def invert_sign(num):
    """Change sign of number or add/remove leading sign of str."""
    if isinstance(num, (float, int)):
        return -1 * num
    elif isinstance(num, str):
        if num.startswith('-'):
            return num[1:]
        return '-{0}'.format(num)
    raise ValueError(
        'Num needs to be numeric value or string, not ' +
        '{0}.'.format(num),
    )