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

[Tobias Ivarsson]

On Tue, Apr 6, 2010 at 11:13 AM, Greg Ewing <greg.ewing at canterbury.ac.nz>wrote:

> Reid Kleckner wrote:
>
>  If I remember correctly, the exec statement is going away in py3k, and
>> calling exec() with one argument can modify the local scope.
>>
>
> I've been kind of wondering what the deal is with exec in py3.
> I always thought the reason for making exec a statement was so
> that locals optimisation could be turned off in its presence,
> so I'm not sure how py3 is getting away with making it a
> function.
>

It looks like py3 does not allow exec to modify the locals:

$ python3
Python 3.1.1 (r311:74543, Aug 24 2009, 18:44:04)
[GCC 4.0.1 (Apple Inc. build 5493)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> def x(a):
...     exec(a)
...     return a
...
>>> x("a = 5")
'a = 5'
>>> # the above statement would have returned 5 if the locals had been
modified


>
> Anyhow, it seems to me that as long as locals() or whatever
> might replace it is able to find the existing value of a local,
> it should also be able to change that value, wherever it
> happens to be stored.
>
> I suppose that might fail if an optimiser decides to keep
> multiple copies of a local for some reason, though.
>
> But even if it has to be read-only, I still think a view object
> would be a more py3ish way of handling locals() and the like.
> You might only want access to a few locals, in which case
> building a dict containing all of them would be wasteful.
>
> --
> Greg
>
>
> --
> Greg
>
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