PyCQA/flake8-bugbear

B023: false positive for nested function inside loop

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#468 opened on Apr 25, 2024

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Description

The following (admittedly bad) code produces what I think must be a false positive:

for _ in range(10):
   foo = []
   def bar():
      foo.append(42)

   bar()

test.py:4:7: B023 Function definition does not bind loop variable 'foo'.

At the very least the warning is confusing because foo is not actually a loop variable.

Using:

flake8 --version
7.0.0 (flake8-bugbear: 24.2.6, flake8-builtins: 2.5.0, mccabe: 0.7.0, pycodestyle: 2.11.1, pyflakes: 3.2.0) CPython 3.10.12 on Linux

Contributor guide

Research direction
Inspect the B023 check logic in flake8 bugbear to understand why 'foo' is incorrectly flagged as a loop variable. Focus on how the linter handles nested functions inside loops and local variables that are not loop iterators.
Tech stack
python
Domain
devtools
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
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Active
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Clear
Prerequisites
PythonGit
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
60