3.14 __call__

This lesson shows the usage of __call__ method of a class.

Suppose we have a function named human.

def human(age):
    print(f"my age is {age}")
    return

Since human is a function, we can call this function using ().

human(12)

my age is 12

What if we want a class (or an instance of a class) to behave like function. To be specific if we want to use an instance of a class as a function, so that when we can call this instance, we will write __call__ method of the class.

class Human:

    def __init__(self, age):
        self.age = age

    def __call__(self):
        print(f"my age is {self.age}")
        return

The Human class has two (user defined) methods i.e. __init__ and __call__. The __init__ method is used/called when we initialize th class and create its instance.

human = Human(12)

Now if we use the instance human as function i.e. if we call human using (), the __call__ method will be executed.

human()

my age is 12

The __call__ can be defined to take any keyword, non-keyword, optional or obligatory arguments.

class Human:

    def __init__(self, age):
        self.age = age

    def __call__(self, characteristic):
        print(f"I am {characteristic}")
        return


human = Human(63)
human("mortal")

I am mortal

class Human:

    def __init__(self, age):
        self.age = age

    def __call__(self, *args, **kwargs):
        print(f"{args} {kwargs}")
        return


human = Human(63)
human()

() {}

human(1)

(1,) {}

human(1, a=2)

(1,) {'a': 2}

Without writing, __call__ method for the Human class, we wouldn’t be able to call the Human class or its instance human.

Question: In the code Human(1)(), what does the second braces (), those which come after (1) represent?

class Human:

    def __init__(self, age):
        self.age = age


human = Human(2)

# uncomment the following line
# human()  # -> TypeError: 'Human' object is not callable.