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to Flatten Array 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.

Take a nested list and return a single flattened list with all values except nil/null.

The challenge is to write a function that accepts an arbitrarily-deep nested list-like structure and returns a flattened structure without any nil/null values.

For Example

input: [1,[2,3,null,4],[null],5]

output: [1,2,3,4,5]

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 flatten_array_test.py
  • Python 3.4+: pytest flatten_array_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 flatten_array_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/flatten-array 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.


Interview Question https://reference.wolfram.com/language/ref/Flatten.html

Submitting Incomplete Solutions

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


import unittest

from flatten_array import flatten

# Tests adapted from `problem-specifications//canonical-data.json` @ v1.2.0

class FlattenArrayTest(unittest.TestCase):

    def test_no_nesting(self):
        self.assertEqual(flatten([0, 1, 2]), [0, 1, 2])

    def test_flatten_integers(self):
        inputs = [1, [2, 3, 4, 5, 6, 7], 8]
        expected = [1, 2, 3, 4, 5, 6, 7, 8]
        self.assertEqual(flatten(inputs), expected)

    def test_five_level_nesting(self):
        inputs = [0, 2, [[2, 3], 8, 100, 4, [[[50]]]], -2]
        expected = [0, 2, 2, 3, 8, 100, 4, 50, -2]
        self.assertEqual(flatten(inputs), expected)

    def test_six_level_nesting(self):
        inputs = [1, [2, [[3]], [4, [[5]]], 6, 7], 8]
        expected = [1, 2, 3, 4, 5, 6, 7, 8]
        self.assertEqual(flatten(inputs), expected)

    def test_with_none_values(self):
        inputs = [0, 2, [[2, 3], 8, [[100]], None, [[None]]], -2]
        expected = [0, 2, 2, 3, 8, 100, -2]
        self.assertEqual(flatten(inputs), expected)

    def test_all_values_are_none(self):
        inputs = [None, [[[None]]], None, None, [[None, None], None], None]
        expected = []
        self.assertEqual(flatten(inputs), expected)

    # Additional tests for this track
    def test_empty_nested_lists(self):
        self.assertEqual(flatten([[()]]), [])

    def test_strings(self):
        self.assertEqual(flatten(['0', ['1', '2']]), ['0', '1', '2'])

if __name__ == '__main__':
def flatten(nested_list):
    return [element for element in flatten_list(nested_list)]

def flatten_list(nested_list):
    for element in nested_list:
        if not isinstance(element, list):
            if element is not None and not isinstance(element, tuple):
                yield element
            for subelement in flatten_list(element):
                    yield subelement

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