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davearonson's solution

to Raindrops in the Python Track

Published at Jul 15 2019 · 0 comments
Test suite


This exercise has changed since this solution was written.

Convert a number to a string, the contents of which depend on the number's factors.

  • If the number has 3 as a factor, output 'Pling'.
  • If the number has 5 as a factor, output 'Plang'.
  • If the number has 7 as a factor, output 'Plong'.
  • If the number does not have 3, 5, or 7 as a factor, just pass the number's digits straight through.


  • 28's factors are 1, 2, 4, 7, 14, 28.
    • In raindrop-speak, this would be a simple "Plong".
  • 30's factors are 1, 2, 3, 5, 6, 10, 15, 30.
    • In raindrop-speak, this would be a "PlingPlang".
  • 34 has four factors: 1, 2, 17, and 34.
    • In raindrop-speak, this would be "34".

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 raindrops_test.py
  • Python 3.4+: pytest raindrops_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 raindrops_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/raindrops 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 Running the Tests.


A variation on a famous interview question intended to weed out potential candidates. http://jumpstartlab.com

Submitting Incomplete Solutions

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


import unittest

from raindrops import convert

# Tests adapted from `problem-specifications//canonical-data.json` @ v1.1.0

class RaindropsTest(unittest.TestCase):
    def test_the_sound_for_1_is_1(self):
        self.assertEqual(convert(1), "1")

    def test_the_sound_for_3_is_pling(self):
        self.assertEqual(convert(3), "Pling")

    def test_the_sound_for_5_is_plang(self):
        self.assertEqual(convert(5), "Plang")

    def test_the_sound_for_7_is_plong(self):
        self.assertEqual(convert(7), "Plong")

    def test_the_sound_for_6_is_pling(self):
        self.assertEqual(convert(6), "Pling")

    def test_2_to_the_power_3_does_not_make_sound(self):
        self.assertEqual(convert(8), "8")

    def test_the_sound_for_9_is_pling(self):
        self.assertEqual(convert(9), "Pling")

    def test_the_sound_for_10_is_plang(self):
        self.assertEqual(convert(10), "Plang")

    def test_the_sound_for_14_is_plong(self):
        self.assertEqual(convert(14), "Plong")

    def test_the_sound_for_15_is_plingplang(self):
        self.assertEqual(convert(15), "PlingPlang")

    def test_the_sound_for_21_is_plingplong(self):
        self.assertEqual(convert(21), "PlingPlong")

    def test_the_sound_for_25_is_plang(self):
        self.assertEqual(convert(25), "Plang")

    def test_the_sound_for_27_is_pling(self):
        self.assertEqual(convert(27), "Pling")

    def test_the_sound_for_35_is_plangplong(self):
        self.assertEqual(convert(35), "PlangPlong")

    def test_the_sound_for_49_is_plong(self):
        self.assertEqual(convert(49), "Plong")

    def test_the_sound_for_52_is_52(self):
        self.assertEqual(convert(52), "52")

    def test_the_sound_for_105_is_plingplangplong(self):
        self.assertEqual(convert(105), "PlingPlangPlong")

    def test_the_sound_for_12121_is_12121(self):
        self.assertEqual(convert(12121), "12121")

if __name__ == '__main__':


NOISES = ((3, "Pling"), (5, "Plang"), (7, "Plong"))

def convert(number):
    if not isinstance(number, int): raise ValueError
    result = [noise for trigger, noise in NOISES if number % trigger == 0]
    return "".join(result) if result else str(number)  # empty list is falsy


import pytest

from raindrops import convert

# basic functionality:

def test_multiple_of_none_becomes_string():
    assert convert(1) == "1"

def test_3_is_pling():
    assert convert(6) == "Pling"

def test_5_is_plang():
    assert convert(5) == "Plang"

def test_7_is_plong():
    assert convert(7) == "Plong"

def test_3_and_5_make_plingplang():  # not plangpling
    assert convert(30) == "PlingPlang"

def test_3_and_7_make_plingplong():  # not plongpling
    assert convert(42) == "PlingPlong"

def test_5_and_7_make_plangplong():  # not plongplang
    assert convert(70) == "PlangPlong"

def test_all_together_make_plingplangplong():
    assert convert(210) == "PlingPlangPlong"  # not any other order

# light bulletproofing:

def test_raising_error_with_non_integer_input():
    with pytest.raises(ValueError):
        assert False, "Failed to raise an error with a non-integer input!"
    assert True

# stuff we may want to consider okay, or not:

# we could possibly consider N.0 equivalent to N, but for now, no
def test_raising_error_with_int_valued_float():
    with pytest.raises(ValueError):
        assert False, "Failed to raise an error with a rounded float!"
    assert True

def test_works_with_zero():
    assert convert(0) == "PlingPlangPlong"

def test_works_with_negative_non_multiples():
    assert convert(-4) == "-4"

def test_works_with_negative_multiples_of_one_trigger():
    assert convert(-50) == "Plang"

def test_works_with_negative_multiples_of_two_triggers():
    assert convert(-42) == "PlingPlong"

def test_works_with_negative_multiples_of_all():
    assert convert(-210) == "PlingPlangPlong"

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