The __call__() method enables class objects to be called like functions. When the method is defined for an object, the object can be considered as a "callable object".   

Some familiar objects that have  this method defined by default includes, functions, lambda functions and classes. This is the method that actually gets invoked when you call such objects by writing their name with parenthesis.

# A demo function
def demo():
    return "Hello, World!"

#Invoke the __call__ method
print(demo.__call__()) #same as func()

#A lambda function to add two numbers
add = lambda a, b: a + b

# Invoke the call method
print(add.__call__(20, 10))

Calling the __call__() method with a class will result with a class object being instantiated. For example:

class Person:
   def __init__(self, name, age):
      self.name = name
      self.age = age

p = Person.__call__("John Doe", 30) # Same as Person("John Doe", 30)

print(p.name,", ", p.age)

Defining the __call__() method for custom objects

Class object are not callable by default, if you want them to be callable,  you must define the __call__() method for the objects.

def __call__(self,  [, args...]):
    #imethod body

Like all other instance methods, the __call__() method must include the self parameter in its definition.  

class Example:
    def __call__(self):
        return "Hello World!"

e = Example()
print(e())

The function can also include any other necessary parameters(alongside self) that should   be given as arguments when the object is being called.

class MathOperation: 
    def __init__(self, num1, num2): 
        self.num1 = num1
        self.num2 = num2
    def __call__(self, operation): 
        output_string = "{} {} {} = {}" #Define a placeholder string for ouputs

        if operation == 'add':
            return output_string.format(self.num1, "+" ,self.num2,  self.num1 + self.num2)
        elif operation == 'sub':
            return  output_string.format(self.num1, "-", self.num2, self.num1 - self.num2)
        elif operation == 'mul':
            return  output_string.format(self.num1, "x", self.num2, self.num1 * self.num2)
        elif operation == 'div': 
            return  output_string.format(self.num1, "/", self.num2,  self.num1 / self.num2)
        else:
            return 'Invalid Operation'

math_obj = MathOperation(20, 10)

#Call the object with various parameters
print(math_obj('add'))
print(math_obj('sub'))
print(math_obj('mul'))
print(math_obj('div'))
print(math_obj('exp'))

Whenever an object contains this method, calling the builtin function, callable() with the object returns True.

class Example:
    def __call__(self):
        return "Hello World!"

e = Example()
print(e())
#The callable function checks whether an object is callable or not.
print(callable(e))

You have now learned how you can make your custom objects callable.