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frame.f_locals for list/dict/set comprehension in module/class scope doesn't contain iteration variables #108732
Description
Bug report
Checklist
- I am confident this is a bug in CPython, not a bug in a third-party project
- I have searched the CPython issue tracker,
and am confident this bug has not been reported before
CPython versions tested on:
CPython main branch
Operating systems tested on:
Linux
Output from running 'python -VV' on the command line:
Python 3.13.0a0 (heads/main:dd32611f4f, Aug 31 2023, 20:23:23) [GCC 10.2.1 20210110]
A clear and concise description of the bug:
The following assertion fails with PEP 709 (c3b595e):
import sys
f_locs = None
[f_locs := sys._getframe().f_locals for a in range(1)]
assert 'a' in f_locsThis was discovered when trying to inspect listcomp iteration variable under pdb, as it makes inspection of these variables impossible:
> /home/radislav/projects/cpython/listcomp.py(1)<module>()
-> [print(a) for a in range(1)]
(Pdb) s
0
> /home/radislav/projects/cpython/listcomp.py(1)<module>()
-> [print(a) for a in range(1)]
(Pdb) print(a)
*** NameError: name 'a' is not defined
(Pdb) s
/home/radislav/projects/cpython/listcomp.py:1: RuntimeWarning: assigning None to unbound local 'a'
[print(a) for a in range(1)]
--Return--
> /home/radislav/projects/cpython/listcomp.py(1)<module>()->None
-> [print(a) for a in range(1)]
As I can see, it happens because iteration vars are now stored in "fast hidden locals" that aren't included into frame->f_locals on _PyFrame_FastToLocalsWithError(frame) call, and thus are not accessible through PyFrameObject f_locals attribute.
cc @carljm
Activity
pythongh-108732: include comprehension locals in frame.f_locals
pythongh-108732: include comprehension locals in frame.f_locals
pythongh-108732: include comprehension locals in frame.f_locals
carljm commentedon Sep 6, 2023
Thanks for the report! Attached PR fixes this. Sorry I wasn't able to get the PR up and merged in time for 3.12.0.rc2; was traveling over the long (US) holiday weekend. I think this probably means the fix will need to wait until 3.12.1, unless @Yhg1s wants to merge it between rc2 and final .0. I think the fix is low-risk, but the bug is also an edge case that is unlikely to affect a lot of users, so I think either way is probably OK.
gh-108732: include comprehension locals in frame.f_locals (#109026)
pythongh-108732: include comprehension locals in frame.f_locals (pyth…
[3.12] gh-108732: include comprehension locals in frame.f_locals (GH-…
P403n1x87 commentedon Sep 20, 2023
Just stumbled upon this and I have noticed something similar with frame stacks. Not sure whether this is considered expected in 3.12
Essentially, frame stacks in 3.12 seem to be missing list (and probably other, not tested) comprehensions.
carljm commentedon Sep 20, 2023
@P403n1x87 Yes, that's expected and called out in PEP 709 as an expected impact. There are not language guarantees about what frames show up in Python frame stacks.