decorator.py

# -*- coding: utf-8 -*-

def class_overrider(cls):
  def __init__(self, value):
    self.value = value
  cls.__init__ = __init__
  return cls

@class_overrider
class Hoge:
  def __init__(self, value):
    self.value = value * 3

  def compute(self):
    return self.value * 2

if __name__ == '__main__':
  hoge = Hoge(3)
  print(hoge.compute())  #=> 6

good_decorator.py

# -*- coding: utf-8 -*-


def instance_return(cls):
    return cls()


@instance_return
class Hoge:
    def compute(self):
        return self.value * 2

if __name__ == "__main__":
    hoge = Hoge
    hoge.value = 3
    print hoge.compute()
    #=>6

inheritance.py

# -*- coding: utf-8 -*-

class HogeBase:
  def __init__(self, value):
    self.value = value

class Hoge(HogeBase):
  def __init__(self, value):
    super(self.__class__, self).__init__(value)
  
  def compute(self):
    return self.value * 2
    
if __name__ == '__main__':
  hoge = Hoge(3)
  print(hoge.compute())  #=> 6