roman.py

def convert_letter(letter):
    if letter == 'I': return 1
    if letter == 'V': return 5
    if letter == 'X': return 10
    if letter == 'L': return 50
    if letter == 'C': return 100
    if letter == 'D': return 500
    if letter == 'M': return 1000
    return -1


def roman(romanNumeral):
    number = 0
    last = 1000  # large number possible

    for letter in romanNumeral:
        current_number = convert_letter(letter)

        number = number + current_number
        if current_number > last:  # subtractive notation
            number = number - (2 * last)

        last = current_number

    return number


def test_single_digit():
    assert roman("I") == 1
    assert roman("V") == 5

def test_many_digits():
    assert roman("VI") == 6
    assert roman("LXVI") == 66

def test_subtractive_notation():
    assert roman("IV") == 4
    assert roman("XIV") == 14