Python: How to copy all attibutes from base class to derived one

Question

I want to achieve the following:

#!/usr/bin/python
class SuperHero(object): 
    def setName(self, name):
        self.name = name
    def getName(self):
            


        

Answer 1

Add a initiator function that copies across the __dict__ attribute:

class SuperMan(SuperHero): 
    def __init__(self, source=None):
        if source is not None:
            self.__dict__.update(source.__dict__)

The __dict__ of an instance holds all instance attributes, the above merely copies over all of those attributes to the new SuperMan instance.

Demo:

>>> class SuperHero(object): 
...     def setName(self, name):
...         self.name = name
...     def getName(self):
...         return self.name
... 
>>> class SuperMan(SuperHero): 
...     def __init__(self, source=None):
...         if source is not None:
...             self.__dict__.update(source.__dict__)
... 
>>> sh = SuperHero()
>>> sh.setName("Clark Kent")
>>> sm = SuperMan(sh)
>>> print sm.getName() 
Clark Kent

Or, for a more terrible hack, you could swap out the class attribute:

sh = SuperHero()
sh.setName("Clark Kent")
sh.__class__ = SuperMan

but that can lead to more interesting bugs as you never called the SuperMan initializer and thus the expected state might be incorrect.

Answer 2

I'd prefer explicit solution - copying one by one. Martijn Pieters's solution is nice, but with time your __dict__ may grow and you may not want to copy all it's content (or worse - forget about this fact and experience some nasty side effects). Besides the Zen of Python says: Explicit is better than implicit..

Side note - you are aware of properties, right? So your code could be more pythonic if you used them:

class SuperHero(object):
    @property
    def name(self):
        return self._name
    @name.setter
    def name(self, name):
        self._name = name

sh = SuperHero()
sh.name = "Clark Kent"