Callable

Learn callable types with Callable.

Callable

Sometimes you take a function as a parameter. Callable declares the type of callable objects (functions, lambdas, classes).

The syntax is Callable[[ArgT1, ArgT2], RetT]. The first list is the parameter types, the second item is the return type.

from typing import Callable

def apply(fn: Callable[[int], int], x: int) -> int:
    return fn(x)

print(apply(lambda n: n * 2, 5))  # 10

For a parameterless function, write Callable[[], int]. The return type is always present.

Higher-order functions

Functions that take a function as a parameter or return one are called higher-order functions. map, filter and sorted are examples.

The Callable hint makes their signatures clear. Example: def map_int(fn: Callable[[int], int], items: List[int]) -> List[int]:. Here what fn does is explicit.

Nested Callables are also possible: Callable[[int], Callable[[int], int]] means a function taking an integer and returning a function.

Exercise apply function with Callable

Write apply(fn: Callable[[int], int], x: int) -> int. The body contains return fn(x). Import Callable from typing. Call print(apply(lambda n: n * 2, 5)).

Show example solution

from typing import Callable

def apply(fn: Callable[[int], int], x: int) -> int:
    return fn(x)

print(apply(lambda n: n * 2, 5))

Test yourself

What kind of object does Callable[[int], int] accept?
- An int value
- A function taking int and returning int
- A list of ints
- Only a class
Show answer

A function taking int and returning int — Callable[[int], int] is the type of a function taking an int and returning an int.

Import Callable

from typing import ____

def apply(fn: Callable[[int], int], x: int) -> int:
    return fn(x)

Show answer

from typing import Callable

def apply(fn: Callable[[int], int], x: int) -> int:
    return fn(x)

Match the Callable parts

Callable[[int], int]

Takes int, returns int
Callable[[], int]

No params, returns int
[int, str]

Parameter types
int (second)

Return type

Next lesson: TypeVar

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