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Narrow context fallback limitation to walrus only #21294

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Merged
ilevkivskyi merged 1 commit into from
Apr 22, 2026
+23 −2
Merged

Narrow context fallback limitation to walrus only #21294

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9 changes: 7 additions & 2 deletions mypy/checker.py
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Original file line number Diff line number Diff line change
Expand Up @@ -547,6 +547,11 @@ def __init__(
self._unique_id = 0
self._variance_dummy_type = None

# Assignment expression is the only *expression* that can have externally
# visible effects via binder. We bump this whenever this happens to handle
# situations when some expressions are accepted multiple times.
self.assignment_expression_effect = 0

@property
def expr_checker(self) -> mypy.checkexpr.ExpressionChecker:
return self._expr_checker
Expand Down Expand Up @@ -4772,7 +4777,7 @@ def infer_rvalue_with_fallback_context(
# context that is ultimately used. This is however tricky with redefinitions.
# For now we simply disable second accept in cases known to cause problems,
# see e.g. testAssignToOptionalTupleWalrus.
binder_version = self.binder.version
assignment_expression_effect = self.assignment_expression_effect

fallback_context_used = False
with (
Expand Down Expand Up @@ -4802,7 +4807,7 @@ def infer_rvalue_with_fallback_context(
union_fallback = (
preferred_context is not None
and isinstance(get_proper_type(lvalue_type), UnionType)
and binder_version == self.binder.version
and assignment_expression_effect == self.assignment_expression_effect
)

# Skip literal types, as they have special logic (for better errors).
Expand Down
3 changes: 3 additions & 0 deletions mypy/checkexpr.py
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Expand Up @@ -4461,7 +4461,10 @@ def check_list_multiply(self, e: OpExpr) -> Type:

def visit_assignment_expr(self, e: AssignmentExpr) -> Type:
value = self.accept(e.value)
binder_version = self.chk.binder.version
self.chk.check_assignment(e.target, e.value)
if self.chk.binder.version != binder_version:
self.chk.assignment_expression_effect += 1
self.chk.check_final(e)
if not has_uninhabited_component(value):
# TODO: can we get rid of this extra store_type()?
Expand Down
13 changes: 13 additions & 0 deletions test-data/unit/check-inference-context.test
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Expand Up @@ -1616,3 +1616,16 @@ async def outer(c: Cls[T]) -> Optional[T]:
return await inner(c)
[builtins fixtures/async_await.pyi]
[typing fixtures/typing-async.pyi]

[case testContextFallbackNarrowingTernaryRValue]
from typing import Iterable, Union

def foo(args: Union[Iterable[Union[str, int]], str, int]) -> Iterable[str]:
if isinstance(args, (str, int)):
args = (args,)
args = (
arg if isinstance(arg, str) else str()
for arg in args
)
return args
[builtins fixtures/isinstancelist.pyi]
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