used-before-assignment false positive with list comprehension and try/except

[The-Compiler]

Bug description

Somewhat similarly to #5965,

def github_upload(filename, all_assets):
    assets = [asset for asset in all_assets if asset.name == filename]

    try:
        pass
    except ValueError:
        asset = assets[0]
        asset.delete()

leads to an used-before-assignment false-positive for the second line, yet asset is always defined there.

I bisected this to bd55b27, part of #5402 ("Fix #4761: Emit used-before-assignment where single assignment only made in except blocks")

Configuration

No response

Command used

pylint -E repro1.py

Pylint output

************* Module repro1
repro1.py:2:47: E0601: Using variable 'asset' before assignment (used-before-assignment)

Expected behavior

No error.

Pylint version

pylint 2.13.2
astroid 2.11.2
Python 3.8.12 (default, Mar  3 2022, 18:19:28) 
[GCC 11.2.0]

(environment in which I originally encountered the bug - output above is from a git install of both astroid and pylint)

Also happens with latest git main (astroid b18c7828, pylint 0bc45e9). Does not happen on pylint 2.12.2 and astroid 2.9.3 which I used previously.

OS / Environment

Archlinux

Additional dependencies

No response

[jacobtylerwalls]

Thanks for the well-written report! Extremely similar to #5586 and #5817, but still missed it before the release.

[SteveHespelt]

in the spirit of possibly being helpful:

somewhat similar list comprehension statement, but not in a try-except block

if isinstance(d, dict):
    dict_keys =  [c for c in d if isinstance(d[c], dict)]

E:191,59: Using variable 'c' before assignment (used-before-assignment)

pylint 2.13.3, pytest-pylint 0.18.0, python 3.8.13, pytest-7.1.1

[jacobtylerwalls]

I can't reproduce with that example:

$ cat a.py
if isinstance(d, dict):
    dict_keys =  [c for c in d if isinstance(d[c], dict)]


$ python3 -m pylint a.py
************* Module a
a.py:1:14: E0602: Undefined variable 'd' (undefined-variable)
a.py:2:29: E0602: Undefined variable 'd' (undefined-variable)
a.py:2:45: E0602: Undefined variable 'd' (undefined-variable)

[SteveHespelt]

my apologies @jacobtylerwalls - my example wasn't meant to be complete, just the minimal snippet showing the statement.
Extremely interesting: the error was reported in a CI pipeline using Python 3.8.13, the other versions I reported earlier.
Not using pytest-pylint, I get the following.
Not being a Python expert/guru, I'm concerned over the difference between the full codebase scan via pytest-pylint and pylint standalone on this snippet. I need to do some more digging.

My immediate take-away: any pytest-pylint reported errors, rerun with standalone pylint to see if any differences.
Interesting...

/tmp/a.py:

d = { 'a': {}, 'b':1 }
dict_keys = []
if isinstance(d, dict):
    dict_keys =  [c for c in d if isinstance(d[c], dict)]
print("dict_keys: " + str(dict_keys))

python3.8 /tmp/a.py

dict_keys: ['a']

python3.7 -m pylint /tmp/a.py

************* Module a
/tmp/a.py:1:0: C0114: Missing module docstring (missing-module-docstring)
/tmp/a.py:7:0: C0206: Consider iterating with .items() (consider-using-dict-items)

With python3.8:

python3.8 -m pylint --version

pylint 2.13.3
astroid 2.11.2
Python 3.8.13 (default, Mar 29 2022, 19:52:48)
[GCC 8.3.0]

root@abb086d6fbf6:/usr/src/app# python3.8 -m pylint /tmp/a.py

************* Module a
/tmp/a.py:1:0: C0114: Missing module docstring (missing-module-docstring)
/tmp/a.py:4:0: C0206: Consider iterating with .items() (consider-using-dict-items)

[jacobtylerwalls]

Thanks, I misunderstood -- thought you were saying you found a case reproducible without a try/except block. I think it's worth adding this case to the tests.

[skshetry]

I can still reproduce this in 2.13.4:

# test.py
def select_dict(data, keys):
    try:
        d = {key: data[key] for key in keys}
    except KeyError as e:
        key, *_ = e.args
        raise Exception(f"{key} not found")

    return d

$ pip install -U pylint==2.13.4 -q && pylint test.py
************* Module test
test.py:3:23: E0601: Using variable 'key' before assignment (used-before-assignment)

------------------------------------------------------------------
Your code has been rated at 2.86/10 (previous run: 2.86/10, +0.00)

[jacobtylerwalls]

Thanks for the report: this one does not involve an if test in the comprehension, so I'm going to open a new issue for it. I have a patch ready.