[Python-Dev] [PEP] += on return of function call result

[Greg Ewing]

Andrew Koenig <ark@research.att.com>:

> So the behavior of
> 
>         foo = d.setdefault(r,'')
>         foo += "test %d\n" % t
> 
> depends on what type foo has, and the OP didn't say.

I assumed that the code snippet was from his actual application, in
which case he *did* want it to work on strings, in which case, even if
he had the feature he wanted, it wouldn't have helped him.

I think the fact that this would only work when the value was mutable
is a good reason to disallow it.  Too big a source of surprises,
otherwise.

Being forced to find another way to update the value in this case is a
feature, because the absence of such a way when the value is immutable
makes it clear that there's no way to do what you're trying to do!

Greg Ewing, Computer Science Dept, +--------------------------------------+
University of Canterbury,	   | A citizen of NewZealandCorp, a	  |
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