalence", I mean mutual substitutability--which is the
> same
> > as identity for mutable objects, but not for immutable ones.
> 
> So what does it mean for immutable objects?

Two immutable objects are equivalent if and only if they have the same type
and all of their attributes are (recursively) equivalent.

> Your raise/except example is bad.  Do you have another one in which
> people could not just use '==' instead of 'is'?

x = ([], [])
y = ([], [])

Here, x == y is true, but x and y are not equivalent.  We can prove this
inequivalence by executing x[0].append(42) and noting that y[0] does not
change.

a = []
b = []
x = (a, b)
y = (a, b)

Here, x == y is true, and x and y are equivalent.