roman_numerals.py

import pytest

BASE = 10
SYMBOLS_DICT = {1: "I", 5: "V", 10: "X", 50: "L", 100: "C", 500: "D", 1000: "M"}


def get_digit_at(num, pos):
    return num // 10 ** pos % 10


def get_symbols_for(pos):
    return (
        SYMBOLS_DICT.get(pos),
        SYMBOLS_DICT.get(pos * 5),
        SYMBOLS_DICT.get(pos * 10, "")
    )


def convert_to_roman(arabic):
    exponent = len(str(arabic))
    result = ""

    while exponent >= 0:
        digit = get_digit_at(arabic, exponent)

        position = BASE ** exponent
        current, next_five, next_ten = get_symbols_for(position)

        if 0 < digit < 4:
            result += current * digit
        elif digit == 4:
            result += current + next_five
        elif digit == 5:
            result += next_five
        elif 5 < digit < 9:
            result += next_five + (current * (digit - 5))
        elif digit == 9:
            result += current + next_ten

        exponent -= 1

    return result

@pytest.mark.parametrize(
    "arabic, roman", [
        (1, "I"),
        (2, "II"),
        (3, "III"),
        (4, "IV"),
        (5, "V"),
        (6, "VI"),
        (7, "VII"),
        (8, "VIII"),
        (9, "IX"),
    ]
)
def test_one_digits(arabic, roman):
    assert convert_to_roman(arabic) == roman


@pytest.mark.parametrize(
    "arabic, roman", [
        (10, "X"),
        (20, "XX"),
        (30, "XXX"),
        (40, "XL"),
        (50, "L"),
        (60, "LX"),
        (70, "LXX"),
        (80, "LXXX"),
        (90, "XC"),
    ]
)
def test_two_digits__ones_zero(arabic, roman):
    assert convert_to_roman(arabic) == roman


@pytest.mark.parametrize(
    "arabic, roman", [
        (100, "C"),
        (200, "CC"),
        (300, "CCC"),
        (400, "CD"),
        (500, "D"),
        (600, "DC"),
        (700, "DCC"),
        (800, "DCCC"),
        (900, "CM"),
    ]
)
def test_three_digits__ones_and_tens_zero(arabic, roman):
    assert convert_to_roman(arabic) == roman


@pytest.mark.parametrize(
    "arabic, roman", [
        (11, "XI"),
        (99, "XCIX"),
        (999, "CMXCIX"),
    ]
)
def test_other_examples(arabic, roman):
    assert convert_to_roman(arabic) == roman