[Python-Dev] Re: accumulator display syntax

[Alex Martelli]

On Sunday 26 October 2003 04:41 pm, Phillip J. Eby wrote:
> 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.
   ...
> 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 +=.

A better architecture than the initial copy.copy I was now thinking of -- 
thanks.  But it doesn't solve Guido's objection as above shown.

> 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?

Only the fact that "there's nothing that guarantees" this, as Guido says.
alist = alist + x only succeds if x is also a list, while alist += x succeeds
also for tuples and other sequences, for example.

Personally, I don't think this would be a problem, but it's not my decision.


Alex