Simple, Clear Python Merge Sort

merge_sort.py

#! /usr/bin/env python3
import unittest

class MergeSort:
    '''Partition list in half, then sort each half and merge the halves.

    Merging does the actual sorting. It looks at the front of the left and right
    halves and picks the smallest value to append to the merged list.

    Time: O(n log n), memory: O(n)
    '''
    def sort(self, iterable):
        if len(iterable) == 1:
            return iterable
        left, right = self._partition(iterable)
        left = self.sort(left)
        right = self.sort(right)
        return self._merge(left, right)

    def _partition(self, iterable):
        split = len(iterable) // 2
        return iterable[:split], iterable[split:]

    def _merge(self, left, right):
        merged = []
        while left and right:
            if left[0] < right[0]:
                merged.append(left[0])
                del left[0]
            else:
                merged.append(right[0])
                del right[0]
        if left:
            merged.extend(left)
        elif right:
            merged.extend(right)
        return merged
      

class SortTests(unittest.TestCase):
    unsorted_list = [9, 6, 3, 3, 8, 1]
    sorted_list = [1, 3, 3, 6, 8, 9] 

    def test_merge_sort(self):
        sorter = MergeSort()
        self._basic_test(sorter.sort)

    def _basic_test(self, sort_method):
        self.assertEqual(
            sort_method(self.unsorted_list),
            self.sorted_list
        )
        

if __name__ == '__main__':
    unittest.main()