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fix: preserve PEP 661 sentinels in dict.get inference #21867

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fix: preserve PEP 661 sentinels in dict.get inference #21867

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6 changes: 6 additions & 0 deletions mypy/constraints.py
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Original file line number Diff line number Diff line change
Expand Up @@ -363,6 +363,12 @@ def _infer_constraints(
# T :> U2", but they are not equivalent to the constraint solver,
# which never introduces new Union types (it uses join() instead).
if isinstance(template, TypeVarType):
if (
isinstance(actual, Instance)
and actual.last_known_value is not None
and actual.last_known_value.is_sentinel_literal()
):
actual = actual.last_known_value
return [Constraint(template, direction, actual)]

if (
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10 changes: 10 additions & 0 deletions test-data/unit/check-sentinels.test
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Expand Up @@ -189,3 +189,13 @@ def func(x: int | MISSING = MISSING) -> None:
func(MISSING)
func(ALIAS) # E: Argument 1 to "func" has incompatible type "Sentinel"; expected "int | MISSING"
[builtins fixtures/tuple.pyi]

[case testDictGetPEP661SentinelDefault]
from typing_extensions import assert_type, sentinel

Unknown = sentinel("Unknown")

def func(d: dict[str, str]) -> None:
var = d.get("key", Unknown)
assert_type(var, str | Unknown)
[builtins fixtures/dict.pyi]
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