merge_sort.py

"""
This is a pure python implementation of the merge sort algorithm

For doctests run following command:
python -m doctest -v merge_sort.py
or
python3 -m doctest -v merge_sort.py

For manual testing run:
python merge_sort.py
"""
from __future__ import print_function


def merge_sort(collection):
    """Pure implementation of the merge sort algorithm in Python

    :param collection: some mutable ordered collection with heterogeneous
    comparable items inside
    :return: the same collection ordered by ascending

    Examples:
    >>> merge_sort([0, 5, 3, 2, 2])
    [0, 2, 2, 3, 5]

    >>> merge_sort([])
    []

    >>> merge_sort([-2, -5, -45])
    [-45, -5, -2]
    """
    def merge(left, right):
        '''merge left and right
        :param left: left collection
        :param right: right collection
        :return: merge result
        '''
        result = []
        while left and right:
            result.append((left if left[0] <= right[0] else right).pop(0))
        return result + left + right
    if len(collection) <= 1:
        return collection
    mid = len(collection) // 2
    return merge(merge_sort(collection[:mid]), merge_sort(collection[mid:]))


if __name__ == '__main__':
    try:
        raw_input          # Python 2
    except NameError:
        raw_input = input  # Python 3

    user_input = raw_input('Enter numbers separated by a comma:\n').strip()
    unsorted = [int(item) for item in user_input.split(',')]
    print(*merge_sort(unsorted), sep=',')