[Python-Dev] Scope object (Re: nonlocals() function?)

[Cesare Di Mauro]

2010/4/6 Antoine Pitrou <solipsis at pitrou.net>

> Greg Ewing <greg.ewing <at> canterbury.ac.nz> writes:
> >
> > Maybe it would be better to deprecate globals() and locals()
> > and replace them with another function called something like
> > scope().
>
> It is useful to distinguish between globals (i.e., module-level variables)
> and
> locals, so replacing them with scope() would not be better IMO.
>
> > It would return a mapping object that looks up
> > names in the current scope. It could also improve on locals()
> > by being writable.
>
> If you can prove that making locals() (or its replacement) writable doesn't
> complicate the interpreter core too much, then why not. Otherwise -1 :-)
>
> Regards
>
> Antoine.
>

It will certainly. There's MUCH that can be optimized to let CPython squeeze
more performance from static analysis (even a gross one) on locals.

Example:

def f():
    a = 1
    b = 2
    return a + b

can be reduced to something similar to:

def f():
    a = 1
    b = 2
    return 3

and, more aggressively, like:

def f():
    return 3

They are just "dummy" examples, but can make it clear how far optimizations
can go with static analysis on locals. Python is a language that make it
possible to use such analysis at compile time, and I think it is a very good
thing.

Obviously the last example brings questions regards the language semantic:
is it right to suppress "unused" or "not useful" local variables? A
"conservative" answer will be clearly NO. But I hope that a future language
specification will fix some aspects, putting clear what you can expect from
the language itself, and what is closet to the implementation.

Cesare
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