Python __call__ special method practical example http://stackoverflow.com/questions/5824881/python-call-special-method-practical-example

gistfile1.py

class Factorial:
    def __init__(self):
        self.cache = {}
    
    def __call__(self, n):
        if n not in self.cache:
            if n == 0:
                self.cache[n] = 1
            else:
                self.cache[n] = n * self.__call__(n - 1)
        return self.cache[n]

fact = Factorial()

for i in xrange(10):
    print '%d! = %d' % (i, fact(i))