[Python-Dev] Re: accumulator display syntax

[Phillip J. Eby]

At 04:16 PM 10/25/03 -0700, Guido van Rossum wrote:
> > > No way.  There's nothing that guarantees that a+=b has the same
> > > semantics as a+b, and in fact for lists it doesn't.
> >
> > You mean because += is more permissive (accepts any sequence
> > RHS while + insists the RHS be specifically a list)?  I don't see how
> > this would make it bad to use += instead of + -- if we let the user
> > sum up a mix of (e.g.) strings and tuples, why are we hurting him?
>
>We specifically decided that sum() wasn't allowed for strings, because
>it's a quadratic algorithm.  Other sequences are just as bad, we just
>didn't expect that to be a common case.
>
>Also see my not-so-far-fetched example of a semantic change.

Maybe I'm confused, but when Alex first proposed this change, I mentally 
assumed that he meant he would change it so that the *first* addition would 
use + (in order to ensure getting a "fresh" object) and then subsequent 
additions would use +=.

If this were the approach taken, it seems to me that there could not be any 
semantic change or side-effects for types that have compatible meaning for 
+ and += (i.e. += is an in-place version of +).

Maybe I'm missing something here?