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to Roman Numerals in the Python Track

Published at Jul 13 2018 · 0 comments
Test suite


This solution was written on an old version of Exercism. The tests below might not correspond to the solution code, and the exercise may have changed since this code was written.

Write a function to convert from normal numbers to Roman Numerals.

The Romans were a clever bunch. They conquered most of Europe and ruled it for hundreds of years. They invented concrete and straight roads and even bikinis. One thing they never discovered though was the number zero. This made writing and dating extensive histories of their exploits slightly more challenging, but the system of numbers they came up with is still in use today. For example the BBC uses Roman numerals to date their programmes.

The Romans wrote numbers using letters - I, V, X, L, C, D, M. (notice these letters have lots of straight lines and are hence easy to hack into stone tablets).

 1  => I
10  => X
 7  => VII

There is no need to be able to convert numbers larger than about 3000. (The Romans themselves didn't tend to go any higher)

Wikipedia says: Modern Roman numerals ... are written by expressing each digit separately starting with the left most digit and skipping any digit with a value of zero.

To see this in practice, consider the example of 1990.

In Roman numerals 1990 is MCMXC:

1000=M 900=CM 90=XC

2008 is written as MMVIII:

2000=MM 8=VIII

See also: http://www.novaroma.org/via_romana/numbers.html

Exception messages

Sometimes it is necessary to raise an exception. When you do this, you should include a meaningful error message to indicate what the source of the error is. This makes your code more readable and helps significantly with debugging. Not every exercise will require you to raise an exception, but for those that do, the tests will only pass if you include a message.

To raise a message with an exception, just write it as an argument to the exception type. For example, instead of raise Exception, you should write:

raise Exception("Meaningful message indicating the source of the error")

Running the tests

To run the tests, run the appropriate command below (why they are different):

  • Python 2.7: py.test roman_numerals_test.py
  • Python 3.4+: pytest roman_numerals_test.py

Alternatively, you can tell Python to run the pytest module (allowing the same command to be used regardless of Python version): python -m pytest roman_numerals_test.py

Common pytest options

  • -v : enable verbose output
  • -x : stop running tests on first failure
  • --ff : run failures from previous test before running other test cases

For other options, see python -m pytest -h

Submitting Exercises

Note that, when trying to submit an exercise, make sure the solution is in the $EXERCISM_WORKSPACE/python/roman-numerals directory.

You can find your Exercism workspace by running exercism debug and looking for the line that starts with Workspace.

For more detailed information about running tests, code style and linting, please see the help page.


The Roman Numeral Kata http://codingdojo.org/cgi-bin/index.pl?KataRomanNumerals

Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.


import unittest

import roman_numerals

# Tests adapted from `problem-specifications//canonical-data.json` @ v1.2.0

class RomanNumeralsTest(unittest.TestCase):
    numerals = {
        1: 'I',
        2: 'II',
        3: 'III',
        4: 'IV',
        5: 'V',
        6: 'VI',
        9: 'IX',
        27: 'XXVII',
        48: 'XLVIII',
        49: 'XLIX',
        59: 'LIX',
        93: 'XCIII',
        141: 'CXLI',
        163: 'CLXIII',
        402: 'CDII',
        575: 'DLXXV',
        911: 'CMXI',
        1024: 'MXXIV',
        3000: 'MMM',

    def test_numerals(self):
        for arabic, numeral in self.numerals.items():
            self.assertEqual(roman_numerals.numeral(arabic), numeral)

if __name__ == '__main__':
class Roman:

    NUMERALS = {
        1: "I",
        4: "IV",
        5: "V",
        9: "IX",
        10: "X",
        40: "XL",
        50: "L",
        90: "XC",
        100: "C",
        400: "CD",
        500: "D",
        900: "CM",
        1000: "M"

    def numeral(cls, arabic):
        return ''.join(map(lambda key: cls.NUMERALS[key],

    def get_components(cls, arabic):
        components = []
        for key in reversed(sorted(cls.NUMERALS.keys())):
            while arabic >= key:
                arabic -= key
        return components

def numeral(arabic):
    return Roman.numeral(arabic)

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