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fix: Narrowing with "tags" on unions (of TypedDicts or normal classes) doesn't work with the match statement #18791

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Merged
hauntsaninja merged 2 commits into from
Mar 14, 2025
Merged

fix: Narrowing with "tags" on unions (of TypedDicts or normal classes) doesn't work with the match statement #18791

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a9c40b8
fix: Narrowing with "tags" on unions (of TypedDicts or normal classes…
codex-maintainers Mar 12, 2025
0a6333a
Address review: drop changelog entry, remove comment
codex-maintainers Mar 14, 2025
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2 changes: 2 additions & 0 deletions mypy/checker.py
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Original file line number Diff line number Diff line change
Expand Up @@ -5527,6 +5527,8 @@ def visit_match_stmt(self, s: MatchStmt) -> None:
pattern_map, else_map = conditional_types_to_typemaps(
named_subject, pattern_type.type, pattern_type.rest_type
)
pattern_map = self.propagate_up_typemap_info(pattern_map)
else_map = self.propagate_up_typemap_info(else_map)
self.remove_capture_conflicts(pattern_type.captures, inferred_types)
self.push_type_map(pattern_map, from_assignment=False)
if pattern_map:
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48 changes: 48 additions & 0 deletions test-data/unit/check-python310.test
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Expand Up @@ -332,6 +332,54 @@ match [SubClass("a"), SubClass("b")]:
reveal_type(rest) # N: Revealed type is "builtins.list[__main__.Example]"
[builtins fixtures/tuple.pyi]

# Narrowing union-based values via a literal pattern on an indexed/attribute subject
# -------------------------------------------------------------------------------
# Literal patterns against a union of types can be used to narrow the subject
# itself, not just the expression being matched. Previously, the patterns below
# failed to narrow the `d` variable, leading to errors for missing members; we
# now propagate the type information up to the parent.

[case testMatchNarrowingUnionTypedDictViaIndex]
from typing import Literal, TypedDict

class A(TypedDict):
tag: Literal["a"]
name: str

class B(TypedDict):
tag: Literal["b"]
num: int

d: A | B
match d["tag"]:
case "a":
reveal_type(d) # N: Revealed type is "TypedDict('__main__.A', {'tag': Literal['a'], 'name': builtins.str})"
reveal_type(d["name"]) # N: Revealed type is "builtins.str"
case "b":
reveal_type(d) # N: Revealed type is "TypedDict('__main__.B', {'tag': Literal['b'], 'num': builtins.int})"
reveal_type(d["num"]) # N: Revealed type is "builtins.int"
[typing fixtures/typing-typeddict.pyi]

[case testMatchNarrowingUnionClassViaAttribute]
from typing import Literal

class A:
tag: Literal["a"]
name: str

class B:
tag: Literal["b"]
num: int

d: A | B
match d.tag:
case "a":
reveal_type(d) # N: Revealed type is "__main__.A"
reveal_type(d.name) # N: Revealed type is "builtins.str"
case "b":
reveal_type(d) # N: Revealed type is "__main__.B"
reveal_type(d.num) # N: Revealed type is "builtins.int"

[case testMatchSequenceUnion-skip]
from typing import List, Union
m: Union[List[List[str]], str]
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