fix(rewrite): prevent walrus operator double evaluation in assertions#14447
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RonnyPfannschmidt wants to merge 5 commits into
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fix(rewrite): prevent walrus operator double evaluation in assertions#14447RonnyPfannschmidt wants to merge 5 commits into
RonnyPfannschmidt wants to merge 5 commits into
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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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Pierre-Sassoulas
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May 23, 2026
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LGTM, but we probably want another person to look at it
bluetech
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Jun 2, 2026
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I haven't reviewed the code yet, before that I dumped the rewritten AST before/after this PR, on the following example:
def side_effect():
return True
def test_walrus_boolop():
assert (x := side_effect())Before
Module(
body=[
Import(
names=[
alias(name='builtins', asname='@py_builtins')]),
Import(
names=[
alias(name='_pytest.assertion.rewrite', asname='@pytest_ar')]),
FunctionDef(
name='side_effect',
args=arguments(),
body=[
Return(
value=Constant(value=True))]),
FunctionDef(
name='test_walrus_boolop',
args=arguments(),
body=[
If(
test=UnaryOp(
op=Not(),
operand=NamedExpr(
target=Name(id='x', ctx=Store()),
value=Call(
func=Name(id='side_effect', ctx=Load())))),
body=[
Assign(
targets=[
Name(id='@py_format1', ctx=Store())],
value=BinOp(
left=BinOp(
left=Constant(value=''),
op=Add(),
right=Constant(value='assert %(py0)s')),
op=Mod(),
right=Dict(
keys=[
Constant(value='py0')],
values=[
IfExp(
test=BoolOp(
op=Or(),
values=[
Compare(
left=Constant(value='x'),
ops=[
In()],
comparators=[
Call(
func=Attribute(
value=Name(id='@py_builtins', ctx=Load()),
attr='locals',
ctx=Load()))]),
Call(
func=Attribute(
value=Name(id='@pytest_ar', ctx=Load()),
attr='_should_repr_global_name',
ctx=Load()),
args=[
NamedExpr(
target=Name(id='x', ctx=Store()),
value=Call(
func=Name(id='side_effect', ctx=Load())))])]),
body=Call(
func=Attribute(
value=Name(id='@pytest_ar', ctx=Load()),
attr='_saferepr',
ctx=Load()),
args=[
NamedExpr(
target=Name(id='x', ctx=Store()),
value=Call(
func=Name(id='side_effect', ctx=Load())))]),
orelse=Constant(value='x'))]))),
Raise(
exc=Call(
func=Name(id='AssertionError', ctx=Load()),
args=[
Call(
func=Attribute(
value=Name(id='@pytest_ar', ctx=Load()),
attr='_format_explanation',
ctx=Load()),
args=[
Name(id='@py_format1', ctx=Load())])]))])])])After
Module(
body=[
Import(
names=[
alias(name='builtins', asname='@py_builtins')]),
Import(
names=[
alias(name='_pytest.assertion.rewrite', asname='@pytest_ar')]),
FunctionDef(
name='side_effect',
args=arguments(),
body=[
Return(
value=Constant(value=True))]),
FunctionDef(
name='test_walrus_boolop',
args=arguments(),
body=[
If(
test=UnaryOp(
op=Not(),
operand=NamedExpr(
target=Name(id='x', ctx=Store()),
value=Call(
func=Name(id='side_effect', ctx=Load())))),
body=[
Assign(
targets=[
Name(id='@py_format1', ctx=Store())],
value=BinOp(
left=BinOp(
left=Constant(value=''),
op=Add(),
right=Constant(value='assert %(py0)s')),
op=Mod(),
right=Dict(
keys=[
Constant(value='py0')],
values=[
IfExp(
test=BoolOp(
op=Or(),
values=[
Compare(
left=Constant(value='x'),
ops=[
In()],
comparators=[
Call(
func=Attribute(
value=Name(id='@py_builtins', ctx=Load()),
attr='locals',
ctx=Load()))]),
Call(
func=Attribute(
value=Name(id='@pytest_ar', ctx=Load()),
attr='_should_repr_global_name',
ctx=Load()),
args=[
Name(id='x', ctx=Load())])]),
body=Call(
func=Attribute(
value=Name(id='@pytest_ar', ctx=Load()),
attr='_saferepr',
ctx=Load()),
args=[
Name(id='x', ctx=Load())]),
orelse=Constant(value='x'))]))),
Raise(
exc=Call(
func=Name(id='AssertionError', ctx=Load()),
args=[
Call(
func=Attribute(
value=Name(id='@pytest_ar', ctx=Load()),
attr='_format_explanation',
ctx=Load()),
args=[
Name(id='@py_format1', ctx=Load())])]))])])])The diff is:
@@ -57,20 +57,14 @@
attr='_should_repr_global_name',
ctx=Load()),
args=[
- NamedExpr(
- target=Name(id='x', ctx=Store()),
- value=Call(
- func=Name(id='side_effect', ctx=Load())))])]),
+ Name(id='x', ctx=Load())])]),
body=Call(
func=Attribute(
value=Name(id='@pytest_ar', ctx=Load()),
attr='_saferepr',
ctx=Load()),
args=[
- NamedExpr(
- target=Name(id='x', ctx=Store()),
- value=Call(
- func=Name(id='side_effect', ctx=Load())))]),
+ Name(id='x', ctx=Load())]),
orelse=Constant(value='x'))]))),
Raise(
exc=Call(This looks good for the issue, since now we no longer run side_effect twice three times.
However if I tweak in this way:
def side_effect():
return True
def test_walrus_boolop():
assert (x := side_effect()) and (x := False)the assertion is
x.py:5: in test_walrus_boolop
assert (x := side_effect()) and (x := False)
E assert (False and False)
which is incorrect (should be assert (True and False)). That said, this also happens in main.
Let me know if you want to tackle this problem in this PR as well, in which I'll wait before reviewing, or if I should open a separate issue for that and review this PR as is.
Member
Author
|
good find, i'll address it in here |
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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