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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def int_to_roman(integer):
 """ Convert an integer into a string of roman numbers. .. code: python reusables.int_to_roman(445) # 'CDXLV' :param integer: :return: roman string """

    if not isinstance(integer, int):
        raise ValueError("Input integer must be of type int")
    output = []
    while integer > 0:
        for r, i in sorted(_roman_dict.items(),
                           key=lambda x: x[1], reverse=True):
            while integer >= i:
                output.append(r)
                integer -= i
    return "".join(output)