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## Summary
Support `as` patterns in reachability analysis:
```py
from typing import assert_never
def f(subject: str | int):
match subject:
case int() as x:
pass
case str():
pass
case _:
assert_never(subject) # would previously emit an error
```
Note that we still don't support inferring correct types for the bound
name (`x`).
Closes https://github.com/astral-sh/ty/issues/928
## Test Plan
New Markdown tests
6.8 KiB
6.8 KiB
Pattern matching
[environment]
python-version = "3.10"
With wildcard
def _(target: int):
match target:
case 1:
y = 2
case _:
y = 3
reveal_type(y) # revealed: Literal[2, 3]
Without wildcard
def _(target: int):
match target:
case 1:
y = 2
case 2:
y = 3
# revealed: Literal[2, 3]
# error: [possibly-unresolved-reference]
reveal_type(y)
Basic match
def _(target: int):
y = 1
y = 2
match target:
case 1:
y = 3
case 2:
y = 4
reveal_type(y) # revealed: Literal[2, 3, 4]
Value match
A value pattern matches based on equality: the first case branch here will be taken if subject
is equal to 2, even if subject is not an instance of int. We can't know whether C here has a
custom __eq__ implementation that might cause it to compare equal to 2, so we have to consider
the possibility that the case branch might be taken even though the type C is disjoint from the
type Literal[2].
This leads us to infer Literal[1, 3] as the type of y after the match statement, rather than
Literal[1]:
from typing import final
@final
class C:
pass
def _(subject: C):
y = 1
match subject:
case 2:
y = 3
reveal_type(y) # revealed: Literal[1, 3]
Class match
A case branch with a class pattern is taken if the subject is an instance of the given class, and
all subpatterns in the class pattern match.
Without arguments
from typing import final
class Foo:
pass
class FooSub(Foo):
pass
class Bar:
pass
@final
class Baz:
pass
def _(target: FooSub):
y = 1
match target:
case Baz():
y = 2
case Foo():
y = 3
case Bar():
y = 4
reveal_type(y) # revealed: Literal[3]
def _(target: FooSub):
y = 1
match target:
case Baz():
y = 2
case Bar():
y = 3
case Foo():
y = 4
reveal_type(y) # revealed: Literal[3, 4]
def _(target: FooSub | str):
y = 1
match target:
case Baz():
y = 2
case Foo():
y = 3
case Bar():
y = 4
reveal_type(y) # revealed: Literal[1, 3, 4]
With arguments
from typing_extensions import assert_never
from dataclasses import dataclass
@dataclass
class Point:
x: int
y: int
class Other: ...
def _(target: Point):
y = 1
match target:
case Point(0, 0):
y = 2
case Point(x=0, y=1):
y = 3
case Point(x=1, y=0):
y = 4
reveal_type(y) # revealed: Literal[1, 2, 3, 4]
def _(target: Point):
match target:
case Point(x, y): # irrefutable sub-patterns
pass
case _:
assert_never(target)
def _(target: Point | Other):
match target:
case Point(0, 0):
reveal_type(target) # revealed: Point
case Point(x=0, y=1):
reveal_type(target) # revealed: Point
case Point(x=1, y=0):
reveal_type(target) # revealed: Point
case Other():
reveal_type(target) # revealed: Other
Singleton match
Singleton patterns are matched based on identity, not equality comparisons or isinstance() checks.
from typing import Literal
def _(target: Literal[True, False]):
y = 1
match target:
case True:
y = 2
case False:
y = 3
case None:
y = 4
reveal_type(y) # revealed: Literal[2, 3]
def _(target: bool):
y = 1
match target:
case True:
y = 2
case False:
y = 3
case None:
y = 4
reveal_type(y) # revealed: Literal[2, 3]
def _(target: None):
y = 1
match target:
case True:
y = 2
case False:
y = 3
case None:
y = 4
reveal_type(y) # revealed: Literal[4]
def _(target: None | Literal[True]):
y = 1
match target:
case True:
y = 2
case False:
y = 3
case None:
y = 4
reveal_type(y) # revealed: Literal[2, 4]
# bool is an int subclass
def _(target: int):
y = 1
match target:
case True:
y = 2
case False:
y = 3
case None:
y = 4
reveal_type(y) # revealed: Literal[1, 2, 3]
def _(target: str):
y = 1
match target:
case True:
y = 2
case False:
y = 3
case None:
y = 4
reveal_type(y) # revealed: Literal[1]
Matching on enums
from enum import Enum
class Answer(Enum):
NO = 0
YES = 1
def _(answer: Answer):
y = 0
match answer:
case Answer.YES:
reveal_type(answer) # revealed: Literal[Answer.YES]
y = 1
case Answer.NO:
reveal_type(answer) # revealed: Literal[Answer.NO]
y = 2
reveal_type(y) # revealed: Literal[1, 2]
Or match
A | pattern matches if any of the subpatterns match.
from typing import Literal, final
def _(target: Literal["foo", "baz"]):
y = 1
match target:
case "foo" | "bar":
y = 2
case "baz":
y = 3
reveal_type(y) # revealed: Literal[2, 3]
def _(target: None):
y = 1
match target:
case None | 3:
y = 2
case "foo" | 4 | True:
y = 3
reveal_type(y) # revealed: Literal[2]
@final
class Baz:
pass
def _(target: int | None | float):
y = 1
match target:
case None | 3:
y = 2
case Baz():
y = 3
reveal_type(y) # revealed: Literal[1, 2]
class Foo: ...
def _(target: None | Foo):
y = 1
match target:
case Baz() | True | False:
y = 2
case int():
y = 3
reveal_type(y) # revealed: Literal[1, 3]
as patterns
def _(target: int | str):
y = 1
match target:
case 1 as x:
y = 2
reveal_type(x) # revealed: @Todo(`match` pattern definition types)
case "foo" as x:
y = 3
reveal_type(x) # revealed: @Todo(`match` pattern definition types)
case _:
y = 4
reveal_type(y) # revealed: Literal[2, 3, 4]
Guard with object that implements __bool__ incorrectly
class NotBoolable:
__bool__: int = 3
def _(target: int, flag: NotBoolable):
y = 1
match target:
# error: [unsupported-bool-conversion] "Boolean conversion is unsupported for type `NotBoolable`"
case 1 if flag:
y = 2
case 2:
y = 3
reveal_type(y) # revealed: Literal[1, 2, 3]