[Python-Dev] PEP 203 Augmented Assignment

Guido van Rossum guido@beopen.com
Thu, 27 Jul 2000 08:09:37 -0500

> > Simply put:
> > 
> >     a += 1 IS NOT ATOMIC!
> > 
> If that's the case, then why do we need augemented assignments
> at all ?
> (I understood the atomicness being the main argument for
> introducing augmented assigns.)

Huh?  Where did you get that idea?  Can you point to a message where
this was indicated?

The reason for introducing it is that it's been the single-most
requested feature ever since Python's inception.  I think the reason
it's been requested so much is that it's a waste of typing skills as
well as (and more importantly) unnecessary extra effort to deduce
what's going on in code like

    a.b.f().x[i] = a.b.f().x[i] + 1

Another important reason is that in cases like

    a[long_and_expensive_call()] = a[long_and_expensive_call()] + 1

you want to avoid doing the long_and_expensive_call() twice.
Introducing a temp variable reduces the readability of the code.

Note that my proposed semantics guarantee that in

    a[long_and_expensive_call()] += 1

the index is computed only once -- but this is not the same as
atomicity (which has to do with other threads, signal handlers and
similar asynchronous things).

--Guido van Rossum (home page: http://www.pythonlabs.com/~guido/)