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Fix inference when unpacking union type #19650

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Fix inference when unpacking union type #19650

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f4c6548
Fix inference when unpacking union type
hauntsaninja Aug 12, 2025
7fd1d1a
[pre-commit.ci] auto fixes from pre-commit.com hooks
pre-commit-ci[bot] Aug 12, 2025
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13 changes: 13 additions & 0 deletions mypy/argmap.py
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Original file line number Diff line number Diff line change
Expand Up @@ -16,6 +16,7 @@
TypedDictType,
TypeOfAny,
TypeVarTupleType,
UnionType,
UnpackType,
get_proper_type,
)
Expand Down Expand Up @@ -211,6 +212,18 @@ def expand_actual_type(
# Just return `Any`, other parts of code would raise
# a different error for improper use.
return AnyType(TypeOfAny.from_error)
elif isinstance(actual_type, UnionType):
item_types = [
self.expand_actual_type(
item,
actual_kind=actual_kind,
formal_name=formal_name,
formal_kind=formal_kind,
allow_unpack=allow_unpack,
)
for item in actual_type.items
]
Comment on lines +217 to +225
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@randolf-scholz randolf-scholz Aug 13, 2025 •
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This doesn't work because the mapper has mutable state (self.tuple_index), so one would need to create multiple copies of the current mapper and apply them independently.

But then this will run into problems if the union is of differently sized tuple, since then you end up with incongruent state across the different copies.

Solving this problem generally seems intractable due to combinatorial explosion: consider f(*x1, *x2, ..., *xn). If each $x_k$ is comprised of a union of $m_k$ differently sized tuples, then there are $m_1⋅m_2⋅…⋅m_k$ possible paths.

So at best I think one can do this union splitting if the Union doesn't contain tuples, or if all contained tuples are of equal length.

return UnionType.make_union(item_types)
elif isinstance(actual_type, TupleType):
# Get the next tuple item of a tuple *arg.
if self.tuple_index >= len(actual_type.items):
Expand Down
17 changes: 17 additions & 0 deletions test-data/unit/check-functions.test
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Expand Up @@ -3708,3 +3708,20 @@ foo(*args) # E: Argument 1 to "foo" has incompatible type "*list[object]"; expe
kwargs: dict[str, object]
foo(**kwargs) # E: Argument 1 to "foo" has incompatible type "**dict[str, object]"; expected "P"
[builtins fixtures/dict.pyi]

[case testUnpackUnionStarArgs]
from __future__ import annotations
from typing import TypeVar
T = TypeVar("T")

def f(*args: T) -> T: ...

def star_union_list(x: list[str | None] | list[str]):
reveal_type([*x]) # N: Revealed type is "builtins.list[Union[builtins.str, None]]"
reveal_type(f(*x)) # N: Revealed type is "Union[builtins.str, None]"

def star_union_list_tuple(x: list[str | None] | tuple[int, int]):
reveal_type([*x]) # N: Revealed type is "builtins.list[Union[builtins.str, None, builtins.int]]"
reveal_type(f(*x)) # N: Revealed type is "Union[builtins.str, None, builtins.int]"

[builtins fixtures/tuple.pyi]
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