Prevent the child class of an abstract class from creating new methods

Question

Is it possible to force a child class to have all the methods of the abstract class?

I have the following example:

class MovementsService(ABC):

  @abstractmethod
  def run():
    pass

and the child class:

class People(MovementsService):

  def run():
    print("im running")

  def move():
    print("im moving")

but I wouldn't want the child class "People" to have the possibility to implement functions that it doesn't have in the abstract class. Is this possible?

I don't know if this is a good practice either, so I am open to hear opposing opinions.

but in my case it would make sense to block the ability to create new functions because I am using the abstract class as an "interface" to one of my project's dependencies...

Answer 1

You can use __init_subclass__ to detect unexpected attributes in a subclass (at least, those appearing in the class's __dict__ attribute). I'm not really sure why this would be necessary, though.

class MovementsService(ABC):

  @abstractmethod
  def run():
    pass

  def __init_subclass__(cls, **kwargs):
      super().__init__(**kwargs)
      for attr in cls.__dict__:
          if attr != "run":
              raise AttributeError(f"Attribute '{attr}' not allowed")

You can refine the check to only disallow callable attributes, only attributes of type function (allowing class methods and static methods), etc.