def get_factor(operator):
    """get the factor to adjust the volume, bass, treble... with

    Returns the parsed number, but accounts for operator-only input.

    >>> get_factor('+10')
    10

    >>> get_factor('+')
    1

    >>> get_factor('-')
    -1
    """
    if not operator.startswith(('+', '-')):
        raise ValueError('Valid operators are + and -')
    if len(operator) == 1:
        operator += '1'
    try:
        return int(operator)
    except ValueError:
        raise ValueError('"{}" is not a number or +/-'.format(operator))
