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Fix isinstance with unions of tuples #20677

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hauntsaninja merged 1 commit into from
Jan 29, 2026
+26 −2
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Fix isinstance with unions of tuples #20677

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5 changes: 3 additions & 2 deletions mypy/checker.py
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Original file line number Diff line number Diff line change
Expand Up @@ -8643,12 +8643,13 @@ def flatten(t: Expression) -> list[Expression]:
def flatten_types(t: Type) -> list[Type]:
"""Flatten a nested sequence of tuples into one list of nodes."""
t = get_proper_type(t)
if isinstance(t, UnionType):
return [b for a in t.items for b in flatten_types(a)]
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@jelle-openai jelle-openai Jan 29, 2026

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hm this seems wrong? Now if you have t: tuple[type[int]] | tuple[type[str]] or something and you do if isinstance(x, t):, won't we incorrectly narrow t in the else branch?

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@hauntsaninja hauntsaninja Jan 29, 2026 •
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Thanks! We won't, but it is possible we might do the incorrect narrowing you suggest for final types in the future. I will add some more test cases to #20675.

The right fix is probably elsewhere, in conditional_types. Over here we are returning a list of types, not a union, and we should distribute over proposed type ranges instead of union-ing here:

mypy/mypy/checker.py

Lines 8380 to 8388 in 8bf049f

proposed_precise_type = UnionType.make_union(
[type_range.item for type_range in proposed_type_ranges if not type_range.is_upper_bound]
)
remaining_type = restrict_subtype_away(
current_type,
proposed_precise_type,
consider_runtime_isinstance=consider_runtime_isinstance,
)
return proposed_type, remaining_type

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@hauntsaninja hauntsaninja Jan 29, 2026

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Tests added here: https://github.com/python/mypy/pull/20675/files#diff-e3de7a75a8a107b4f462b164cdf4945d50505c5e9f7092b753c4add0c01530bbR3006

if isinstance(t, TupleType):
return [b for a in t.items for b in flatten_types(a)]
elif is_named_instance(t, "builtins.tuple"):
return [t.args[0]]
else:
return [t]
return [t]


def expand_func(defn: FuncItem, map: dict[TypeVarId, Type]) -> FuncItem:
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23 changes: 23 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 @@ -2938,3 +2938,26 @@ def foo(x: object, t: type[Any]):
if isinstance(x, t):
reveal_type(x) # N: Revealed type is "Any"
[builtins fixtures/isinstance.pyi]

[case testIsInstanceUnionOfTuples]
# flags: --strict-equality --warn-unreachable
from __future__ import annotations
from typing import TypeVar, Iterator

T1 = TypeVar("T1")
T2 = TypeVar("T2")
T3 = TypeVar("T3")

def extract(
values: object,
ts: (
tuple[type[T1]]
| tuple[type[T1], type[T2]]
| tuple[type[T1], type[T2], type[T3]]
)
) -> Iterator[T1 | T2 | T3]:
if isinstance(values, ts):
reveal_type(values) # N: Revealed type is "T1`-1 | T2`-2 | T3`-3"
yield values
raise
[builtins fixtures/primitives.pyi]