Python 3.5+Annotations & typingterm

Callable[[arg], ret]

callable

Annotation for callable objects. Callable[[A1, A2], R] takes A1, A2 and returns R. Callable[..., R] accepts any arguments.

Syntax

from typing import Callable
Callable[[ArgType, ...], ReturnType]

The inner brackets list the argument types, and only positional ones: keyword parameters, defaults and *args simply cannot be expressed with Callable — use a Protocol with a __call__ method instead. The ellipsis form means "any arguments at all", not "no arguments"; a no-argument callable is written with an empty inner list. Since Python 3.9 typing.Callable is deprecated in favour of collections.abc.Callable.

Examples

from typing import Callable
def apply(f: Callable[[int], int], x: int) -> int:
    return f(x)
print(apply(lambda x: x * 2, 5))  # → 10

from typing import Callable
def run(cb: Callable[[], None]) -> None:
    cb()
run(lambda: print('called'))  # → called

from typing import Callable
def transform(data: list, fn: Callable[[int], int]) -> list:
    return [fn(x) for x in data]
print(transform([1, 2, 3], lambda x: x ** 2))  # → [1, 4, 9]
print(callable(print))  # → True

See also

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