fix B031 false positive with if-else branches#557
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Only one branch of an if-else executes at runtime, so using the groupby variable in both branches shouldn't trigger B031. Previously, ast.walk() visited both branches and counted the variable twice, causing a false positive.
cooperlees
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Jun 15, 2026
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Hi - many thanks for the potential bug fix. Can you add some example code into B031's tests to show it now working and the code so I can try understand the bug more please.
Also, please make sure pre-commit passes and let's see if copilot finds anything.
Thanks again.
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Pull request overview
This PR updates the B031 check (check_for_b031) to avoid flagging itertools.groupby() iterators as “used multiple times” when the iterator is referenced in both branches of an if/else, since only one branch executes at runtime.
Changes:
- Added parent-tracking within the loop subtree to enable upward traversal from
ast.Namenodes. - Introduced helper logic to detect
if/elseconstructs where both branches reference the group iterator, and attempted to suppress B031 counting in that scenario. - Adjusted B031 usage counting to skip
group_nameoccurrences detected as part of such anif/else.
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| body_contains = any( | ||
| isinstance(n, ast.Name) and n.id == name | ||
| for n in ast.walk(if_node.body[0]) | ||
| ) | ||
| else_contains = ( | ||
| bool(if_node.orelse) | ||
| and any( | ||
| isinstance(n, ast.Name) and n.id == name | ||
| for n in ast.walk(if_node.orelse[0]) | ||
| ) | ||
| ) | ||
| return body_contains and else_contains |
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| # Skip if this name is in an if-else where the other | ||
| # branch also uses it (only one branch executes) | ||
| if is_in_if_branch_where_other_branch_has( | ||
| node, group_name | ||
| ): |
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| # Build parent map for this subtree so we can walk up | ||
| parent_map: dict[ast.AST, ast.AST] = {} | ||
|
|
||
| class ParentTracker(ast.NodeVisitor): | ||
| def __init__(self) -> None: | ||
| super().__init__() |
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when the groupby variable is used inside an if-else, only one branch executes at runtime, so using it in both branches shouldn't trigger B031.