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Improve match inline list subject inference #19744

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8 changes: 6 additions & 2 deletions mypy/fastparse.py
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Original file line number Diff line number Diff line change
Expand Up @@ -396,6 +396,7 @@ def __init__(
# 'C' for class, 'D' for function signature, 'F' for function, 'L' for lambda
self.class_and_function_stack: list[Literal["C", "D", "F", "L"]] = []
self.imports: list[ImportBase] = []
self.match_stmt_subject = False

self.options = options
self.is_stub = is_stub
Expand Down Expand Up @@ -1758,7 +1759,7 @@ def visit_Name(self, n: Name) -> NameExpr:
# List(expr* elts, expr_context ctx)
def visit_List(self, n: ast3.List) -> ListExpr | TupleExpr:
expr_list: list[Expression] = [self.visit(e) for e in n.elts]
if isinstance(n.ctx, ast3.Store):
if isinstance(n.ctx, ast3.Store) or self.match_stmt_subject:
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@ilevkivskyi ilevkivskyi Aug 27, 2025

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Wouldn't this cause problems in situations like this:

def foo(list_only: list[int]) -> None: ...

match foo([1, 2]):  # type error because foo() got a tuple?
    case whatever:
        ...

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@cdce8p cdce8p Aug 27, 2025

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Good catch, missed that one. Guess it's one reason to recommend inline tuples over lists for match subjects then. Going to close this one.

# [x, y] = z and (x, y) = z means exactly the same thing
e: ListExpr | TupleExpr = TupleExpr(expr_list)
else:
Expand All @@ -1779,8 +1780,11 @@ def visit_Slice(self, n: ast3.Slice) -> SliceExpr:

# Match(expr subject, match_case* cases) # python 3.10 and later
def visit_Match(self, n: Match) -> MatchStmt:
self.match_stmt_subject = True
subject = self.visit(n.subject)
self.match_stmt_subject = False
node = MatchStmt(
self.visit(n.subject),
subject,
[self.visit(c.pattern) for c in n.cases],
[self.visit(c.guard) for c in n.cases],
[self.as_required_block(c.body) for c in n.cases],
Expand Down
26 changes: 24 additions & 2 deletions test-data/unit/check-python310.test
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Original file line number Diff line number Diff line change
Expand Up @@ -318,6 +318,7 @@ match x:
pass

[case testMatchSequencePatternWithInvalidClassPattern]
# flags: --warn-unreachable
class Example:
__match_args__ = ("value",)
def __init__(self, value: str) -> None:
Expand All @@ -327,11 +328,32 @@ SubClass: type[Example]

match [SubClass("a"), SubClass("b")]:
case [SubClass(value), *rest]: # E: Expected type in class pattern; found "type[__main__.Example]"
reveal_type(value) # E: Cannot determine type of "value" \
# N: Revealed type is "Any"
reveal_type(value) # E: Statement is unreachable
reveal_type(rest)
case [Example(value), *rest]:
reveal_type(value) # N: Revealed type is "builtins.str"
reveal_type(rest) # N: Revealed type is "builtins.list[__main__.Example]"
[builtins fixtures/tuple.pyi]

[case testMatchSequencePatternSequenceSubject]
a: int
b: str
match a, b:
case 1, "Hello":
reveal_type(a) # N: Revealed type is "Literal[1]"
reveal_type(b) # N: Revealed type is "Literal['Hello']"

match (a, b):
case (1, "Hello"):
reveal_type(a) # N: Revealed type is "Literal[1]"
reveal_type(b) # N: Revealed type is "Literal['Hello']"

match [a, b]:
case [1, "Hello"]:
reveal_type(a) # N: Revealed type is "Literal[1]"
reveal_type(b) # N: Revealed type is "Literal['Hello']"
Comment on lines +351 to +354
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@cdce8p cdce8p Aug 27, 2025

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Without the change here, this would be inferred as int and str respectively. I.e. just the types from a and b without any narrowing applied.

[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
Expand Down
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