python assignment

[Tim Peters]

[Tim]
>> It would be very unusual (because bad design) for the __iadd__
>> method of a mutable type to return a pre-existing object, though.

[Juha Autero]
> I'm confused. I thought that the idea of __iadd__ is that for
> *mutable* types it modifies existing object instead of returning new
> one.

Bjorn added more appropriate words -- it would be bad design for the
__iadd__ method of a mutable type to return a pre-existing object other than
self.

>>> a = [1, 2]
>>> b = [1]
>>> b += [2]

While "a == b" must be true at this point, it would be a nightmare if "a is
b" could be true at this point.  For immutable types it doesn't matter:

>>> a = (1, 2)
>>> b = (1,)
>>> b += (2,)

It so happens that "a is b" is not true at this point under any Python
released so far, but it could be true someday without causing harm.

Sorry for the confusion!