def increment_string(s):
    """
    Your job is to write a function which increments a string.

    To create a new string. If the string already ends with a number,
    the number should be incremented by 1. If the string does not end
    with a number the number 1 should be appended to the new string.
    """
    if s and s[-1].isdigit():
        num = s[len(s.rstrip("0123456789")):]
        return s[:-len(num)] + str(int(num) + 1).zfill(len(num))

    return s + "1"