What about mix-ins or abstract base classes?

class VecMixin:
    def length(self):
        return math.hypot(*self)

class Vec2D(NamedTuple, VecMixin):
    x: float
    y: float

class Vec3D(NamedTuple, VecMixin):
    x: float
    y: float
    z: float

Currently you need to use the following trick to get a similar result:

class Vec2D(NamedTuple):
    x: float
    y: float

class Vec2D(Vec2D, VecMixin):
    pass