Update merge_sort.py (#2356)

* Update merge_sort.py

* Update merge_sort.py

sorts/merge_sort.py

@@ -1,44 +1,40 @@
 """
 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
 """
 
 
-def merge_sort(collection):
+def merge_sort(collection: list) -> list:
     """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):
+    def merge(left: list, right: list) -> list:
         """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
+        def _merge():
+            while left and right:
+                yield (left if left[0] <= right[0] else right).pop(0)
+            yield from left
+            yield from right
+        return list(_merge())
 
     if len(collection) <= 1:
         return collection
@@ -47,6 +43,8 @@ def merge_sort(collection):
 
 
 if __name__ == "__main__":
+    import doctest
+    doctest.testmod()
     user_input = input("Enter numbers separated by a comma:\n").strip()
     unsorted = [int(item) for item in user_input.split(",")]
     print(*merge_sort(unsorted), sep=",")