From cd402bd5ca6b37c089dc7298a1a17e3b45c92ca4 Mon Sep 17 00:00:00 2001
From: AuroraAeon <auroraeon@users.noreply.github.com>
Date: Wed, 23 Sep 2026 17:24:16 +0800
Subject: [PATCH] types(unknown_sort): constrain items to Comparable

Bind merge_sort's element type to a Comparable Protocol so the signature
says "a list of items that can be compared with each other" instead of a
bare list, and keeps the element type in the return. start/end are now
annotated list[T] so the generic survives the local bindings.

Adds doctests for a comparable non-int type (strings, floats) and for the
failure mode: mixing non-comparable items must raise TypeError rather than
silently mis-sort.

The test battery picks the sort up for the shared cases and for the
rejection check. It is imported under an alias because sorts/merge_sort.py
already exports a merge_sort; the parametrize id therefore shows up as
merge_sort0/merge_sort1, the same way shrink_shell_sort already shares the
shell_sort id.
---
 sorts/unknown_sort.py | 22 ++++++++++++++++++++--
 tests/test_sorts.py   |  3 +++
 2 files changed, 23 insertions(+), 2 deletions(-)

diff --git a/sorts/unknown_sort.py b/sorts/unknown_sort.py
index 3545da68ea80..de9c1e73ad7f 100644
--- a/sorts/unknown_sort.py
+++ b/sorts/unknown_sort.py
@@ -5,8 +5,14 @@
 already O(n)
 """
 
+from typing import Any, Protocol
 
-def merge_sort(collection: list) -> list:
+
+class Comparable(Protocol):
+    def __lt__(self, other: Any, /) -> bool: ...
+
+
+def merge_sort[T: Comparable](collection: list[T]) -> list[T]:
     """Pure implementation of the fastest merge sort algorithm in Python
 
     :param collection: some mutable ordered collection with heterogeneous
@@ -22,8 +28,20 @@ def merge_sort(collection: list) -> list:
 
     >>> merge_sort([-2, -5, -45])
     [-45, -5, -2]
+
+    >>> merge_sort(["banana", "apple", "cherry"])
+    ['apple', 'banana', 'cherry']
+
+    >>> merge_sort([3.14, 1.5, 2.7])
+    [1.5, 2.7, 3.14]
+
+    >>> merge_sort([1, "a"])  # doctest: +ELLIPSIS
+    Traceback (most recent call last):
+    ...
+    TypeError: ...
     """
-    start, end = [], []
+    start: list[T] = []
+    end: list[T] = []
     while len(collection) > 1:
         min_one, max_one = min(collection), max(collection)
         start.append(min_one)
diff --git a/tests/test_sorts.py b/tests/test_sorts.py
index d18be1c22b83..6dadd6127ec0 100644
--- a/tests/test_sorts.py
+++ b/tests/test_sorts.py
@@ -49,6 +49,7 @@
 from sorts.shrink_shell_sort import shell_sort as shrink_shell_sort
 from sorts.stooge_sort import stooge_sort
 from sorts.strand_sort import strand_sort
+from sorts.unknown_sort import merge_sort as unknown_merge_sort
 
 
 def test_heap_sort() -> None:
@@ -85,6 +86,7 @@ def test_heap_sort() -> None:
     shrink_shell_sort,
     stooge_sort,
     strand_sort,
+    unknown_merge_sort,
 )
 
 
@@ -153,6 +155,7 @@ def test_rec_insertion_sort(case) -> None:
         selection_sort,
         shrink_shell_sort,
         strand_sort,
+        unknown_merge_sort,
     ],
     ids=lambda f: f.__name__,
 )