Recursively calculate the digital root of a non-negative integer

digital_root.py

def digital_root(n: int, original=None) -> int:
    """
    Return the digital root of a non-negative integer `n`. Note that this is
    basically the same as `n % 9` except in the special case when `n` is a
    multiple of 9.

    @param n:  A non-negative integer
    @type  n:  int

    @return:  The digital root of `n`
    @rtype :  int
    >>> digital_root(274)
    4
    >>> digital_root(99999943)
    7
    >>> digital_root(999999)
    9
    >>> digital_root(0)
    0
    >>> digital_root(108)
    9
    """
    assert n >= 0, "`n` must be a non-negative integer"
    original = n
    r = 0
    while n:
        r += n % 10
        n //= 10

    if r < 10:
        return 9 if r == 0 and original > 0 else r
    else:
        return digital_root(r, original)