roman.py

def convert_letter(letter):
    if letter == "I":
        return 1
    if letter == "V":
        return 5
    if letter == "X":
        return 10

    return -1

def roman(roman_numeral):
    number = 0

    for letter in roman_numeral:
        current_number = convert_letter(letter)
        number = number + current_number

    return number

roman_test.py

from roman import roman

def test_single_digit():
    assert roman("I") == 1
    assert roman("V") == 5
    assert roman("X") == 10

def test_multiple_digits():
    assert roman("II") == 2
    assert roman("III") == 3
    assert roman("VIII") == 8

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