Using __setattr__ and __getattr__ for delegation with __slots__ without triggering infinite recursion

class A:
    __slots__ = ("a",)
    def __init__(self) -> None:
        self.a = 1

class B1:
    __slots__ = ("b",)
    def __init__(self, b) -> None:
        self.b = b

    def __getattr__(self, k):
        return getattr(self.b, k)

    def __setattr__(self, k, v):
        setattr(self.b, k, v)

class B2:
    __slots__ = ("b",)
    def __init__(self, b) -> None:
        self.b = b

    def __getattr__(self, k):
        return getattr(super().__getattr__("b"), k)

    def __setattr__(self, k, v):
        setattr(super().__getattr__("b"), k, v)

class B3:
    __slots__ = ("b",)
    def __init__(self, b) -> None:
        self.b = b

    def __getattr__(self, k):
        return getattr(getattr(super(), "b"), k)

    def __setattr__(self, k, v):
        setattr(getattr(super(), "b"), k, v)

a = A()
b = B1(a)
print(b.a) # RecursionError: maximum recursion depth exceeded

b = B2(a)
print(b.a) # AttributeError: 'super' object has no attribute '__getattr__'

b = B3(a)
print(b.a) # AttributeError: 'super' object has no attribute 'b'

A more proper way is to check if the attribute name is in any of the available __slots__ up the class hierarchy before delegating:

class BCorrect(object):
    __slots__ = ('b',)

    def __init__(self, b) -> None:
        self.b = b

    def _in_slots(self, attr) -> bool:
        for cls in type(self).__mro__:
            if attr in getattr(cls, '__slots__', []):
                return True
        return False

    def __getattr__(self, attr):
        if self._in_slots(attr):
            return object.__getattr__(self, attr)
        return getattr(self.b, attr)

    def __setattr__(self, attr, value):
        if self._in_slots(attr):
            object.__setattr__(self, attr, value)
            return
        setattr(self.b, attr, value)

This has the advantages that it does not break inheritance and does not need any magic in __init__.