10.12. Functional Callable

In Python, a callable is an object that can be called like a function. It can be any of the following:

Functions: Defined using the def keyword or lambda keyword.
Methods: Functions that are defined inside a class.
Classes: A class can be called to create an instance of that class.

4. Objects: If an object has a __call__() method, it can be called like a function.

5. Built-in functions: Functions that are built into the Python language, such as print() and len().

6. Built-in methods: Methods that are built into objects, such as append() for lists.

To check if an object is callable, you can use the callable() function. It returns True if the object is callable, and False otherwise.

Here's an example of using the callable() function:

>>> def my_function():
...     print('Hello, world!')
>>>
>>> class MyClass:
...     def __call__(self):
...         print('Hello from MyClass!')
>>>
>>> obj = MyClass()
>>>
>>> callable(my_function)
True
>>> callable(obj)
True
>>> callable('Hello')
False

In this example, my_function and obj are callable objects, while the string "Hello" is not callable. The __call__() method in the MyClass class allows instances of the class to be called like functions.

>>> def hello():
...     return 'Hello World'
>>>
>>>
>>> callable(hello)
True
>>>
>>> type(hello)
<class 'function'>
>>>
>>> hello  
<function hello at 0x...>
>>>
>>> hello()
'Hello World'

10.12.1. Calling Call Method

__call__() method makes object callable

>>> def hello():
...     return 'Hello World'
>>>
>>>
>>> hello()
'Hello World'
>>>
>>> hello.__call__()
'Hello World'

10.12.2. Overloading Call Method

>>> data = str('Mark Watney')
>>>
>>> data()
Traceback (most recent call last):
TypeError: 'str' object is not callable
>>>
>>> callable(data)
False

>>> class str(str):
...     def __call__(self):
...         print('Calling str')
>>>
>>>
>>> data = str('Mark Watney')
>>>
>>> data()
Calling str
>>>
>>> callable(data)
True

10.12.3. Typing

>>> from typing import Callable
>>>
>>>
>>> def add(a: int, b: int) -> int:
...     return a + b
>>>
>>>
>>> total: Callable = add
>>> total: Callable[[int, int], int] = add

>>> from typing import Callable
>>>
>>>
>>> def lower() -> str:
...     return 'hello'
>>>
>>>
>>> def higher() -> Callable:
...     return lower

>>> from typing import Callable
>>>
>>>
>>> def fetch(url: str,
...           on_success: Callable = lambda: ...,
...           on_error: Callable = lambda: ...) -> None:
...     ...

>>> from typing import Callable
>>>
>>>
>>> def fetch(url: str,
...           on_success: Callable[[int,int], int],
...           on_error: Callable) -> callable:
...     ...
>>>
>>> def ok(x: int, y: int) -> int:
...     ...
>>>
>>> fetch('https://...',
...         on_success=ok,
...         on_error=lambda: ...)

>>> from typing import Callable, Iterator, Iterable
>>>
>>>
>>> def map(func: Callable, data: Iterable) -> Iterator:
...     ...
>>>
>>> def filter(func: Callable, data: Iterable) -> Iterator:
...     ...