# some recursive functions on nested lists

def nested_depth(L):
    """
    Return if L is non-list, or 1 + maximum of depths of elements list L.
    Not defined for infinitely nested lists.

    >>> nested_depth(3)
    0
    >>> nested_depth([1, 2, 3])
    1
    >>> nested_depth([1, [2, 3], 4])
    2
    """

    return (1 + max([nested_depth(x) for x in L] + [0])
            if isinstance(L, list) else 0)

def rec_max(L):
    """
    Return the maximum number in possibly nested list of numbers.

    >>> rec_max([17, 21, 0])
    21
    >>> rec_max([17, [21, 24], 0])
    24
    """

    return max([rec_max(x) if isinstance(x, list) else x for x in L])


if __name__ == '__main__':
    import doctest
    doctest.testmod()