sorts: make quick_sort work with any Comparable, not just int

Use a Comparable Protocol + TypeVar bound so that quick_sort
correctly expresses it can sort any orderable type, not just
integers. Simplifies partitioning to cleaner lesser/equal/greater
lists and adds string + float doctest examples.

Part of #15234

Author: Pradhyut21

sorts/quick_sort.py

@@ -11,9 +11,17 @@
 from __future__ import annotations
 
 from random import randrange
+from typing import Any, Protocol, TypeVar
 
 
-def quick_sort(collection: list) -> list:
+class Comparable(Protocol):
+    def __lt__(self, other: Any, /) -> bool: ...
+
+
+T = TypeVar("T", bound=Comparable)
+
+
+def quick_sort(collection: list[T]) -> list[T]:
     """A pure Python implementation of quicksort algorithm.
 
     :param collection: a mutable collection of comparable items
@@ -26,27 +34,27 @@ def quick_sort(collection: list) -> list:
     []
     >>> quick_sort([-2, 5, 0, -45])
     [-45, -2, 0, 5]
+    >>> quick_sort(["z", "a", "m", "b"])
+    ['a', 'b', 'm', 'z']
+    >>> quick_sort([3.14, -1.0, 2.71])
+    [-1.0, 2.71, 3.14]
+    >>> quick_sort([0, 5, 3, 2, 2]) == sorted([0, 5, 3, 2, 2])
+    True
+    >>> quick_sort(["z", "a", "m"]) == sorted(["z", "a", "m"])
+    True
     """
     # Base case: if the collection has 0 or 1 elements, it is already sorted
     if len(collection) < 2:
         return collection
-
-    # Randomly select a pivot index and remove the pivot element from the collection
     pivot_index = randrange(len(collection))
-    pivot = collection.pop(pivot_index)
-
-    # Partition the remaining elements into two groups: lesser or equal, and greater
-    lesser = [item for item in collection if item <= pivot]
+    pivot = collection[pivot_index]
+    lesser = [item for item in collection if item < pivot]
+    equal = [item for item in collection if item == pivot]
     greater = [item for item in collection if item > pivot]
-
-    # Recursively sort the lesser and greater groups, and combine with the pivot
-    return [*quick_sort(lesser), pivot, *quick_sort(greater)]
+    return [*quick_sort(lesser), *equal, *quick_sort(greater)]
 
 
 if __name__ == "__main__":
-    # Get user input and convert it into a list of integers
     user_input = input("Enter numbers separated by a comma:\n").strip()
     unsorted = [int(item) for item in user_input.split(",")]
-
-    # Print the result of sorting the user-provided list
     print(quick_sort(unsorted))