81 – Structural validation and homogenisation

81 – Structural validation and homogenisation#

Some functions can accept arguments in many different formats. In that case, structural pattern matching can be used to validate the argument and to homogenise it.

For example, padding in CSS can be specified as a single integer, or as a tuple of 1, 2, or 4 integers. But in CSS the padding is always unwrapped into 4-item tuples.

Some valid examples and the conversions:

10 is converted to (10, 10, 10, 10)
(5, 10) is converted to (5, 10, 5, 10)
(1, 2, 3, 4) is converted to (1, 2, 3, 4)

Using the match statement you can easily write a function that validates this structure and does the conversion:

def unpack_padding(pad) -> tuple[int, int, int, int]:
    match pad:
        case int(p) | (int(p),):
            return (p, p, p, p)
        case (int(vert), int(horz)):
            return (vert, horz, vert, horz)
        case (int(top), int(right), int(bottom), int(left)):
            return (top, right, bottom, left)
        case _:
            raise ValueError(f"1, 2 or 4 integers required for padding; got {pad!r}.")