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Re: PyEval_GetLocals and unreferenced variables

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  Re: PyEval_GetLocals and unreferenced variables Kasper Peeters <kasper@phi-sci.com> - 2014-12-03 11:18 +0000
    Re: PyEval_GetLocals and unreferenced variables Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2014-12-04 01:28 +1300
      Re: PyEval_GetLocals and unreferenced variables Ian Kelly <ian.g.kelly@gmail.com> - 2014-12-04 08:42 -0700

#81852 — Re: PyEval_GetLocals and unreferenced variables

> I'm not sure how you think you're adding a local from C
> code. If you're using PyEval_GetLocals(), that only gives
> you a dict containing a *copy* of the locals; modifying
> that dict doesn't change the locals in the function's frame.

That may have been the design plan, but in Python 2.7.6, I definitely
am able to inject locals via PyEval_GetLocals() and have them be visible
both from the C and Python side; see also

http://stackoverflow.com/questions/22276502/create-python-object-in-local-scope-from-within-c

> If the bytecode of the nested function doesn't reference a given
> variable in the outer function, it doesn't get passed in.

Ok, that's good to know because it rules out doing this without having
an explicit reference to the variable in the inner scope. Thanks.

Cheers,
Kasper

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#81859

Kasper Peeters wrote:
> That may have been the design plan, but in Python 2.7.6, I definitely
> am able to inject locals via PyEval_GetLocals() and have them be visible
> both from the C and Python side;

What seems to be happening is that the dict created by
PyEval_GetLocals() is kept around, so you can change it
and have the changes be visible through locals() in
Python.

However, that doesn't create a local *name* that's
visible from Python. Or if the local name exists,
changes made through locals() aren't reflected in
the value of the local name.

Existing local name is not changed:

 >>> def f():
...  a = 42
...  locals()['a'] = 17
...  print a
...
 >>> f()
42

Can't create a local name:

 >>> def g():
...  locals()['a'] = 17
...  print a
...
 >>> g()
Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
   File "<stdin>", line 3, in g
NameError: global name 'a' is not defined

Changes to locals() persist:

 >>> def h():
...  locals()['a'] = 17
...  print locals()['a']
...
 >>> h()
17

-- 
Greg

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#81911

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On Wed, Dec 3, 2014 at 5:28 AM, Gregory Ewing <greg.ewing@canterbury.ac.nz>
wrote:
>
> Kasper Peeters wrote:
>>
>> That may have been the design plan, but in Python 2.7.6, I definitely
>> am able to inject locals via PyEval_GetLocals() and have them be visible
>> both from the C and Python side;
>
> What seems to be happening is that the dict created by
> PyEval_GetLocals() is kept around, so you can change it
> and have the changes be visible through locals() in
> Python.

That the dict is cached is an implementation detail. It's not advisable to
rely upon this, and as stated in the Python docs, the locals dict should
not be modified.

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