Parametrized inheritance in Python?

Question

I am curious whether there is a better way to achieve something like this? My intentions are to avoid unnecessary boilerplate. The provided example is obviously just simple enough to let the others understand what I had on my mind.

def create_parametrized_class(animal):
    class SomeClass:
        def __init__(self, name):
            self.name = name

        def __str__(self):
            return "{}: {}".format(animal, self.name)

    return SomeClass

class Cat(create_parametrized_class("Cat")):
    pass

class Dog(create_parametrized_class("Dog")):
    pass

cat = Cat("Micka")
dog = Dog("Rex")
assert str(cat) == "Cat: Micka", "Cats..."
assert str(dog) == "Dog: Rex", "Dogs..."

Answer 1

I'm going to presume that type(self).__name__ won't suffice here (for both your example classes that value is equal to the parameter value you passed in).

To set up per-class values at class-definition time, as of Python 3.6 you can use the __init_subclass__ classmethod:

class Animal:
    def __init_subclass__(cls, animal_name, **kw):
        super().__init_subclass__(**kw)
        self._animal_name = animal_name

    def __str__(self):
        return "{}: {}".format(self._animal_name, self.name)


class Cat(Animal, animal_name='Cat'):
    pass

class Dog(Animal, animal_name='Dog'):
    pass

__init_subclass__ is called for all new subclasses, and any parameters you specify in the class Subclass(...) line are passed into that method, letting you parameterise that specific subclass.