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Add support for PEP 661 (sentinels) #21454

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Add support for PEP 661 (sentinels) #21454

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c641321
Add support for PEP 661 (sentinels)
JelleZijlstra May 10, 2026
4d00721
simplify
JelleZijlstra May 10, 2026
271a07c
[pre-commit.ci] auto fixes from pre-commit.com hooks
pre-commit-ci[bot] May 10, 2026
190b7c5
test isinstance
JelleZijlstra May 10, 2026
cfa09f4
Fix sentinel cache restore
JelleZijlstra May 11, 2026
835bb3b
maybe fix
JelleZijlstra May 11, 2026
a2caece
[pre-commit.ci] auto fixes from pre-commit.com hooks
pre-commit-ci[bot] May 11, 2026
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simplify
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@JelleZijlstra
JelleZijlstra committed May 10, 2026
commit 4d00721893f73aac635b20d76ef8920a218183f9
6 changes: 1 addition & 5 deletions mypy/semanal.py
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Expand Up @@ -3423,11 +3423,7 @@ def sentinel_type_for_var(self, var: Var, rvalue: Expression) -> Instance | None
assert isinstance(rvalue, CallExpr)
callee = rvalue.callee
assert isinstance(callee, RefExpr)
if callee.fullname == "typing_extensions.sentinel":
fallback_fullname = "typing_extensions.Sentinel"
else:
fallback_fullname = callee.fullname
typ = self.named_type_or_none(fallback_fullname)
typ = self.named_type_or_none(callee.fullname)
if typ is None:
return None
name = f"{self.type.name}.{var.name}" if self.type is not None else var.name
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2 changes: 2 additions & 0 deletions test-data/unit/check-sentinels.test
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Expand Up @@ -36,6 +36,8 @@ MISSING = Sentinel("MISSING")
SPECIAL = Sentinel("SPECIAL")

def func(x: int | MISSING | SPECIAL) -> None:
# We could reasonably do narrowing here, but currently we're pretty conservative
# about narrowing on ==.
if x == MISSING:
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assert_type(x, int | MISSING | SPECIAL)
else:
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49 changes: 5 additions & 44 deletions test-data/unit/fixtures/sentinel.pyi
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@@ -1,60 +1,21 @@
# Builtins stub used in sentinel-related test cases.

import _typeshed
import types
from typing import Iterable, Iterator, TypeVar, Generic, Sequence, Optional, overload, Tuple, Type, Self

_T = TypeVar("_T")
_Tco = TypeVar('_Tco', covariant=True)
from typing import Self

class object:
def __init__(self) -> None: pass
def __new__(cls) -> Self: ...

class type:
def __init__(self, *a: object) -> None: pass
def __call__(self, *a: object) -> object: pass
class tuple(Sequence[_Tco], Generic[_Tco]):
def __hash__(self) -> int: ...
def __new__(cls: Type[_T], iterable: Iterable[_Tco] = ...) -> _T: ...
def __iter__(self) -> Iterator[_Tco]: pass
def __contains__(self, item: object) -> bool: pass
@overload
def __getitem__(self, x: int) -> _Tco: pass
@overload
def __getitem__(self, x: slice) -> Tuple[_Tco, ...]: ...
def __mul__(self, n: int) -> Tuple[_Tco, ...]: pass
def __rmul__(self, n: int) -> Tuple[_Tco, ...]: pass
def __add__(self, x: Tuple[_Tco, ...]) -> Tuple[_Tco, ...]: pass
def count(self, obj: object) -> int: pass
class type: pass
class function:
__name__: str
class ellipsis: pass
class classmethod: pass

# We need int and slice for indexing tuples.
class int:
def __neg__(self) -> 'int': pass
def __pos__(self) -> 'int': pass
class float: pass
class slice: pass
class int: pass
class bool(int): pass
class str: pass # For convenience
class str: pass

class sentinel:
def __init__(self, name: str, /) -> None: ...
class bytes: pass
class bytearray: pass

class list(Sequence[_T], Generic[_T]):
@overload
def __getitem__(self, i: int) -> _T: ...
@overload
def __getitem__(self, s: slice) -> list[_T]: ...
def __contains__(self, item: object) -> bool: ...
def __iter__(self) -> Iterator[_T]: ...

def isinstance(x: object, t: type) -> bool: pass

class BaseException: pass

class dict: pass
2 changes: 1 addition & 1 deletion test-data/unit/lib-stub/typing_extensions.pyi
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Expand Up @@ -45,7 +45,7 @@ Never: _SpecialForm
class Sentinel:
def __init__(self, name: str, repr: str | None = None) -> None: ...
def __eq__(self, other: object) -> bool: ...
def sentinel(name: str, repr: str | None = None) -> Sentinel: ...
sentinel = Sentinel

TypeVarTuple: _SpecialForm
Unpack: _SpecialForm
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