fix(rewrite): prevent walrus operator double evaluation in assertions#14447
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RonnyPfannschmidt wants to merge 5 commits intopytest-dev:mainfrom
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fix(rewrite): prevent walrus operator double evaluation in assertions#14447RonnyPfannschmidt wants to merge 5 commits intopytest-dev:mainfrom
RonnyPfannschmidt wants to merge 5 commits intopytest-dev:mainfrom
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Fixes pytest-dev#14445 - assertion rewriting evaluated NamedExpr (:=) expressions multiple times, causing side effects to fire repeatedly. The root cause was the `variables_overwrite` mechanism which stored and re-evaluated NamedExpr AST nodes in subsequent assertions, in `_call_reprcompare`'s results tuple, and in explanation formatting. The fix: - visit_NamedExpr: reference the target variable in explanations instead of re-evaluating the full expression - visit_Compare: assign left-side NamedExpr to a temp before right-side hoisting; freeze left_res when a comparator walrus targets the same name; replace NamedExpr entries in `results` with target variables - visit_BoolOp: capture short-circuit condition in a stable temp for the explanation path; remove walrus target rename logic - visit_Call: remove variables_overwrite substitution (walrus now properly assigns to user variables in its natural evaluation position) - Remove variables_overwrite, scope tracking, Sentinel class Co-authored-by: Cursor AI <ai@cursor.sh> Co-authored-by: Anthropic Claude Sonnet 4 <claude@anthropic.com>
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Pull request overview
Fixes assertion rewriting so walrus (:=) expressions are not evaluated multiple times, preventing side effects from running twice and producing incorrect rewritten-assert behavior (per #14445).
Changes:
- Removes the prior
variables_overwrite/scope-tracking mechanism and adjustsNamedExprhandling to avoid re-evaluation in explanations. - Updates BoolOp/Compare rewriting to stabilize conditions/operands for explanation formatting.
- Adds new regression tests for walrus side-effect/double-evaluation cases and updates existing expected assertion output.
Reviewed changes
Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.
| File | Description |
|---|---|
src/_pytest/assertion/rewrite.py |
Refactors assertion-rewrite AST generation around NamedExpr, BoolOp, and Compare to avoid walrus re-evaluation. |
testing/test_assertrewrite.py |
Updates expected assertion output and adds regression tests for #14445 scenarios. |
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Co-authored-by: Cursor AI <ai@cursor.sh> Co-authored-by: Anthropic Claude Sonnet 4 <claude@anthropic.com>
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Add tests for two remaining walrus double-evaluation scenarios: - Bare NamedExpr as BoolOp operand evaluated twice via condition check - Same walrus target in chained comparison evaluated multiple times Co-authored-by: Cursor AI <ai@cursor.sh> Co-authored-by: Anthropic Claude Sonnet 4 <claude@anthropic.com>
Use the already-assigned res_var to build the short-circuit condition instead of the raw visitor result, preventing bare NamedExpr operands from being evaluated a second time when checking truthiness. Co-authored-by: Cursor AI <ai@cursor.sh> Co-authored-by: Anthropic Claude Sonnet 4 <claude@anthropic.com>
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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Summary
Fixes #14445 — assertion rewriting evaluated walrus operator (
:=) expressions multiple times, causing incorrect test results when the expression had side effects.Root cause: The
variables_overwritemechanism storedNamedExprAST nodes and re-evaluated them in subsequent assertions, in_call_reprcompare's results tuple, and in explanation formatting.Fix: Remove
variables_overwriteentirely and instead:assign()when a comparator walrus targets the same nameTest plan
test_walrus_in_assertion_basicandtest_walrus_running_counter)test_assertrewrite.pysuite passes (118 tests; only pre-existing subprocess env failures excluded)TestIssue14445Made with Cursor