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pierrefrncs's solution

to Resistor Color in the Python Track

Published at Oct 19 2020 · 0 comments
Test suite


This exercise has changed since this solution was written.

Resistors have color coded bands, where each color maps to a number. The first 2 bands of a resistor have a simple encoding scheme: each color maps to a single number.

These colors are encoded as follows:

  • Black: 0
  • Brown: 1
  • Red: 2
  • Orange: 3
  • Yellow: 4
  • Green: 5
  • Blue: 6
  • Violet: 7
  • Grey: 8
  • White: 9

Mnemonics map the colors to the numbers, that, when stored as an array, happen to map to their index in the array: Better Be Right Or Your Great Big Values Go Wrong.

More information on the color encoding of resistors can be found in the Electronic color code Wikipedia article

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 pytest resistor_color_test.py

Alternatively, you can tell Python to run the pytest module: python -m pytest resistor_color_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/resistor-color 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.


Maud de Vries, Erik Schierboom https://github.com/exercism/problem-specifications/issues/1458

Submitting Incomplete Solutions

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


import unittest

from resistor_color import color_code, colors

# Tests adapted from `problem-specifications//canonical-data.json` @ v1.0.0

class ResistorColorTest(unittest.TestCase):
    def test_black(self):
        self.assertEqual(color_code("black"), 0)

    def test_white(self):
        self.assertEqual(color_code("white"), 9)

    def test_orange(self):
        self.assertEqual(color_code("orange"), 3)

    def test_colors(self):
        expected = [
        self.assertEqual(colors(), expected)

if __name__ == "__main__":
color_dict = {"black": 0,
              "brown": 1,
              "red": 2,
              "orange": 3,
              "yellow": 4,
              "green": 5,
              "blue": 6,
              "violet": 7,
              "grey": 8,
              "white": 9}

def color_code(color):
    return color_dict[color]

def colors():
    return [str for str in color_dict.keys()]

What can you learn from this solution?

A huge amount can be learned from reading other people’s code. This is why we wanted to give exercism users the option of making their solutions public.

Here are some questions to help you reflect on this solution and learn the most from it.

  • What compromises have been made?
  • Are there new concepts here that you could read more about to improve your understanding?