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to Grains in the Python Track

Published at May 01 2021 · 0 comments
Test suite

Calculate the number of grains of wheat on a chessboard given that the number on each square doubles.

There once was a wise servant who saved the life of a prince. The king promised to pay whatever the servant could dream up. Knowing that the king loved chess, the servant told the king he would like to have grains of wheat. One grain on the first square of a chess board, with the number of grains doubling on each successive square.

There are 64 squares on a chessboard (where square 1 has one grain, square 2 has two grains, and so on).

Write code that shows:

  • how many grains were on a given square, and
  • the total number of grains on the chessboard

For bonus points

Did you get the tests passing and the code clean? If you want to, these are some additional things you could try:

  • Optimize for speed.
  • Optimize for readability.

Then please share your thoughts in a comment on the submission. Did this experiment make the code better? Worse? Did you learn anything from it?

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 grains_test.py

Alternatively, you can tell Python to run the pytest module: python -m pytest grains_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/grains 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.


JavaRanch Cattle Drive, exercise 6 http://www.javaranch.com/grains.jsp

Submitting Incomplete Solutions

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


import unittest

from grains import square, total

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

class GrainsTest(unittest.TestCase):
    def test_1(self):
        self.assertEqual(square(1), 1)

    def test_2(self):
        self.assertEqual(square(2), 2)

    def test_3(self):
        self.assertEqual(square(3), 4)

    def test_4(self):
        self.assertEqual(square(4), 8)

    def test_16(self):
        self.assertEqual(square(16), 32768)

    def test_32(self):
        self.assertEqual(square(32), 2147483648)

    def test_64(self):
        self.assertEqual(square(64), 9223372036854775808)

    def test_square_0_raises_an_exception(self):
        with self.assertRaisesWithMessage(ValueError):

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

    def test_square_greater_than_64_raises_an_exception(self):
        with self.assertRaisesWithMessage(ValueError):

    def test_returns_the_total_number_of_grains_on_the_board(self):
        self.assertEqual(total(), 18446744073709551615)

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

if __name__ == "__main__":
def square(number):
    if 1 <= number <= 64:
        return 2 ** (number-1)
    raise ValueError('constraint: 1 <= number <= 64')

def total():
    return 18446744073709551615

Community comments

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What can you learn from this solution?

A huge amount can be learned from reading other peopleโ€™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?