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Merged
hauntsaninja merged 2 commits into from
Apr 25, 2026
+82 −8
Merged

Narrow more sequence parents #21327

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bb5479c
Narrow more sequence parents
hauntsaninja Apr 25, 2026
2213e61
more test
hauntsaninja Apr 25, 2026
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25 changes: 17 additions & 8 deletions mypy/checker.py
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Original file line number Diff line number Diff line change
Expand Up @@ -7143,14 +7143,23 @@ def replay_lookup(new_parent_type: ProperType) -> Type | None:
if int_literals is not None:

def replay_lookup(new_parent_type: ProperType) -> Type | None:
if not isinstance(new_parent_type, TupleType):
return None
try:
assert int_literals is not None
member_types = [new_parent_type.items[key] for key in int_literals]
except IndexError:
return None
return make_simplified_union(member_types)
if isinstance(new_parent_type, TupleType):
try:
assert int_literals is not None
member_types = [
new_parent_type.items[key] for key in int_literals
]
except IndexError:
return None
return make_simplified_union(member_types)
if (
isinstance(new_parent_type, Instance)
and new_parent_type.type.fullname
in ("builtins.list", "builtins.tuple")
and new_parent_type.args
):
return new_parent_type.args[0]
return None

else:
return output
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65 changes: 65 additions & 0 deletions test-data/unit/check-isinstance.test
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Original file line number Diff line number Diff line change
Expand Up @@ -53,6 +53,71 @@ def f(x: Union[int, str, List]) -> None:
reveal_type(x) # N: Revealed type is "builtins.int | builtins.str | builtins.list[Any]"
[builtins fixtures/isinstancelist.pyi]

[case testIsinstanceNarrowsSequenceParentFromElement]
# flags: --warn-unreachable
from __future__ import annotations

def f1(x: list[str] | list[int]) -> None:
if isinstance(x[0], str):
reveal_type(x) # N: Revealed type is "builtins.list[builtins.str]"
reveal_type(x[0]) # N: Revealed type is "builtins.str"
else:
reveal_type(x) # N: Revealed type is "builtins.list[builtins.int]"
reveal_type(x[0]) # N: Revealed type is "builtins.int"

def f2(x: tuple[str, ...] | tuple[int, ...]) -> None:
if isinstance(x[0], str):
reveal_type(x) # N: Revealed type is "builtins.tuple[builtins.str, ...]"
reveal_type(x[0]) # N: Revealed type is "builtins.str"
else:
reveal_type(x) # N: Revealed type is "builtins.tuple[builtins.int, ...]"
reveal_type(x[0]) # N: Revealed type is "builtins.int"

def f3(x: tuple[str, str] | tuple[int, int]) -> None:
if isinstance(x[0], str):
reveal_type(x) # N: Revealed type is "tuple[builtins.str, builtins.str]"
else:
reveal_type(x) # N: Revealed type is "tuple[builtins.int, builtins.int]"

class A: ...
class B: ...
class C(A, B): ...

def f4(x: list[A] | list[B]):
# Technically this is unsound in the presence of multiple inheritance,
# but we already do this kind of narrowing for tuples
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@ilevkivskyi ilevkivskyi Apr 25, 2026

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What about a situation like this:

class Super: ...
class Sub(Super): ...

def foo(x: list[Sub] | list[Super]) -> None:
    if isinstance(x[0], Sub):
        ...

Will this cause a narrowing here? I would say we should not narrow. Same question about list[str] | list[str | None] with x[0] is not None.

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@hauntsaninja hauntsaninja Apr 25, 2026

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No it won't (from the pre-existing logic). I can add a test case for it though

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@ilevkivskyi ilevkivskyi Apr 25, 2026

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OK, thanks!

if isinstance(x[0], A):
reveal_type(x) # N: Revealed type is "builtins.list[__main__.A]"
else:
reveal_type(x) # N: Revealed type is "builtins.list[__main__.B]"

def f5(x: tuple[A, ...] | tuple[B, ...]):
if isinstance(x[0], A):
reveal_type(x) # N: Revealed type is "builtins.tuple[__main__.A, ...]"
else:
reveal_type(x) # N: Revealed type is "builtins.tuple[__main__.B, ...]"

def f6(x: tuple[A, A] | tuple[B, B]):
if isinstance(x[0], A):
reveal_type(x) # N: Revealed type is "tuple[__main__.A, __main__.A]"
else:
reveal_type(x) # N: Revealed type is "tuple[__main__.B, __main__.B]"

class AA(A): ...

def f7(x: list[A] | list[AA]):
if isinstance(x[0], AA):
reveal_type(x) # N: Revealed type is "builtins.list[__main__.A] | builtins.list[__main__.AA]"
else:
reveal_type(x) # N: Revealed type is "builtins.list[__main__.A] | builtins.list[__main__.AA]"

def f8(x: list[str] | list[str | None]):
if x[0] is not None:
reveal_type(x) # N: Revealed type is "builtins.list[builtins.str] | builtins.list[builtins.str | None]"
else:
reveal_type(x) # N: Revealed type is "builtins.list[builtins.str | None]"
[builtins fixtures/tuple.pyi]

[case testClassAttributeInitialization]
# flags: --warn-unreachable
class A:
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