def prepend(value: str, char="_"):
    """Prepend a string to a value if the value doesn't already start with that string

    Examples:
        >>> prepend('a')
        '_a'
        >>> prepend('_a')
        '_a'
        >>> prepend('my_str', '--')
        '--my_str'
        >>> prepend('---my_str', '-')
        '---my_str'
    """
    if value.strip() == "":
        return ""
    return value if value.startswith(char) else f"{char}{value}"