I don't understand the problem with your first example. The __init__ method isn't special and B.__init__ is just a function. Your code boils down to:

    def some_function(obj):
      obj.prop = 2

    class Foo:
      def __init__(self):
        some_function(self)

    # or really just like

    class Foo:
      def __init__(self):
        self.prop = 2

Your examples genuinely haven't even scratched the surface of the weird stuff you can do when you take control of Python's machinery—self is just a convention, you can remove __init__ entirely, types are made up and the points don't matter. Foo() isn't even special it's just __call__ on the classes type and you can make that do anything.

With the assumptions typical of static class-based OO (but which may or may not apply in programs in Python), this naively seems like a type error, an even when it isn't it introduces a coupling where the class where the call is made likely depends on the internal implementation (not just the public interface) of the called class, which is...definitely an opportunity to introduce unexpected bugs easily.