[Python] Problem if two inherited __init_subclass__s have the same argument name

I'm trying to use two classes, A and B, as mixing classes of AB. They both have __init_subclass__ methods. The problem is that both __init__sublass__ methods have the same argument msg. Therefore I've used an adaptor class B_ to rename B's argument msg_b. But I'm having trouble!

The nearest I have got is:

class A:
def __init_subclass__(cls, msg, **kwargs):
print(f'{cls=} A: {msg}')
super().__init_subclass__(**kwargs)

class B:
def __init_subclass__(cls, msg, **kwargs):
print(f'{cls=} B: {msg}')
super().__init_subclass__(**kwargs)

# Adaptor class `B_` needed because both `A` and `B` have an argument `msg`.
class B_:
# Rename `msg` to `msg_b`.
def __init_subclass__(cls, msg_b, **kwargs):
# `B.__init_subclass__(msg_b, **kwargs)` sets the subclass as `B` not `cls`, but otherwise works.
B.__init_subclass__.__func__(cls, msg=msg_b, **kwargs)

# Still need a `B`.
def __init__(self, *args, **kwargs):
self.b = B(*args, **kwargs)

# Forward all the attributes to `self.b`.
def __getattr__(self, item):
return getattr(self.b, item)

class AB(A, B_, msg='Hello.', msg_b='Also, hello.' ):
...

print(f'{AB()=}, {isinstance(AB(), A)=}, {isinstance(AB(), B_)=}, {isinstance(AB(), B)=}')


Which does call both __init_sublass__s with the correct class argument, it prints:

cls=<class '__main__.AB'> A: Hello.
cls=<class '__main__.AB'> B: Also, hello.


But then you get the error:

Traceback (most recent call last):
File <definition of AB>, in <module>
class AB(A, B_, msg='Hello.', msg_b='Also, hello.' ):
File <A's>, in __init_subclass__
super().__init_subclass__(**kwargs)
File <B_'s>, in __init_subclass__
B.__init_subclass__.__func__(cls, msg=msg_b, **kwargs)
File <B's>, in __init_subclass__
super().__init_subclass__(**kwargs)
^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: super(type, obj): obj must be an instance or subtype of type


Presumably because B is not a super type of AB (B_ is). I’m not clear why super wants subtypes matching though!

Any ideas on how to fix this?

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