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rootulp's solution

to Grains 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.

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. Two grains on the next. Four on the third, and so on.

There are 64 squares on a chessboard.

Write code that shows:

  • how many grains were on each square, and
  • the total number of grains

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 the appropriate command below (why they are different):

  • Python 2.7: py.test grains_test.py
  • Python 3.4+: pytest grains_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 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 the help page.


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 (

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

class GrainsTest(unittest.TestCase):
    def test_square_1(self):
        self.assertEqual(on_square(1), 1)

    def test_square_2(self):
        self.assertEqual(on_square(2), 2)

    def test_square_3(self):
        self.assertEqual(on_square(3), 4)

    def test_square_4(self):
        self.assertEqual(on_square(4), 8)

    def test_square_16(self):
        self.assertEqual(on_square(16), 32768)

    def test_square_32(self):
        self.assertEqual(on_square(32), 2147483648)

    def test_square_64(self):
        self.assertEqual(on_square(64), 9223372036854775808)

    def test_square_0_raises_exception(self):
        with self.assertRaisesWithMessage(ValueError):
        with self.assertRaisesWithMessage(ValueError):

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

    def test_square_gt_64_raises_exception(self):
        with self.assertRaisesWithMessage(ValueError):
        with self.assertRaisesWithMessage(ValueError):

    def test_total(self):
        self.assertEqual(total_after(64), 18446744073709551615)

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

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

if __name__ == '__main__':
def on_square(x):
    return 2 ** (x - 1)

def total_after(x):
    return 1 if x == 1 else on_square(x) + total_after(x - 1)

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What can you learn from this solution?

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