For instance:

nums = "IVXLCDM"

class RomanNumerals:

    def encode(this, value):
        if (value < 1 or value > 3999):
            raise ValueError("Value should be between 1 and 3999 inclusive")

        roman = ""

        x = value
        numoff = 0
        while x > 0:
            d = x % 10
            if d < 4:
                roman = nums[numoff] * d + roman
            elif d == 4:
                roman = nums[numoff] + nums[numoff + 1] + roman
            elif d < 9:
                roman = nums[numoff] * (d - 5) + roman
                roman = nums[numoff + 1] + roman
            else:
                roman = nums[numoff] + nums[numoff + 2] + roman
            x = x // 10
            numoff += 2
        return roman

    def decode(this, roman):
        if len(roman) == 0:
            raise ValueError("Roman encoded value is empty")

        res = 0

        tail = False
        for c in roman:
            # raises ValueError if not found
            i = nums.index(c)

            # power of ten for each two, but multiply with 5 if we're in between
            cv = 10**(i // 2) * (1 if i % 2 == 0 else 5)

            if tail:
                # decrease if the next value is larger instead of smaller (e.g. IV instead of VI)
                if cv > lv:
                    res -= lv
                else:
                    res += lv
            else:
                tail = True
            lv = cv
        res += lv

        # check for correctness the way the Roman numeral is formatted doing the reverse
        if roman != this.encode(res):
            raise ValueError("Roman encoding is not canonical")

        return res

Note that this is for demonstrating the idea only (and for me to practice some Python).