Is there a way to call a method on definition of a subclass in Python?

Question

The __init__ method defines what is done on creating an instance of a class. Can I do something equivalent when a subclass is created?

Let's say I have the abstract class Entity:

class Entity:
    def __onsubclasscreation__(cls):
        for var in cls.__annotations__:
            cls.__dict__[var] = property(lambda self:self.vars[var])

This would mean that whenever I define a new class inheriting from Entity, all annotated variables of that class would receive a getter:

class Train(Entity):
    wagons: int
    color: str

>>> t = Train()
>>> t.vars["wagons"] = 5
>>> t.wagons
5

I can't do this on instantiation because properties need to be defined in the class, and I can't do it in the superclass because I don't know which attributes will be needed. Is there any way to do something dynamically on subclass creation?

Answer 1

You are describing the basic usage of __init_subclass__ hook (docs):

Whenever a class inherits from another class, __init_subclass__ is called on that class. This way, it is possible to write classes which change the behavior of subclasses.

>>> class A: 
...     def __init_subclass__(cls): 
...         print(f"init {cls}") 
...
>>> class B(A): 
...     pass 
...
init <class '__main__.B'>

See PEP 487 -- Simpler customisation of class creation for more information.

Note: This is a 3.6+ feature. In older Python versions, use the metaclass __new__ to achieve same:

>>> class MyMeta(type):
...     def __new__(meta, name, bases, class_dict):
...         print("MyMeta.__new__", meta, name, bases, class_dict)
...         return type.__new__(meta, name, bases, class_dict)
...
>>> class A(metaclass=MyMeta):
...     pass
...
MyMeta.__new__ <class '__main__.MyMeta'> A () {'__module__': '__main__', '__qualname__': 'A'}
>>> class B(A):
...     pass
...
MyMeta.__new__ <class '__main__.MyMeta'> B (<class '__main__.A'>,) {'__module__': '__main__', '__qualname__': 'B'}