I have a type alias that I call point2d and a related recursive type alias.

point2d = tuple[float, float]

point2d_nested = (
    point2d
    | list["point2d_nested"]
    | tuple["point2d_nested", ...]
)

Now I want to write a function that will scale points in any nested structure by a factor. For example, scale_nested((1,1), 2) == (2,2) or scale_nested([(1,1), (1,1)]) == [(2,2), (2,2)]. Here’s an implementation:

def scale_nested(points: point2d_nested, factor: float) -> point2d_nested:
    """
    Scales a list of lists of points by a given factor.
    Weird things happen if you supply 3D points or if your point is not a tuple.
    """
    if not points:
        return points

    # We've bottomed out at a point
    if (
        isinstance(points, tuple)
        and len(points) == 2
        and isinstance(points[0], (int, float))
        and isinstance(points[1], (int, float))
    ):
        return points[0] * factor, points[1] * factor

    if isinstance(points, list):
        return [scale_nested(p, factor) for p in points]

    if isinstance(points, tuple):
        return tuple(
            scale_nested(p, factor) for p in points if not isinstance(p, (int, float))
        )

    raise ValueError(f"Something is an invalid type: {points}")

Unfortunately, this doesn’t work:

knots = [((1.0, 2.0),)]
scale_nested(knots, 2) # <- knots look red

Argument of type "list[tuple[tuple[float, float]]]" cannot be assigned to parameter "points" of type "point2d_nested" in function "scale_nested"
  Type "list[tuple[tuple[float, float]]]" cannot be assigned to type "point2d_nested"
    "list[tuple[tuple[float, float]]]" is incompatible with "point2d"
    "list[tuple[tuple[float, float]]]" is incompatible with "list[point2d_nested]"
      Type parameter "_T@list" is invariant, but "tuple[tuple[float, float]]" is not the same as "point2d_nested"
      Consider switching from "list" to "Sequence" which is covariant
    "list[tuple[tuple[float, float]]]" is incompatible with "tuple[point2d_nested, ...]"PylancereportGeneralTypeIssues
(variable) knots: list[tuple[tuple[float, float]]]