如何从传递给 python 中的超类的字符串创建子类？

Question

Given a super class with two (or many) sub classes, for the sake of simplicity lets call them Super, Sub1 and Sub2 respectively. I would like to instantiate Sub1 and Sub2 as follows:

s1 = Super('Sub1')
s2 = Super('Sub2')

ie, passing the name of the sub classes as strings to the constructor of the super class.

Something that came to mind was defining a class variable in Super with the names of the sub classes and with a couple of if statements in the constructor of the Super class the corresponding sub class constructor could be called. I'm not completely sure if that would work, but it just seems messy to me. Any suggestion on how to tackle this problem with a clean and pythonic approach is welcomed.

Answer 1

I don't really think this is a good idea. A super class is not meant as a container for a finite list of possible subclasses, that pretty much entirely goes against the point of inheritence. If I were you I would instead create a simple factory from which I can construct the objects I want, eg:

class Fruit():
  ...

class Apple(Fruit):
  ...

class Pear(Fruit):
  ...


fruits = {
   "apple": Apple,
   "pear": Pear       
}

def make_fruit(name, **kwargs):
    return fruits[name](**kwargs)

apple = make_fruit("apple", colour="red")
pear = make_fruit("pear", size="small)

Answer 2

This requires overriding Super.__new__ to make it into a factory function.

class Super:
    def __new__(cls, subclass_name, *args, **kwargs):
        for sc in cls.__subclasses__():
            if sc.__name__ == subclass_name:
                return super().__new__(sc, *args, **kwargs)
        raise ValueError("No such subclass")    


class Sub1(Super):
    pass


class Sub2(Super):
    pass


assert type(Super('Sub1')) is Sub1

This requires more work to allow you to define a subclass directly (as Sub1() will invoke Super.__new__ , since Sub1.__new__ is not defined).

As such, I would prefer a dedicated class method that takes a class name, rather than overriding __new__ .

class Super:
    @classmethod
    def subclass_by_name(cls, name, *args, **kwargs):
        for sc in cls.__subclasses__():
            if sc.__name__ == name:
                return sc(*args, **kwargs)
        raise ValueError("No such subclass")


assert type(Super.subclass_by_name('Sub1')) is Sub1