pythonic way of decorating classmethods

Question

Given the following example:

class A:
    def f(self, x):
        return 2*x 

I would like to write another method which uses f above but adds a constant, i.e.

class A:
        def f(self, x):
            return 2*x
        def g(self, x):
            return self.f(x) + 10

This would be one way. However, this smells very much like decorating! What would be the proper pythonic way to do this?

Answer 1

You can write a simple function outside the class:

def add_val(f):
  def wrapper(cls, _x):
    return f(cls, _x) + 10
  return wrapper


class A:
  @add_val
  def f(self, x):
    return 2*x 
print(A().f(20))

Output:

50

Edit: you can utilize functools.wraps with a classmethod in Python3. The wrapped function f will return x*2 and the decorator g will add 10 to the returned result of the function passed to it. However, to be able to save the original functionality of f, you can utilize the __wrapped__ attribute:

import functools
def g(f):
  @functools.wraps(f)
  def wrapper(cls, _x):
    return f(cls, _x)+10
  return wrapper

class A:
   @classmethod
   @g
   def f(cls, x):
     return x*2

f_1 = functools.partial(A.f.__wrapped__, A)
print(A.f(4))
print(f_1(4))

Output:

18
8