Refine Shell Sort implementation and documentation

sorts/shell_sort.py

@@ -1,40 +1,42 @@
 """
+Shell sort implementation.
+
+Reference:
 https://en.wikipedia.org/wiki/Shellsort#Pseudocode
 """
 
 
 def shell_sort(collection: list[int]) -> list[int]:
-    """Pure implementation of shell sort algorithm in Python
-    :param collection:  Some mutable ordered collection with heterogeneous
-    comparable items inside
-    :return:  the same collection ordered by ascending
+    """
+    Sort a list of integers in ascending order using Shell sort.
+
+    :param collection: A list of integers to sort
+    :return: The same list sorted in ascending order
 
     >>> shell_sort([0, 5, 3, 2, 2])
     [0, 2, 2, 3, 5]
     >>> shell_sort([])
     []
     >>> shell_sort([-2, -5, -45])
     [-45, -5, -2]
+    >>> shell_sort([3, "a"])  # doctest: +IGNORE_EXCEPTION_DETAIL
+    Traceback (most recent call last):
+    TypeError
     """
-    # Marcin Ciura's gap sequence
-
     gaps = [701, 301, 132, 57, 23, 10, 4, 1]
+
     for gap in gaps:
-        for i in range(gap, len(collection)):
-            insert_value = collection[i]
-            j = i
-            while j >= gap and collection[j - gap] > insert_value:
-                collection[j] = collection[j - gap]
-                j -= gap
-            if j != i:
-                collection[j] = insert_value
-    return collection
-
-
-if __name__ == "__main__":
-    from doctest import testmod
-
-    testmod()
-    user_input = input("Enter numbers separated by a comma:\n").strip()
-    unsorted = [int(item) for item in user_input.split(",")]
-    print(shell_sort(unsorted))
+        for index in range(gap, len(collection)):
+            insert_value = collection[index]
+            current_index = index
+
+            while (
+                current_index >= gap
+                and collection[current_index - gap] > insert_value
+            ):
+                collection[current_index] = collection[current_index - gap]
+                current_index -= gap
+
+            collection[current_index] = insert_value
+
+    return collection