罗马数字与阿拉伯数字相互转换

#!/usr/bin/env python3
'''
thousands - 0 to 3 Ms
 hundreds - 900 (CM),400 (CD),0-300 (0 to 3 Cs),
            or 500-800 (D,followed by 0 to 3 Cs)
     tens - 90 (XC),40 (XL),0-30 (0 to 3 Xs)
            or 50-80 (L,followed by 0 to 3 Xs)
     ones - 9 (IX),4 (IV),0-3 (0 to 3 Is)
            or 5-8 (V,fowllowed by 0 to 3 Is)

the range of the number need to be converted is 1-3999
'''
import re


def isRoman(num):
    pattern = '^M{0,3}(CM|CD|D?C{0,3})(XC|XL|L?X{0,3})(IX|IV|V?I{0,3})'
    if re.search(pattern, num):
        return 1
    else:
        return 0


def arabic2roman(num):
    if num > 3999 or num < 1:
        print('num is out of range (1-3999)')
        return
    dict = {
        0: ('', 'M', 'MM', 'MMM'),
        1: ('', 'C', 'CC', 'CCC', 'CD', 'D', 'DC', 'DCC', 'DCCC', 'CM'),
        2: ('', 'X', 'XX', 'XXX', 'XL', 'L', 'LX', 'LXX', 'LXXX', 'XC'),
        3: ('', 'I', 'II', 'III', 'IV', 'V', 'VI', 'VII', 'VIII', 'IX')
    }

    roman = ''
    roman += dict[0][num // 1000 % 10]
    roman += dict[1][num // 100 % 10]
    roman += dict[2][num // 10 % 10]
    roman += dict[3][num % 10]
    return roman


def roman2arabic(roman):
    if isRoman(roman) == 0:
        print('it is not a valid roman number !')
        return
    num = 0
    dict = {'M': 1000, 'D': 500, 'C': 100, 'L': 50, 'X': 10, 'V': 5, 'I': 1}
    for i in range(len(roman)-1):
        if dict[roman[i]] < dict[roman[i+1]]:
            num -= dict[roman[i]]
        else:
            num += dict[roman[i]]
    return num+dict[roman[-1]]