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feat(rewrite): address assertion rewriting blind spots with systematic tests #14448

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feat(rewrite): address assertion rewriting blind spots with systematic tests #14448

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fd53440
fix(rewrite): prevent walrus operator double evaluation in assertions
RonnyPfannschmidt May 8, 2026
9f371c7
Add changelog fragment for #14445
RonnyPfannschmidt May 8, 2026
df0e0a9
test(rewrite): add xfail tests for remaining walrus edge cases
RonnyPfannschmidt May 8, 2026
60f8e24
fix(rewrite): avoid double evaluation of walrus in BoolOp condition
RonnyPfannschmidt May 8, 2026
ffdc372
fix(rewrite): assign walrus comparators to temps in chained comparisons
RonnyPfannschmidt May 8, 2026
806b27e
test(rewrite): add systematic assertion rewriting test infrastructure
RonnyPfannschmidt May 8, 2026
04fcadb
test(rewrite): add introspection matrix covering all expression types
RonnyPfannschmidt May 8, 2026
0b8a681
test(rewrite): add single-evaluation test suite for all expression types
RonnyPfannschmidt May 8, 2026
3b2d44b
feat(rewrite): add visit_Subscript for container[key] introspection
RonnyPfannschmidt May 8, 2026
79d59da
feat(rewrite): add visit_IfExp for ternary expression introspection
RonnyPfannschmidt May 8, 2026
6369f54
feat(rewrite): flatten method call display to remove bound-method noise
RonnyPfannschmidt May 8, 2026
5531662
test(rewrite): add edge case and regression tests for new visitors
RonnyPfannschmidt May 8, 2026
dc7af37
fix(rewrite): update test_assert_matches for flat method call format
RonnyPfannschmidt May 8, 2026
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fix(rewrite): assign walrus comparators to temps in chained comparisons 
In a chained comparison like `(x := f()) < (x := g()) < (x := h())`,
each NamedExpr comparator is now assigned to a temp variable so it
evaluates exactly once. Previously the raw NamedExpr node would be
reused as left_res in the next iteration, causing double evaluation.

Co-authored-by: Cursor AI <ai@cursor.sh>
Co-authored-by: Anthropic Claude Sonnet 4 <claude@anthropic.com>
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@RonnyPfannschmidt @claude
3 people committed May 8, 2026
commit ffdc372a3613faed3af8f6d9e86c030223d945ed
11 changes: 5 additions & 6 deletions src/_pytest/assertion/rewrite.py
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Expand Up @@ -1095,6 +1095,11 @@ def visit_Compare(self, comp: ast.Compare) -> tuple[ast.expr, str]:
next_res, next_expl = self.visit(next_operand)
if isinstance(next_operand, ast.Compare | ast.BoolOp):
next_expl = f"({next_expl})"
# Assign NamedExpr comparators to a temp so each walrus evaluates
# exactly once — critical for chained comparisons where the same
# node would otherwise be re-evaluated as left_res next iteration.
if isinstance(next_res, ast.NamedExpr):
next_res = self.assign(next_res)
results.append(next_res)
sym = BINOP_MAP[op.__class__]
syms.append(ast.Constant(sym))
Expand All @@ -1103,12 +1108,6 @@ def visit_Compare(self, comp: ast.Compare) -> tuple[ast.expr, str]:
res_expr = ast.copy_location(ast.Compare(left_res, [op], [next_res]), comp)
self.statements.append(ast.Assign([store_names[i]], res_expr))
left_res, left_expl = next_res, next_expl
# Replace NamedExpr entries in results with their target variable
# to avoid re-evaluating walrus operators in the explanation path.
results = [
ast.Name(r.target.id, ast.Load()) if isinstance(r, ast.NamedExpr) else r
for r in results
]
# Use pytest.assertion.util._reprcompare if that's available.
expl_call = self.helper(
"_call_reprcompare",
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1 change: 0 additions & 1 deletion testing/test_assertrewrite.py
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Expand Up @@ -1928,7 +1928,6 @@ def test_walrus_boolop():
result = pytester.runpytest()
assert result.ret == 0

@pytest.mark.xfail(reason="Chained compare re-evaluates walrus with same target")
def test_walrus_no_double_eval_chained_compare(self, pytester: Pytester) -> None:
"""Same walrus target in chained comparison must evaluate each once."""
pytester.makepyfile(
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