Python Callables with Hysterersis

gistfile1.py

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