class decorator to inherit annotations from base-classes

inherit_annotations.py

def inherit_annotations(cls):
    """
    Inherit annotations from all base classes according to method-resolution-order.
    This is the same way that type checkers will interpret annotations.

    This allows for other class decorators such as attr.define() or dataclasses.dataclass()
    to see the inherited annotations from plain-vanilla python classes.  This, in turn,
    allows base classes defining common fields to be shared among different class-decorator-annotation
    libraries.
    """
    annotations = {}
    for base in cls.__mro__[::-1]:  # go in reverse order so "closer" updates come later and override
        if not hasattr(base, "__dict__"):
            continue
        if "__annotations__" not in base.__dict__:
            continue  # in python 3.9 and earlier we might unintentionally get parent classes annotations
        annotations.update(base.__annotations__)
    if "__annotations__" not in cls.__dict__:
        cls.__annotations__ = {}  # ensure __annotations__ exists for 3.9 and earlier
    cls.__annotations__.update(annotations)
    return cls


def test():
    from dataclasses import dataclass

    class Base:
        a: int
        b: int = 3
    
    @dataclass
    @inherit_annotations
    class Derived(Base):
        pass
    
    assert Derived.__annotations__ == Base.__annotations__
    assert Derived(a=1).b == 3


    class A:
        a: int
    
    class B(A):
        a: str
    
    assert B.__annotations__['a'] is str


if __name__ == "__main__":
    test()