__call__

Why would one use the special method __call__ instead of defining a new method for an object in Python (Quora):

• When you need to create an object that acts like a function.

  One example is when writing function decorators. A function decorator needs to return an object that acts like a function, so __call__ can be useful. It’s awkward to write a memoizing decorator without using __call__.

  Another example is when dealing with third-party libraries that take functions as arguments. Using __call__ is often the cleanest way to interact with these libraries in a way that preserves state.

• Most of the other times it would be tempting to use __call__ I think it would be more appropriate to use either a bound method or a lambda. If you can conveniently use either a bound method or a lambda instead of __call__ I think you probably should.

Sample

class Animal(object):
    def __init__(self, name, legs):
        self.name = name
        self.legs = legs
        self.stomach = []

    def __call__(self, food):
        self.stomach.append(food)

    def __str__(self):
        return '%s has %s legs and has eaten %s' % (
            self.name, self.legs, ' and '.join(self.stomach)
        )


cow = Animal('cow', 4)
cow('Oats')
cow('Silage')
print cow

hen = Animal('hen', 2)
hen('Corn')
print hen

Output

cow has 4 legs and has eaten Oats and Silage
hen has 2 legs and has eaten Corn