Python handles globals badly.

Ian Kelly ian.g.kelly at gmail.com
Fri Sep 11 17:49:47 CEST 2015


On Fri, Sep 11, 2015 at 9:44 AM, Chris Angelico <rosuav at gmail.com> wrote:
> On Sat, Sep 12, 2015 at 1:27 AM, Ian Kelly <ian.g.kelly at gmail.com> wrote:
>> The exec still happily runs; it's just using its own private locals namespace.
>>
>> Tangent: does the help for exec need to be updated? It currently reads:
>>
>>     The globals and locals are dictionaries, defaulting to the current
>>     globals and locals.  If only globals is given, locals defaults to it.
>>
>> Which would seem to indicate that if called from within a function
>> with no globals or locals, the locals from the function would be used.
>
> And that's the thing... I think. It's using locals(), which starts out
> as a copy of the function's locals (in this example, empty), but
> without assignment affecting anything. Which is more than a little
> weird:
>
>>>> def f():
> ...     x = [1]
> ...     exec("print(x); x[0] = 2; print(x); x = [3]; print(x)")
> ...     print(x)
> ...
>>>> f()
> [1]
> [2]
> [3]
> [2]

Ah, that makes sense. It's writing into the dict that is created and
returned by locals(), but not actually updating the frame locals which
are the source of truth.


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