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rootulp's solution

to Nth Prime 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.

Given a number n, determine what the nth prime is.

By listing the first six prime numbers: 2, 3, 5, 7, 11, and 13, we can see that the 6th prime is 13.

If your language provides methods in the standard library to deal with prime numbers, pretend they don't exist and implement them yourself.

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 nth_prime_test.py
  • Python 3.4+: pytest nth_prime_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 nth_prime_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/nth-prime 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.


A variation on Problem 7 at Project Euler http://projecteuler.net/problem=7

Submitting Incomplete Solutions

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


import unittest

from nth_prime import nth_prime

# Tests adapted from `problem-specifications//canonical-data.json` @ v2.1.0

class NthPrimeTest(unittest.TestCase):
    def test_first_prime(self):
        self.assertEqual(nth_prime(1), 2)

    def test_second_prime(self):
        self.assertEqual(nth_prime(2), 3)

    def test_sixth_prime(self):
        self.assertEqual(nth_prime(6), 13)

    def test_big_prime(self):
        self.assertEqual(nth_prime(10001), 104743)

    def test_there_is_no_zeroth_prime(self):
        with self.assertRaisesWithMessage(ValueError):

    # additional track specific test
    def test_first_twenty_primes(self):
        self.assertEqual([2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31,
                          37, 41, 43, 47, 53, 59, 61, 67, 71],
                         [nth_prime(n) for n in range(1, 21)])

    # Utility functions
    def setUp(self):
        except AttributeError:
            self.assertRaisesRegex = self.assertRaisesRegexp

    def assertRaisesWithMessage(self, exception):
        return self.assertRaisesRegex(exception, r".+")

if __name__ == '__main__':
import math

class prime:

    def nth_prime(cls, n):
        primes = []
        possible = cls.possible_primes()
        while len(primes) < n:
            x = next(possible)
            if cls.is_prime(x):
        return primes[n - 1]

    def is_prime(x):
        for i in range(2, int(math.sqrt(x)) + 1):
            if x % i == 0:
                return False
        return True

    def possible_primes():
        yield 2
        n = 3
        while True:
            yield n
            n += 2

def nth_prime(n):
    return prime.nth_prime(n)

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