Add subclass to super class' list when subclass is created in Python?

Question

Suppose I have a base class as following:

class User:
    user_types = []
    def __init__(self, name, age):
        self.name = name
        self.age = age

And then I want to create a subclass, which will then be added to user_types

class Farmer(User):
    def harvest(self):
        print("%s just harvested his plantation!" %self.name)

Now I can achieve what I'm trying to do, by simply saying

User.user_types.append(Farmer)

But how could I achieve this if I wanted it to be automatic, and to be done for every subclass? Let's say there will be 400 subclasses, it would be a pain in the ass to manually do that for all of them.

Answer 1

New style classes have this functionality:

>>> User.__subclasses__()
[<class '__main__.Farmer'>]

Just need to make User inherit from object in case you're using Python 2.x.

If you need all the subclasses of each subclass, you can still avoid metaclasses with this:

def get_subclasses(c):
    subs = set(c.__subclasses__())
    return subs.union(*(get_subclasses(i) for i in subs))