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Better match narrowing for irrefutable mapping patterns #20906

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Merged
hauntsaninja merged 2 commits into from
Mar 4, 2026
Merged

Better match narrowing for irrefutable mapping patterns #20906

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6e84a67
Better match narrowing for irrefutable mapping patterns
hauntsaninja Feb 25, 2026
b3706b4
Special case Any
hauntsaninja Feb 26, 2026
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Better match narrowing for irrefutable mapping patterns
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@hauntsaninja
hauntsaninja committed Feb 26, 2026
commit 6e84a6727d88250ec214b192736e451031c2d782
12 changes: 11 additions & 1 deletion mypy/checkpattern.py
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Original file line number Diff line number Diff line change
Expand Up @@ -508,12 +508,22 @@ def visit_mapping_pattern(self, o: MappingPattern) -> PatternType:

captures[o.rest] = rest_type

else_type = current_type
if can_match:
# We can't narrow the type here, as Mapping key is invariant.
new_type = self.type_context[-1]
if not o.keys:
# Match cannot be refuted, so narrow the remaining type
mapping = self.chk.named_type("typing.Mapping")
new_type, else_type = self.chk.conditional_types_with_intersection(
current_type,
[TypeRange(mapping, is_upper_bound=False)],
o,
default=current_type,
)
else:
new_type = UninhabitedType()
return PatternType(new_type, current_type, captures)
return PatternType(new_type, else_type, captures)

def get_mapping_item_type(
self, pattern: MappingPattern, mapping_type: Type, key: Expression
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40 changes: 39 additions & 1 deletion test-data/unit/check-python310.test
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Expand Up @@ -620,7 +620,45 @@ match m:
reveal_type(r) # N: Revealed type is "builtins.dict[builtins.str, builtins.int]"
[builtins fixtures/dict.pyi]

-- Mapping patterns currently do not narrow --
[case testMatchMappingPatternNarrowing]
from typing import Mapping, Sequence

def f1(x: dict[str, str] | list[str] | str) -> None:
match x:
case {}:
reveal_type(x) # N: Revealed type is "builtins.dict[builtins.str, builtins.str]"
case [*_]:
reveal_type(x) # N: Revealed type is "builtins.list[builtins.str]"
case _:
reveal_type(x) # N: Revealed type is "builtins.str"

def f1_rest(x: dict[str, str] | list[str] | str) -> None:
match x:
case {**rest}:
reveal_type(x) # N: Revealed type is "builtins.dict[builtins.str, builtins.str]"
case [*_]:
reveal_type(x) # N: Revealed type is "builtins.list[builtins.str]"
case _:
reveal_type(x) # N: Revealed type is "builtins.str"

def f2(x: Mapping[str, str] | Sequence[str] | str) -> None:
match x:
case {}:
reveal_type(x) # N: Revealed type is "typing.Mapping[builtins.str, builtins.str]"
case [*_]:
reveal_type(x) # N: Revealed type is "typing.Sequence[builtins.str]"
case _:
reveal_type(x) # N: Revealed type is "builtins.str"

def f2_rest(x: Mapping[str, str] | Sequence[str] | str) -> None:
match x:
case {**rest}:
reveal_type(x) # N: Revealed type is "typing.Mapping[builtins.str, builtins.str]"
case [*_]:
reveal_type(x) # N: Revealed type is "typing.Sequence[builtins.str]"
case _:
reveal_type(x) # N: Revealed type is "builtins.str"
[builtins fixtures/dict.pyi]

-- Class Pattern --

Expand Down