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ruff/crates/ty_python_semantic/resources/mdtest/enums.md
David Peter 2a00eca66b
[ty] Exhaustiveness checking & reachability for match statements (#19508) 
## Summary

Implements proper reachability analysis and — in effect — exhaustiveness
checking for `match` statements. This allows us to check the following
code without any errors (leads to *"can implicitly return `None`"* on
`main`):

```py
from enum import Enum, auto

class Color(Enum):
    RED = auto()
    GREEN = auto()
    BLUE = auto()

def hex(color: Color) -> str:
    match color:
        case Color.RED:
            return "#ff0000"
        case Color.GREEN:
            return "#00ff00"
        case Color.BLUE:
            return "#0000ff"
```

Note that code like this already worked fine if there was a
`assert_never(color)` statement in a catch-all case, because we would
then consider that `assert_never` call terminal. But now this also works
without the wildcard case. Adding a member to the enum would still lead
to an error here, if that case would not be handled in `hex`.

What needed to happen to support this is a new way of evaluating match
pattern constraints. Previously, we would simply compare the type of the
subject expression against the patterns. For the last case here, the
subject type would still be `Color` and the value type would be
`Literal[Color.BLUE]`, so we would infer an ambiguous truthiness.

Now, before we compare the subject type against the pattern, we first
generate a union type that corresponds to the set of all values that
would have *definitely been matched* by previous patterns. Then, we
build a "narrowed" subject type by computing `subject_type &
~already_matched_type`, and compare *that* against the pattern type. For
the example here, `already_matched_type = Literal[Color.RED] |
Literal[Color.GREEN]`, and so we have a narrowed subject type of `Color
& ~(Literal[Color.RED] | Literal[Color.GREEN]) = Literal[Color.BLUE]`,
which allows us to infer a reachability of `AlwaysTrue`.

<details>

<summary>A note on negated reachability constraints</summary>

It might seem that we now perform duplicate work, because we also record
*negated* reachability constraints. But that is still important for
cases like the following (and possibly also for more realistic
scenarios):

```py
from typing import Literal

def _(x: int | str):
    match x:
        case None:
            pass # never reachable
        case _:
            y = 1

    y
```

</details>

closes https://github.com/astral-sh/ty/issues/99

## Test Plan

* I verified that this solves all examples from the linked ticket (the
first example needs a PEP 695 type alias, because we don't support
legacy type aliases yet)
* Verified that the ecosystem changes are all because of removed false
positives
* Updated tests
2025-07-23 22:45:45 +02:00

16 KiB
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Enums

Basic

from enum import Enum

class Color(Enum):
    RED = 1
    GREEN = 2
    BLUE = 3

reveal_type(Color.RED)  # revealed: Literal[Color.RED]
# TODO: This could be `Literal[1]`
reveal_type(Color.RED.value)  # revealed: Any

# TODO: Should be `Color` or `Literal[Color.RED]`
reveal_type(Color["RED"])  # revealed: Unknown

# TODO: Could be `Literal[Color.RED]` to be more precise
reveal_type(Color(1))  # revealed: Color

reveal_type(Color.RED in Color)  # revealed: bool

Enum members

Basic

Simple enums with integer or string values:

from enum import Enum
from ty_extensions import enum_members

class ColorInt(Enum):
    RED = 1
    GREEN = 2
    BLUE = 3

# revealed: tuple[Literal["RED"], Literal["GREEN"], Literal["BLUE"]]
reveal_type(enum_members(ColorInt))

class ColorStr(Enum):
    RED = "red"
    GREEN = "green"
    BLUE = "blue"

# revealed: tuple[Literal["RED"], Literal["GREEN"], Literal["BLUE"]]
reveal_type(enum_members(ColorStr))

When deriving from IntEnum

from enum import IntEnum
from ty_extensions import enum_members

class ColorInt(IntEnum):
    RED = 1
    GREEN = 2
    BLUE = 3

# revealed: tuple[Literal["RED"], Literal["GREEN"], Literal["BLUE"]]
reveal_type(enum_members(ColorInt))

Declared non-member attributes

Attributes on the enum class that are declared are not considered members of the enum:

from enum import Enum
from ty_extensions import enum_members

class Answer(Enum):
    YES = 1
    NO = 2

    non_member_1: int

    # TODO: this could be considered an error:
    non_member_1: str = "some value"

# revealed: tuple[Literal["YES"], Literal["NO"]]
reveal_type(enum_members(Answer))

Enum members are allowed to be marked Final (without a type), even if unnecessary:

from enum import Enum
from typing import Final
from ty_extensions import enum_members

class Answer(Enum):
    YES: Final = 1
    NO: Final = 2

# revealed: tuple[Literal["YES"], Literal["NO"]]
reveal_type(enum_members(Answer))

Non-member attributes with disallowed type

Methods, callables, descriptors (including properties), and nested classes that are defined in the class are not treated as enum members:

from enum import Enum
from ty_extensions import enum_members
from typing import Callable, Literal

def identity(x) -> int:
    return x

class Descriptor:
    def __get__(self, instance, owner):
        return 0

class Answer(Enum):
    YES = 1
    NO = 2

    def some_method(self) -> None: ...
    @staticmethod
    def some_static_method() -> None: ...
    @classmethod
    def some_class_method(cls) -> None: ...

    some_callable = lambda x: 0
    declared_callable: Callable[[int], int] = identity
    function_reference = identity

    some_descriptor = Descriptor()

    @property
    def some_property(self) -> str:
        return ""

    class NestedClass: ...

# revealed: tuple[Literal["YES"], Literal["NO"]]
reveal_type(enum_members(Answer))

enum.property

Enum attributes that are defined using enum.property are not considered members:

[environment]
python-version = "3.11"

from enum import Enum, property as enum_property
from ty_extensions import enum_members

class Answer(Enum):
    YES = 1
    NO = 2

    @enum_property
    def some_property(self) -> str:
        return "property value"

# revealed: tuple[Literal["YES"], Literal["NO"]]
reveal_type(enum_members(Answer))

types.DynamicClassAttribute

Attributes defined using types.DynamicClassAttribute are not considered members:

from enum import Enum
from ty_extensions import enum_members
from types import DynamicClassAttribute

class Answer(Enum):
    YES = 1
    NO = 2

    @DynamicClassAttribute
    def dynamic_property(self) -> str:
        return "dynamic value"

# revealed: tuple[Literal["YES"], Literal["NO"]]
reveal_type(enum_members(Answer))

In stubs

Stubs can optionally use ... for the actual value:

from enum import Enum
from ty_extensions import enum_members
from typing import cast

class Color(Enum):
    RED = ...
    GREEN = cast(int, ...)
    BLUE = 3

# revealed: tuple[Literal["RED"], Literal["GREEN"], Literal["BLUE"]]
reveal_type(enum_members(Color))

Aliases

Enum members can have aliases, which are not considered separate members:

from enum import Enum
from ty_extensions import enum_members

class Answer(Enum):
    YES = 1
    NO = 2

    DEFINITELY = YES

# revealed: tuple[Literal["YES"], Literal["NO"]]
reveal_type(enum_members(Answer))

reveal_type(Answer.DEFINITELY)  # revealed: Literal[Answer.YES]

If a value is duplicated, we also treat that as an alias:

from enum import Enum

class Color(Enum):
    RED = 1
    GREEN = 2

    red = 1
    green = 2

# revealed: tuple[Literal["RED"], Literal["GREEN"]]
reveal_type(enum_members(Color))

# revealed: Literal[Color.RED]
reveal_type(Color.red)

Using auto()

from enum import Enum, auto
from ty_extensions import enum_members

class Answer(Enum):
    YES = auto()
    NO = auto()

# revealed: tuple[Literal["YES"], Literal["NO"]]
reveal_type(enum_members(Answer))

Combining aliases with auto():

from enum import Enum, auto

class Answer(Enum):
    YES = auto()
    NO = auto()

    DEFINITELY = YES

# TODO: This should ideally be `tuple[Literal["YES"], Literal["NO"]]`
# revealed: tuple[Literal["YES"], Literal["NO"], Literal["DEFINITELY"]]
reveal_type(enum_members(Answer))

member and nonmember

[environment]
python-version = "3.11"

from enum import Enum, auto, member, nonmember
from ty_extensions import enum_members

class Answer(Enum):
    YES = member(1)
    NO = member(2)
    OTHER = nonmember(17)

# revealed: tuple[Literal["YES"], Literal["NO"]]
reveal_type(enum_members(Answer))

member can also be used as a decorator:

from enum import Enum, member
from ty_extensions import enum_members

class Answer(Enum):
    yes = member(1)
    no = member(2)

    @member
    def maybe(self) -> None:
        return

# revealed: tuple[Literal["yes"], Literal["no"], Literal["maybe"]]
reveal_type(enum_members(Answer))

Class-private names

An attribute with a class-private name (beginning with, but not ending in, a double underscore) is treated as a non-member:

from enum import Enum
from ty_extensions import enum_members

class Answer(Enum):
    YES = 1
    NO = 2

    __private_member = 3
    __maybe__ = 4

# revealed: tuple[Literal["YES"], Literal["NO"], Literal["__maybe__"]]
reveal_type(enum_members(Answer))

Ignored names

An enum class can define a class symbol named _ignore_. This can be a string containing a whitespace-delimited list of names:

from enum import Enum
from ty_extensions import enum_members

class Answer(Enum):
    _ignore_ = "IGNORED _other_ignored       also_ignored"

    YES = 1
    NO = 2

    IGNORED = 3
    _other_ignored = "test"
    also_ignored = "test2"

# revealed: tuple[Literal["YES"], Literal["NO"]]
reveal_type(enum_members(Answer))

_ignore_ can also be a list of names:

class Answer2(Enum):
    _ignore_ = ["MAYBE", "_other"]

    YES = 1
    NO = 2

    MAYBE = 3
    _other = "test"

# TODO: This should be `tuple[Literal["YES"], Literal["NO"]]`
# revealed: tuple[Literal["YES"], Literal["NO"], Literal["MAYBE"], Literal["_other"]]
reveal_type(enum_members(Answer2))

Special names

Make sure that special names like name and value can be used for enum members (without conflicting with Enum.name and Enum.value):

from enum import Enum
from ty_extensions import enum_members

class Answer(Enum):
    name = 1
    value = 2

# revealed: tuple[Literal["name"], Literal["value"]]
reveal_type(enum_members(Answer))

reveal_type(Answer.name)  # revealed: Literal[Answer.name]
reveal_type(Answer.value)  # revealed: Literal[Answer.value]

Iterating over enum members

from enum import Enum

class Color(Enum):
    RED = 1
    GREEN = 2
    BLUE = 3

for color in Color:
    reveal_type(color)  # revealed: Color

# TODO: Should be `list[Color]`
reveal_type(list(Color))  # revealed: list[Unknown]

Methods / non-member attributes

Methods and non-member attributes defined in the enum class can be accessed on enum members:

from enum import Enum

class Answer(Enum):
    YES = 1
    NO = 2

    def is_yes(self) -> bool:
        return self == Answer.YES
    constant: int = 1

reveal_type(Answer.YES.is_yes())  # revealed: bool
reveal_type(Answer.YES.constant)  # revealed: int

class MyEnum(Enum):
    def some_method(self) -> None:
        pass

class MyAnswer(MyEnum):
    YES = 1
    NO = 2

reveal_type(MyAnswer.YES.some_method())  # revealed: None

Accessing enum members from type[…]

from enum import Enum

class Answer(Enum):
    YES = 1
    NO = 2

def _(answer: type[Answer]) -> None:
    reveal_type(answer.YES)  # revealed: Literal[Answer.YES]
    reveal_type(answer.NO)  # revealed: Literal[Answer.NO]

Calling enum variants

from enum import Enum
from typing import Callable
import sys

class Printer(Enum):
    STDOUT = 1
    STDERR = 2

    def __call__(self, msg: str) -> None:
        if self == Printer.STDOUT:
            print(msg)
        elif self == Printer.STDERR:
            print(msg, file=sys.stderr)

Printer.STDOUT("Hello, world!")
Printer.STDERR("An error occurred!")

callable: Callable[[str], None] = Printer.STDOUT
callable("Hello again!")
callable = Printer.STDERR
callable("Another error!")

Properties of enum types

Implicitly final

An enum with one or more defined members cannot be subclassed. They are implicitly "final".

from enum import Enum

class Color(Enum):
    RED = 1
    GREEN = 2
    BLUE = 3

# error: [subclass-of-final-class] "Class `ExtendedColor` cannot inherit from final class `Color`"
class ExtendedColor(Color):
    YELLOW = 4

def f(color: Color):
    if isinstance(color, int):
        reveal_type(color)  # revealed: Never

An Enum subclass without any defined members can be subclassed:

from enum import Enum
from ty_extensions import enum_members

class MyEnum(Enum):
    def some_method(self) -> None:
        pass

class Answer(MyEnum):
    YES = 1
    NO = 2

# revealed: tuple[Literal["YES"], Literal["NO"]]
reveal_type(enum_members(Answer))

Meta-type

from enum import Enum

class Answer(Enum):
    YES = 1
    NO = 2

reveal_type(type(Answer.YES))  # revealed: <class 'Answer'>

class NoMembers(Enum): ...

def _(answer: Answer, no_members: NoMembers):
    reveal_type(type(answer))  # revealed: <class 'Answer'>
    reveal_type(type(no_members))  # revealed: type[NoMembers]

Cyclic references

from enum import Enum
from typing import Literal
from ty_extensions import enum_members

class Answer(Enum):
    YES = 1
    NO = 2

    @classmethod
    def yes(cls) -> "Literal[Answer.YES]":
        return Answer.YES

# revealed: tuple[Literal["YES"], Literal["NO"]]
reveal_type(enum_members(Answer))

Custom enum types

Enum classes can also be defined using a subclass of enum.Enum or any class that uses enum.EnumType (or a subclass thereof) as a metaclass. enum.EnumType was called enum.EnumMeta prior to Python 3.11.

Subclasses of Enum

from enum import Enum, EnumMeta

class CustomEnumSubclass(Enum):
    def custom_method(self) -> int:
        return 0

class EnumWithCustomEnumSubclass(CustomEnumSubclass):
    NO = 0
    YES = 1

reveal_type(EnumWithCustomEnumSubclass.NO)  # revealed: Literal[EnumWithCustomEnumSubclass.NO]
reveal_type(EnumWithCustomEnumSubclass.NO.custom_method())  # revealed: int

Enums with (subclasses of) EnumMeta as metaclass

[environment]
python-version = "3.9"

from enum import Enum, EnumMeta

class EnumWithEnumMetaMetaclass(metaclass=EnumMeta):
    NO = 0
    YES = 1

reveal_type(EnumWithEnumMetaMetaclass.NO)  # revealed: Literal[EnumWithEnumMetaMetaclass.NO]

class SubclassOfEnumMeta(EnumMeta): ...

class EnumWithSubclassOfEnumMetaMetaclass(metaclass=SubclassOfEnumMeta):
    NO = 0
    YES = 1

reveal_type(EnumWithSubclassOfEnumMetaMetaclass.NO)  # revealed: Literal[EnumWithSubclassOfEnumMetaMetaclass.NO]

# Attributes like `.value` can *not* be accessed on members of these enums:
# error: [unresolved-attribute]
EnumWithSubclassOfEnumMetaMetaclass.NO.value

Enums with (subclasses of) EnumType as metaclass

[environment]
python-version = "3.11"

from enum import Enum, EnumType

class EnumWithEnumMetaMetaclass(metaclass=EnumType):
    NO = 0
    YES = 1

reveal_type(EnumWithEnumMetaMetaclass.NO)  # revealed: Literal[EnumWithEnumMetaMetaclass.NO]

class SubclassOfEnumMeta(EnumType): ...

class EnumWithSubclassOfEnumMetaMetaclass(metaclass=SubclassOfEnumMeta):
    NO = 0
    YES = 1

reveal_type(EnumWithSubclassOfEnumMetaMetaclass.NO)  # revealed: Literal[EnumWithSubclassOfEnumMetaMetaclass.NO]

# error: [unresolved-attribute]
EnumWithSubclassOfEnumMetaMetaclass.NO.value

Function syntax

To do: https://typing.python.org/en/latest/spec/enums.html#enum-definition

Exhaustiveness checking

if statements

from enum import Enum
from typing_extensions import assert_never

class Color(Enum):
    RED = 1
    GREEN = 2
    BLUE = 3

def color_name(color: Color) -> str:
    if color is Color.RED:
        return "Red"
    elif color is Color.GREEN:
        return "Green"
    elif color is Color.BLUE:
        return "Blue"
    else:
        assert_never(color)

# No `invalid-return-type` error here because the implicit `else` branch is detected as unreachable:
def color_name_without_assertion(color: Color) -> str:
    if color is Color.RED:
        return "Red"
    elif color is Color.GREEN:
        return "Green"
    elif color is Color.BLUE:
        return "Blue"

def color_name_misses_one_variant(color: Color) -> str:
    if color is Color.RED:
        return "Red"
    elif color is Color.GREEN:
        return "Green"
    else:
        assert_never(color)  # error: [type-assertion-failure] "Argument does not have asserted type `Never`"

class Singleton(Enum):
    VALUE = 1

def singleton_check(value: Singleton) -> str:
    if value is Singleton.VALUE:
        return "Singleton value"
    else:
        assert_never(value)

match statements

[environment]
python-version = "3.10"

from enum import Enum
from typing_extensions import assert_never

class Color(Enum):
    RED = 1
    GREEN = 2
    BLUE = 3

def color_name(color: Color) -> str:
    match color:
        case Color.RED:
            return "Red"
        case Color.GREEN:
            return "Green"
        case Color.BLUE:
            return "Blue"
        case _:
            assert_never(color)

def color_name_without_assertion(color: Color) -> str:
    match color:
        case Color.RED:
            return "Red"
        case Color.GREEN:
            return "Green"
        case Color.BLUE:
            return "Blue"

def color_name_misses_one_variant(color: Color) -> str:
    match color:
        case Color.RED:
            return "Red"
        case Color.GREEN:
            return "Green"
        case _:
            assert_never(color)  # error: [type-assertion-failure] "Argument does not have asserted type `Never`"

class Singleton(Enum):
    VALUE = 1

def singleton_check(value: Singleton) -> str:
    match value:
        case Singleton.VALUE:
            return "Singleton value"
        case _:
            assert_never(value)

References