I know that __call__
method in a class is triggered when the instance of a class is called. However, I have no idea when I can use this special method, because one can simply create a new method and perform the same operation done in __call__
method and instead of calling the instance, you can call the method.
I would really appreciate it if someone gives me a practical usage of this special method.
This example uses memoization, basically storing values in a table (dictionary in this case) so you can look them up later instead of recalculating them.
Here we use a simple class with a __call__
method to calculate factorials (through a callable object) instead of a factorial function that contains a static variable (as that's not possible in Python).
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()
Now you have a fact
object which is callable, just like every other function. For example
for i in xrange(10):
print("{}! = {}".format(i, fact(i)))
# output
0! = 1
1! = 1
2! = 2
3! = 6
4! = 24
5! = 120
6! = 720
7! = 5040
8! = 40320
9! = 362880
And it is also stateful.