What is the use of call method and when to use it?

[ichabod801]

It's a way to create functions out of class instances.

In the expression bar = foo(), the parentheses are an operator. Specifically, they are the function call operator. Normally you only see this with functions:

def foo(x):
    return 2 * x + 1
bar = foo(5)
#        ^ function call operator

The __call__ method of a class allows you to override that operator, just as the __add__ method allows you to override the + operator. This allows your class to operate as if it were a function. A typical example is to make an efficient factorial function by storing previously calculated values:

class Factorial(object):

    def __init__(self):
        self.facts = [1, 1]

    def __call__(self, x):
        try:
            return self.facts[x]
        except IndexError:
            while len(self.facts) <= x:
                self.facts.append(self.facts[-1] * len(self.facts))
            return self.facts[-1]

You can now create an instance of the factorial object and use it as a function:

Output:
>>> factorial = Factorial()
>>> factorial(5)    # adds items to factorial.facts
120
>>> factorial(3)    # Use factorial.facts[3]
6