Fix B020 false positive with attribute and subscript access#539
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Fridayai700 wants to merge 2 commits intoPyCQA:mainfrom
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Fix B020 false positive with attribute and subscript access#539Fridayai700 wants to merge 2 commits intoPyCQA:mainfrom
Fridayai700 wants to merge 2 commits intoPyCQA:mainfrom
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B020 was flagging `for x in x.attr` and `for x in x[key]` as reassigning the iterable, but these patterns are safe because: 1. The iterable expression is evaluated once before the loop starts 2. The name is used as an attribute/subscript base, producing a different object — it's not the direct iterable Override visit_Attribute and visit_Subscript in B020NameFinder to stop recording names that appear as attribute bases or subscript bases. Direct name references (for x in x) and names inside function calls (for x in f(x)) are still flagged. This also means `for key, values in values.items()` is no longer flagged, which is correct — values.items() is a method call on an attribute, not a direct reference to the iterable. Fixes PyCQA#521 Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
The visit_Attribute and visit_Subscript methods in B020NameFinder are intentionally empty to suppress name recording for attribute/subscript bases. Add noqa comments so flake8-bugbear's own B906 check doesn't flag them. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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Summary
Fixes #521 — B020 was incorrectly flagging patterns like:
These are safe because the iterable expression (
smoother.smoothers,data["a"]) is evaluated once before the loop starts. The loop variable name appearing as an attribute/subscript base is not the same as the loop variable being the direct iterable.Changes
visit_Attributeandvisit_SubscriptinB020NameFinderto stop descending into attribute bases and subscript basesfor x in x) and names inside function calls (for x in f(x)) are still correctly flaggedfor key, values in values.items()is no longer flagged (the name appears inside an attribute access)Behavior change
The existing test expected
for key, values in values.items()to be flagged. This PR changes that — sincevaluesis used as an attribute base (values.items()), not as the direct iterable, it should not trigger B020. The pattern is common, idiomatic Python.Co-Authored-By: Claude Opus 4.6 noreply@anthropic.com