How to avoid parallel class hierarchy in python

Question

I\'ve been running into a weird little smell in my python code lately and I think it has something to do with parallel inheritance. Here is a small example I concoc

Answer 1

IF habits need to a class attribute, rather than instance attributes, this may actually be a good use for metaclasses.

Habits need not be a simple list, it could be something else, as long as there is the notion of addition to previous and return new. (__add__ or __radd__ on a Habits class would do the trick I think)

class DogType(type):

    def __init__(cls, name, bases, attrs):
        """ this is called at the Dog-class creation time.  """

        if not bases:
            return

        #pick the habits of direct ancestor and extend it with 
        #this class then assign to cls.
        if "habits" in attrs:
            base_habits = getattr(bases[0], "habits", [])
            cls.habits = base_habits + cls.habits


class Dog(metaclass=DogType):
    habits = ["licks butt"]

    def __repr__(self):
        return f"My name is {self.name}.  I am a {self.__class__.__name__} %s and I like to {self.habits}"

    def __init__(self, name):
        """ dog instance can have all sorts of instance variables"""
        self.name = name

class Sheperd(Dog):
    habits = ["herds sheep"]

class GermanSheperd(Sheperd):
    habits = ["bites people"]

class Poodle(Dog):
    habits = ["barks stupidly"]

class StBernard(Dog):
    pass


for ix, cls in enumerate([GermanSheperd, Poodle, StBernard]):
    name = f"dog{ix}"
    dog = cls(name)
    print(dog)

output:

My name is dog0.  I am a GermanSheperd %s and I like to ['licks butt', 'herds sheep', 'bites people']
My name is dog1.  I am a Poodle %s and I like to ['licks butt', 'barks stupidly']
My name is dog2.  I am a StBernard %s and I like to ['licks butt']