python factory functions compared to class

Question

Just working through learning python and started to look at nested/factory functions (simple example):

def maker(N):
    def action(X):
        return X * N
    return action

Are there any advantages of factory functions over creating a class? performance? memory? clean up?

Answer 1

What I like most about nested functions is that it is less verbose than classes. The equivalent class definition to your maker function is:

class clsmaker(object):
    def __init__(self, N):
        self.N = N
    def __call__(self, X):
        return X * self.N

That doesn't seem so bad until you start adding more arguments to the constructor. Then doing it the class way takes an extra line for each argument, while the function just gets the extra args.

It turns out that there is a speed advantage to the nested functions as well:

>>> T1 = timeit.Timer('maker(3)(4)', 'from __main__ import maker')
>>> T1.timeit()
1.2818338871002197
>>> T2 = timeit.Timer('clsmaker(3)(4)', 'from __main__ import clsmaker')
>>> T2.timeit()
2.2137160301208496

This may be due to there being fewer opcodes involved in the nested functions version:

>>> dis(clsmaker.__call__)
  5           0 LOAD_FAST                1 (X)
              3 LOAD_FAST                0 (self)
              6 LOAD_ATTR                0 (N)
              9 BINARY_MULTIPLY     
             10 RETURN_VALUE        
>>> act = maker(3)
>>> dis(act)
  3           0 LOAD_FAST                0 (X)
              3 LOAD_DEREF               0 (N)
              6 BINARY_MULTIPLY     
              7 RETURN_VALUE  

Answer 2

Comparing a function factory to a class is comparing apples and oranges. Use a class if you have a cohesive collection of data and functions, together called an object. Use a function factory if you need a function, and want to parameterize its creation.

Your choice of the two techniques should depend on the meaning of the code.