Use __init_subclass__ to patch abstract methods while preserving the abc check

I would like to wrap an abstractmethod for all subclasses of an abc. I tried doing that by implementing __init_subclass__ as below:

import abc


class Base(abc.ABC):

  @abc.abstractmethod
  def foo(self) -> str:
    pass

  def __init_subclass__(cls):
    super().__init_subclass__()
    orig_foo = cls.foo
    cls.foo = lambda s: orig_foo(s) + 'def'


class Derived(Base):

  def foo(self):
    return 'abc'

This works and if I do something like:

derived = Derived()
derived.foo()  # -> 'abcdef'

which is expected. Unfortunately, I noticed that this approach does not invoke the abc check, so if I forget to implement foo on Derived:

class Derived(Base):
  pass

I can still create it:

derived = Derived()  # This works
derived.foo()        # -> TypeError: unsupported operand type(s) for +: 'NoneType' and 'str'

Is there a way to do the above wrapping, but not break abc.abstractmethod checks?

Solution

I was able to solve it by overriding cls.foo in __init_subclass__ only if cls.foo differs from Base.foo (i.e. is already overridden).

This yields the following Base implementation:

class Base(abc.ABC):

  @abc.abstractmethod
  def foo(self) -> str:
    pass

  def __init_subclass__(cls):
    super().__init_subclass__()
    orig_foo = cls.foo
    if orig_foo != Base.foo:  # Already overridden.
      cls.foo = lambda s: orig_foo(s) + 'def'