Convert to and from roman numerals in python

roman_numerals.py

#!/usr/bin/python3

import unittest

class ToRoman(int):
    def __new__(cls, number):
        if number > 3999:
            raise ValueError('Values over 3999 are not allowed: {}'.format(number))
        if number < 0:
            raise ValueError('Negative values are not allowed: {}'.format(number))
        return super().__new__(cls, number)

    def __init__(self, number):
        to_roman = {1: 'I', 2: 'II', 3: 'III', 4: 'IV', 5: 'V',
                6: 'VI', 7: 'VII', 8: 'VIII', 9: 'IX', 10: 'X', 20: 'XX',
                30: 'XXX', 40: 'XL', 50: 'L', 60: 'LX', 70: 'LXX', 80: 'LXXX',
                90: 'XC', 100: 'C', 200: 'CC', 300: 'CCC', 400: 'CD', 500: 'D',
                600: 'DC', 700: 'DCC', 800: 'DCCC', 900: 'CM', 1000: 'M',
                2000: 'MM', 3000: 'MMM'}
        self.roman = ''.join([to_roman.get(num) for num in self][::-1])

    def __iter__(self):
        number = self.__str__()
        count = 1
        for digit in number[::-1]:
            if digit != '0':
                yield int(digit) * count
            count *= 10

class ToArabic(str):
    def __init__(self, roman):
        roman = self.check_valid(roman)
        keys = ['IV', 'IX', 'XL', 'XC', 'CD', 'CM', 'I', 'V', 'X', 'L', 'C', 'D', 'M']
        to_arabic = {'IV': '4', 'IX': '9', 'XL': '40', 'XC': '90', 'CD': '400', 'CM': '900',
                'I': '1', 'V': '5', 'X': '10', 'L': '50', 'C': '100', 'D': '500', 'M': '1000'}
        for key in keys:
            if key in roman:
                roman = roman.replace(key, ' {}'.format(to_arabic.get(key)))
        self.arabic = sum(int(num) for num in roman.split())

    def check_valid(self, roman):
        roman = roman.upper()
        invalid = ['IIII', 'VV', 'XXXX', 'LL', 'CCCC', 'DD', 'MMMM']
        if any(sub in roman for sub in invalid):
            raise ValueError('Numerus invalidus est: {}'.format(roman))
        return roman

def convert(number):
    if isinstance(number, int):
        num = ToRoman(number)
        return num.roman
    num = ToArabic(number)
    return num.arabic

class TestRomanConversion(unittest.TestCase):
    def setUp(self):
        self.numbers = [(1, 'I'), (3, 'III'), (4, 'IV'), (27, 'XXVII'), (44, 'XLIV'),
                (93, 'XCIII'), (141, 'CXLI'), (402, 'CDII'), (575, 'DLXXV'),
                (1024, 'MXXIV'), (3000, 'MMM')]

    def test_to_roman(self):
        for num in self.numbers:
            self.assertEqual(num[0], convert(num[1]))

    def test_to_arabic(self):
        for num in self.numbers:
            self.assertEqual(num[1], convert(num[0]))

if __name__ == '__main__':
    unittest.main()

Watch my Roman conversion to arabic. It does use many if:

https://github.com/rockcesar/python_code/
https://github.com/rockcesar/python_code/blob/master/roman_numbers.py

Random note: this chokes on invalid numbers like IC (returns 101 instead of expected 99). Assuming we actually want to reject this input, this can easily be fixed by checking afterwards if the Arabic number, if converted back to Roman, yields the input again.