NoReturn & Never

NoReturn is used to annotate the return type for functions that never return. Never is the bottom type, representing the empty set of Python objects. These two annotations can be used interchangeably.

Function Return Type Annotation

from typing import NoReturn

def stop() -> NoReturn:
    raise RuntimeError("no way")

# revealed: Never
reveal_type(stop())

Assignment

from typing_extensions import NoReturn, Never, Any

# error: [invalid-type-form] "Type `typing.Never` expected no type parameter"
x: Never[int]
a1: NoReturn
a2: Never
b1: Any
b2: int

def f():
    # revealed: Never
    reveal_type(a1)
    # revealed: Never
    reveal_type(a2)

    # Never is assignable to all types.
    v1: int = a1
    v2: str = a1
    # Other types are not assignable to Never except for Never (and Any).
    v3: Never = b1
    v4: Never = a2
    v5: Any = b2
    # error: [invalid-assignment] "Object of type `Literal[1]` is not assignable to `Never`"
    v6: Never = 1

`typing.Never`

typing.Never is only available in Python 3.11 and later.

Python 3.11

[environment]
python-version = "3.11"

from typing import Never

reveal_type(Never)  # revealed: typing.Never

Python 3.10

[environment]
python-version = "3.10"

# error: [unresolved-import]
from typing import Never

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