[Python-Dev] variable name resolution in exec is incorrect

[Guido van Rossum]

On Wed, May 26, 2010 at 5:37 PM, Colin H <hawkett at gmail.com> wrote:
> Mark Dickinson wrote:
>
>> Seems to me the whole idea of being able to specify
>> separate global and local scopes for top-level code is
>> screwy in the first place. Are there any use cases for
>> it? Maybe the second scope argument to exec() should
>> be deprecated?
>
> It is running as class namespace that makes the argument that there's
> no use case. I agree - I can't think of any decent use cases for
> exec() as class namespace - defining functions and classes only works
> for a subset of function and class definitions
>
> However, if exec() ran as function namespace instead, then the locals
> dictionary will contain all the definitions from the exec()'d code
> block, and only those definitions. Very useful.

Hmm... I see your point, but fear that the implementation is tricky,
since it would mean every exec'ed block would have to be compiled as
if it were a function body, and that probably violates some other
invariants.

> This is a major use
> case for exec() - defining code from strings (e.g. enabling you to
> store python code in the database), and using it at runtime. It seems
> to me this must have been the point of locals in the first place.
>
> If you just use globals, then your definitions exist amongst a whole
> bunch of other python stuff, and unless you know in advance what was
> defined in your code block, its very difficult to extract them.

Check the reconstruction of your use case I just posted. I think you
re thinking context == globals(), which makes sense.

-- 
--Guido van Rossum (python.org/~guido)