A 'broken' roman numeral converter

roman_numerals_to_arabic.py

roman_to_arabic = {"I": 1,"V": 5,"X": 10,"L": 50,"C": 100,"D": 500,"M": 1000}
valid_subtractions = {"IV", "IX", "XL", "XC", "CD", "CM"}
def solution(s):
    s = "".join(list(s.upper()))
    for c in s:
        if c not in roman_to_arabic.keys():
            return 0
    rta = roman_to_arabic
    total = 0
    skipnext = False
    for i in range(len(s)):
        if skipnext:
            skipnext = False
            continue
        try:
            if rta[s[i]] < rta[s[i+1]]:
                if str(s[i]+s[i+1]) in valid_subtractions:
                    total += rta[s[i+1]] - rta[s[i]]
                    skipnext = True
            else:
                total += rta[s[i]]
        except IndexError as e:
            total += rta[s[i]]
    return total

assert(solution("MCMXCVIIII") == 1999)
assert(solution("MCMXCIX") == 1999)
assert(solution("MDCCCCLXXXXVIIII") == 1999)
assert(solution("XX") == 20)
assert(solution("XIV") == 14)
assert(solution("VII") == 7)
assert(solution("XXXIV") == 34)
assert(solution("XXXVII") == 37)
assert(solution("XLIII") == 43)
assert(solution("XLVIII") == 48)
assert(solution("LII") == 52)
assert(solution("CXL") == 140)
assert(solution("CXLV") == 145)
assert(solution("XIII") == 13)
assert(solution("XIV") == 14)
assert(solution("XVIII") == 18)
assert(solution("XXVI") == 26)
assert(solution("XXXIX") == 39)
assert(solution("XXXXII") == 42)
assert(solution("LXI") == 61)
assert(solution("DXLV") == 545)
assert(solution("CCXVII") == 217)
assert(solution("MLXVI") == 1066)
assert(solution("I") == 1)
assert(solution("II") == 2)
assert(solution("III") == 3)
assert(solution("V") == 5)
assert(solution("VI") == 6)
assert(solution("VII") == 7)
assert(solution("VIII") == 8)
assert(solution("IV") == 4)
assert(solution("X") == 10)
assert(solution("IX") == 9)
assert(solution("XIV") == 14)
assert(solution("XIX") == 19)
assert(solution("XI") == 11)
assert(solution("XIII") == 13)
assert(solution("XVIII") == 18)
assert(solution("XX") == 20)
assert(solution("XXVIII") == 28)
assert(solution("XXIX") == 29)
assert(solution("XXXIX") == 39)
assert(solution("L") == 50)
assert(solution("LIII") == 53)
assert(solution("LVII") == 57)
assert(solution("XL") == 40)
assert(solution("XLIX") == 49)
assert(solution("LIX") == 59)
assert(solution("LXXIX") == 79)
assert(solution("C") == 100)
assert(solution("XC") == 90)
assert(solution("XCIX") == 99)
assert(solution("CC") == 200)
assert(solution("D") == 500)
assert(solution("CDXCIX") == 499)
assert(solution("CMXCIX") == 999)
assert(solution("MMCMXCIX") == 2999)
assert(solution("MMMCMXCIX") == 3999)
assert(solution("CMI") == 901)
assert(solution("CMXXVI") == 926)
assert(solution("CMLI") == 951)
assert(solution("CMLXXVI") == 976)
assert(solution("CMII") == 902)
assert(solution("CMXXVII") == 927)