Callable
- Python course
- Advanced
- Lesson 10/83
- 5 min
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)) # 10For 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.
Practice 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)).
Starter code
# import Callable from typing
# apply(fn: Callable[[int], int], x: int) -> int: call fn with x
# print apply with lambda n: n * 2 and 5Sample solution
from typing import Callable
def apply(fn: Callable[[int], int], x: int) -> int:
return fn(x)
print(apply(lambda n: n * 2, 5))Expected output
10
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What kind of object does Callable[[int], int] accept?
- An int value
- A function taking int and returning int Correct answer
- A list of ints
- Only a class
Explanation: Callable[[int], int] is the type of a function taking an int and returning an int.
2 Import Callable
from typing import ____
def apply(fn: Callable[[int], int], x: int) -> int:
return fn(x)from typing import Callable
def apply(fn: Callable[[int], int], x: int) -> int:
return fn(x)Explanation: Callable comes from typing; it declares the callable type.
3 Match the Callable parts
| Term | Meaning |
|---|---|
Callable[[int], int] |
Takes int, returns int |
Callable[[], int] |
No params, returns int |
[int, str] |
Parameter types |
int (second) |
Return type |