Convert roman numerals to integer in python3

roman2int.py

#!/usr/bin/env python3

def roman2int(roman):
    sum = 0
    previous = 0
    roman_numerals = { 'I': 1, 'V': 5, 'X': 10, 'L': 50, 'C': 100, 'D': 500, 'M': 1000 }
    for c in [*roman][::-1]:
        current = roman_numerals[c]
        if previous > current:
            sum -= current
        else:
            sum += current
        previous = current
    return sum


def main():
    roman = input("Enter a roman numeral: ")
    print(roman2int(roman))

if __name__ == "__main__":
    main()

roman2int.test.py

#!/usr/bin/env python3

import unittest
from roman2int import roman2int

class TestRoman2Int(unittest.TestCase):
    def test_convert(self):
        cases = { 'III': 3, 'IV': 4, 'IX': 9, 'LVIII': 58, 'MCMXCIV': 1994 }
        for roman, integer in cases.items():
            self.assertEqual(roman2int(roman), integer)

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