mypy-protocol-demo.py

from typing import Protocol

class InterfaceAB(Protocol):
    def method_a(self) -> None: ...
    def method_b(self) -> None: ...

class ClassA:
    def method_a(self) -> None:
        print("a")

class ClassB:
    def method_b(self: InterfaceAB) -> None:
        print("b")
        self.method_a()

# if I remove ClassA here, I get a type checking error!
class AB(ClassA, ClassB): pass

ab = AB()
ab.method_b()

# % mypy --version
# mypy 0.761
# % mypy mypy-protocol-demo.py
# Success: no issues found in 1 source file