Why return None?

[Paul Rubin]

Antoon Pardon <apardon at forel.vub.ac.be> writes:
> but having operators like '+=' which have a different kind of result
> depending on whether you have a mutable or immutable object is IMO
> not such a good design and I wonder what design principle inspired them.

+= was added to Python fairly recently (in 2.0, I think) and there was
a lot of agonizing about whether to add it.  It was one of those
things like the ?: construction.  On the one hand there was the
abstract objection you raise.  On the other hand there were lots of
users frustrated at being unable to say v[f(x)] += 1 instead of having
to call f twice or introduce a temp variable or something.
Eventually, practicality beat purity.