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fix(rewrite): freeze walrus and starred operands too - #14814

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fix(rewrite): freeze walrus and starred operands too#14814
RonnyPfannschmidt wants to merge 11 commits into
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RonnyPfannschmidt:ronny/fix-walrus-raw-operands

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@RonnyPfannschmidt RonnyPfannschmidt commented Jul 31, 2026

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Stacked on #14447, which is stacked on #14813. Its diff includes theirs.

visit_operand() (added in #14447) only froze a bare name, so two other unhoisted operands kept being evaluated after everything that follows them:

assert collect((x := 1), identity(x := 2)) == (1, 2)      # passes (1, 2) to plain Python, (2, 2) rewritten
assert collect(*items, identity(items := [9])) == (1, [9]) # passes (1, [9]) plain, (9, [9]) rewritten

A walrus operator left in place assigns only once the enclosing expression is assembled, which is after the later arguments have run — so the earlier argument saw the later assignment. A starred argument hid its value inside an ast.Starred, where the existing Name check could not reach it.

Closes the order-starred-argument group and the last order-call-argument entry in the coverage matrix from #14813: 16 xfails to 14.

The starred case was found by the matrix, not by hand — it is what the order-* axis is for.

@psf-chronographer psf-chronographer Bot added the bot:chronographer:provided (automation) changelog entry is part of PR label Jul 31, 2026
@RonnyPfannschmidt
RonnyPfannschmidt force-pushed the ronny/fix-walrus-raw-operands branch from 16b94a1 to cc6afa4 Compare July 31, 2026 19:04
This was referenced Jul 31, 2026
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RonnyPfannschmidt and others added 11 commits July 31, 2026 22:12
@RonnyPfannschmidt
test(rewrite): systematic coverage matrix for assertion rewriting
ff0f7c0 
The rewriter is tested through its failure messages, one regression test
per issue.  That makes it hard to say whether a change improved anything:
there is no place that records what the rewriter cannot do yet.

Add a matrix over expression types with four axes -- introspection depth,
semantic equivalence, single evaluation, and evaluation order -- and
record every known gap as a strict xfail tagged with a group name.  A
change that closes one names the group and flips its markers; strictness
means it cannot forget.

The evaluation-order axis is new.  Comparing pass/fail is not enough: an
operand read after a later walrus operator rebound its name runs exactly
once and can still fail the assertion, having compared the wrong value.
assert_evaluation_order() compares what check() returns, because the
fragile operand is a bare name -- wrapping it in a call to observe it
would hoist it and hide the bug.

Behaviour is unchanged; this only describes it.
@RonnyPfannschmidt Cursor AI @claude
fix(rewrite): prevent walrus operator double evaluation in assertions
c7b8908 
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>
@RonnyPfannschmidt Cursor AI @claude
Add changelog fragment for pytest-dev#14445
b2dfadc 
Co-authored-by: Cursor AI <ai@cursor.sh>
Co-authored-by: Anthropic Claude Sonnet 4 <claude@anthropic.com>
@RonnyPfannschmidt Cursor AI @claude
test(rewrite): add xfail tests for remaining walrus edge cases
400a848 
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>
@RonnyPfannschmidt Cursor AI @claude
fix(rewrite): avoid double evaluation of walrus in BoolOp condition
f997370 
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>
@RonnyPfannschmidt Cursor AI @claude
fix(rewrite): assign walrus comparators to temps in chained comparisons
487fe38 
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>
@RonnyPfannschmidt Cursor AI @claude
fix(rewrite): show correct walrus values in BoolOp explanations
1189dae 
When multiple walrus operators target the same variable in a BoolOp
(e.g., `assert (x := side_effect()) and (x := False)`), the assertion
explanation previously showed the final value of `x` for all operands
because the format context evaluated lazily after all operands ran.

Fix by tracking Name/NamedExpr operand values in stable @py_assert
variables (via self.assign) immediately after evaluation, then pointing
the explanation format context at the tracked copy. This uses the same
value-tracking mechanism already used by visit_Call, visit_Attribute, etc.

Fixes the case reported by @bluetech in PR review.

Co-authored-by: Cursor AI <ai@cursor.sh>
Co-authored-by: Anthropic Claude Sonnet 4 <claude@anthropic.com>
@RonnyPfannschmidt Cursor AI @claude
scripts: add assert-rewrite dump and diff tools
cc256d1 
Add scripts to inspect and compare assertion rewriting across pytest
versions, useful for tracking changes in the rewriter output.

- dump-assert-rewrite.py: dumps the rewritten form of a Python file
  using any pytest version (via uv ephemeral env) or the local worktree.
  Supports source, ast, and compact output formats.
- diff-assert-rewrite.py: compares rewrite output between two versions
  (or worktree) with colored unified diff, runs both sides in parallel.
- example_asserts.py: example file covering common assertion patterns
  including bluetech's walrus-in-BoolOp case from pytest-dev#14445.

Co-authored-by: Cursor AI <ai@cursor.sh>
Co-authored-by: Anthropic Claude Opus 4 <claude@anthropic.com>
@RonnyPfannschmidt Cursor AI @claude
refactor(rewrite): minimal snapshots in BoolOp for walrus conflicts
e6931af 
Replace the blanket snapshot-all-operands approach with a targeted one:
pre-scan the BoolOp to find walrus targets, then only snapshot operands
whose value a later walrus would corrupt.

Snapshot rules:
- NamedExpr (non-last): always, to avoid re-evaluating side effects
- Name with later walrus conflict: to freeze the pre-overwrite value
- Everything else: use res directly (stable @py_assert or plain name)

Non-walrus BoolOps now generate identical code to 8.3.5 (no snapshots).

Co-authored-by: Cursor AI <ai@cursor.sh>
Co-authored-by: Anthropic Claude Opus 4 <claude@anthropic.com>
@RonnyPfannschmidt
fix(rewrite): freeze operands a later walrus would clobber
6caf74a 
The rewriter hoists each operand into its own statement, but a plain
name is left as a bare load evaluated when the enclosing expression is
assembled -- after the statements of the operands that follow it.  A
walrus operator in a later operand rebinds the name in between, so both
the value used and the value reported were the post-walrus one, while
Python evaluates the earlier operand first:

    assert value != identity(value := value.lower())

visit_BoolOp already guarded against this; extract its pre-scan as
_walrus_targets() and add visit_operand() to apply the same freeze in
visit_Compare, visit_Call and visit_BinOp.  visit_Compare previously
matched only a comparator that *was* a NamedExpr, missing walrus
operators nested inside it; visit_Call did not guard at all, so an
earlier argument saw a later argument's assignment.

These cases predate the walrus rework -- they fail on main too.

Closes the single-eval-walrus, order-compare-left, order-call-argument
and order-binop-left groups in the coverage matrix.  order-call-argument
keeps one entry: a bare walrus argument is still substituted into a
later one, which visit_operand does not yet see because the operand is a
NamedExpr rather than a Name.

Reported-by: Denis Scapin
@RonnyPfannschmidt
fix(rewrite): freeze walrus and starred operands too
d271ad5 
visit_operand() only froze a bare name, so two other unhoisted operands
kept being evaluated after everything that follows them:

    assert collect((x := 1), identity(x := 2)) == (1, 2)
    assert collect(*items, identity(items := [9])) == (1, [9])

A walrus operator left in place assigns once the enclosing expression is
assembled, which is after the later arguments have run -- so the earlier
argument saw the later assignment.  A starred argument hid its value
inside an ast.Starred, where the existing Name check could not see it.

Closes the order-starred-argument group and the remaining
order-call-argument entry in the coverage matrix.
@RonnyPfannschmidt
RonnyPfannschmidt force-pushed the ronny/fix-walrus-raw-operands branch from cc6afa4 to d271ad5 Compare July 31, 2026 20:18
This was referenced Jul 31, 2026
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