roman_numerals.py

def add(first, second):
    smooshed = ''.join(sorted(first + second, reverse=True))

    smooshed = smooshed.replace('IIIII', 'V')
    smooshed = smooshed.replace('VV', 'X')
    smooshed = smooshed.replace('XXXXX', 'L')

    return smooshed

test_roman_numerals.py

from roman_numerals import add
import pytest


def test_one_plus_one():
     assert add('I', 'I') == 'II'


def test_one_plus_two():
    assert add('I', 'II') == 'III'
    assert add('II', 'I') == 'III'


def test_three_plus_two():
    assert add('III', 'II') == 'V'


def test_five_plus_one():
    assert add('V', 'I') == 'VI'


def test_five_plus_file():
    assert add('V', 'V') == 'X'


def test_seven_plus_three():
    assert add('VII', 'III') == 'X'


def test_thirty_plus_twenty():
    assert add('XXX', 'XX') == 'L'