def substring(string: str, start: int, end: int = 0) -> str:
    """
    A substring function
    :param string: The string
    :param start: The starting index
    :param end: The ending index. If None, this is set to the length of the substring     :return:
    """
    if not isinstance(string, str):
        raise ValueError("No string provided")
    if not isinstance(start, int):
        raise ValueError("No starting index provided")

    if end is None:
        end = len(str)

    result = ""

    for i in range(start, end):
        result += string[i]

    return result