> Dataclasses is notable because it's the only example (I'm aware of) of type hints effecting runtime behavior as part of the stdlib.

FWIW, `typing.NamedTuple` did this in Python 3.5, three years before dataclasses was introduced in 3.7.

    class Foo(typing.NamedTuple):
        a: int
        b: str

    f = Foo(a=1, b="hello")
    print(f.b)  # "hello"