How to automatically create all child objects of a parent class in python?

Question

I want to automate the creation of objects for all child classes of a parent class.For example: This is the parent class

from abc import ABC,abstractmethod
class Parent(ABC):
    @abstractmethod
    def fetch(self):
        # some code here

Now I create child classes for this parent class

#child classes
class obj1(Parent):
    def fetch(self):
       # some code

class obj2(Parent):
    def fetch(self):
       # some code

class obj3(Parent):
    def fetch(self):
       # some code

Now my problem is I want to create one object for all these children classed here automatically and save them in an objectList.So that even if someone creates a new child class later, the code will automatically create an object for that new child class and append it in the objectList.

I can do this manually by:

objectList = []
objectList.append(obj1())
objectList.append(obj2())
objectList.append(obj3())

But if someone creates a new child for example obj4() I have to manually append obj4. How to create an object of all child classes automatically?

Answer 1

In python3, subclasses are registered automatically, and made available via the __subclasses__ method:

objectList = [cls() for cls in Parent.__subclasses__()]

Could also use a Metaclass, although then you end up with all the classes in your list, including Parent:

class MetaParent(type):
  subclasses = []
  def __new__(cls, name, bases, kwargs):
    kls = super().__new__(cls, name, bases, kwargs)
    cls.subclasses.append(kls())
    return kls

class Parent(metaclass=MetaParent):
  pass

class ChildA(Parent):
  pass

class ChildB(Parent):
  pass

Now MetaParent.subclasses will have all your instances. Filter out instances of Parent if you want: [cls for cls in MetaParent.subclasses if not type(Parent) == cls]