Python: Fix pattern matching false positives and add template auto-parenthesization#6814
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knutwannheden merged 1 commit intomainfrom Feb 25, 2026
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Two fixes in the Python template system:
1. PatternMatchingComparator: Change default fallthrough from `return True`
to `return False` so unhandled node types reject matches instead of
silently accepting. Add explicit handlers for Empty (sentinel) and
ArrayAccess nodes.
2. PlaceholderReplacementVisitor: Auto-wrap substituted expressions in
parentheses when they have lower operator precedence than the
surrounding context. For example, `template("{a} and {b}")` with
`b = x or y` now correctly produces `a and (x or y)` instead of
`a and x or y`. Handles Binary, Python Binary, and Unary (not).
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…renthesization (#6814) Two fixes in the Python template system: 1. PatternMatchingComparator: Change default fallthrough from `return True` to `return False` so unhandled node types reject matches instead of silently accepting. Add explicit handlers for Empty (sentinel) and ArrayAccess nodes. 2. PlaceholderReplacementVisitor: Auto-wrap substituted expressions in parentheses when they have lower operator precedence than the surrounding context. For example, `template("{a} and {b}")` with `b = x or y` now correctly produces `a and (x or y)` instead of `a and x or y`. Handles Binary, Python Binary, and Unary (not).
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Summary
Two fixes in the Python template system discovered while running cleanup recipes across 11 open-source Python projects:
PatternMatchingComparator: The default fallthrough for unhandled node types returned
True, causing false positive matches. For example,pattern("{x} == None")would matchx == b"hello"becauseArrayAccessnodes weren't handled and any two nodes of an unrecognized type silently matched. Changed toreturn Falseand added explicit handlers forEmpty(sentinel) andArrayAccessnodes.PlaceholderReplacementVisitor: Template substitution didn't account for operator precedence, producing semantically incorrect output. For example,
template("{a} and {b}")withbcaptured asx or ywould producea and x or yinstead ofa and (x or y). Added auto-parenthesization invisit_binary,visit_python_binary, andvisit_unarythat wraps substituted expressions when they have lower precedence than the surrounding context.Test plan
test_comparator.pyandtest_replacement.pycovering:orinand,andinand(no parens), arithmetic precedence, Pythoninoperator,notwithand/oroperands,notwith identifiers/comparisons (no parens)