If I make a explicit subclass of a protocol that also inherits from a mixin and calls super() in order to initialize the mixin, I get the "Protocols cannot be instantiated" exception.

This case arises when having a hierarchy of both protocols and concrete classes that implement the protocols.

A simple example is:

import typing


class P(typing.Protocol):
    def m1(self) -> None: ...

class M:
    def __init__(self) -> None:
        super().__init__()
        self.o = True

class C(M, P):
    def __init__(self) -> None:
        super().__init__()
        self.op = True

    def m1(self) -> None:
        pass

c = C()

I can resolve this in particular cases by not invoking super in the mixin or putting a special no-super class in the hierarchy. However, that is not a general solution and once the class hierarchy gets more complicated, it fails to work.

Am I missing any known solution to this issue?

This looks like it a regression specific to Python 3.9.7 and has been fixed.

https://bugs.python.org/issue45081
https://github.com/python/cpython/pull/28132.