Behavior of += (was Re: [Python-Dev] Customization docs)

[Donn Cave]

Quoth huaiyu at gauss.almadan.ibm.com (Huaiyu Zhu):
...
| That is one proposal.  My proposal is to disallow += for immutables.
| Another symbol would be used for rebinding operations.  (Or maybe swap the
| role of these symbols.)  Thus for mutables both operations would be
| available.

That suits me, except the mutable/immutable distinction.  That's
how we have been describing the distinction, but it's worth noting
that it isn't really what we mean.

For a silly example, suppose that in 2.3, None supports +=, kind of
like the UNIX /dev/null.  This would not make it mutable in any
reasonable sense of the word, but it's in the "in place" category.
A class instance that implements __iadd__ to return something other
than self, would not be, though it's certainly mutable.  In any
normal case, "in place" and "mutable" go together, but they aren't
the same thing.

	Donn Cave, donn at drizzle.com