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.

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]

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

    # TODO: with exhaustiveness checking, this should be Literal[2, 3]
    reveal_type(y)  # revealed: Literal[1, 2, 3]

def _(target: bool):
    y = 1

    match target:
        case True:
            y = 2
        case False:
            y = 3
        case None:
            y = 4

    # TODO: with exhaustiveness checking, this should be Literal[2, 3]
    reveal_type(y)  # revealed: Literal[1, 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

    # TODO: with exhaustiveness checking, this should be Literal[2, 4]
    reveal_type(y)  # revealed: Literal[1, 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]

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

    # TODO: with exhaustiveness, this should be Literal[2, 3]
    reveal_type(y)  # revealed: Literal[1, 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]

def _(target: None | str):
    y = 1

    match target:
        case Baz() | True | False:
            y = 2
        case int():
            y = 3

    reveal_type(y)  # revealed: Literal[1, 3]

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]

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