From f8cf09ab8ce64ed887bd4a38acdcc24c3552692f Mon Sep 17 00:00:00 2001
From: "J. Nick Koston" <nick@koston.org>
Date: Thu, 12 Dec 2024 14:57:05 +0100
Subject: [PATCH] Raise `TypeError` when setting `StreamResponse.last_modified`
 to an unsupported type (#10146)
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<!-- Thank you for your contribution! -->

## What do these changes do?

closes #10143

## Are there changes in behavior for the user?

Currently this would silently fail and discard the new value.

<img width="685" alt="Screenshot 2024-12-08 at 12 32 49 PM"
src="https://github.com/user-attachments/assets/b0cf00c3-cd2e-4be6-af94-d993491bd296">

(cherry picked from commit a818e51cbba7daf8c7cebd246d9a0ba214ebeba3)
---
 CHANGES/10146.misc.rst     | 1 +
 aiohttp/web_response.py    | 3 +++
 tests/test_web_response.py | 7 +++++++
 3 files changed, 11 insertions(+)
 create mode 100644 CHANGES/10146.misc.rst

diff --git a/CHANGES/10146.misc.rst b/CHANGES/10146.misc.rst
new file mode 100644
index 00000000000..bee4ef68fb3
--- /dev/null
+++ b/CHANGES/10146.misc.rst
@@ -0,0 +1 @@
+Setting :attr:`aiohttp.web.StreamResponse.last_modified` to an unsupported type will now raise :exc:`TypeError` instead of silently failing -- by :user:`bdraco`.
diff --git a/aiohttp/web_response.py b/aiohttp/web_response.py
index e498a905caf..e068b11f8e1 100644
--- a/aiohttp/web_response.py
+++ b/aiohttp/web_response.py
@@ -370,6 +370,9 @@ def last_modified(
             )
         elif isinstance(value, str):
             self._headers[hdrs.LAST_MODIFIED] = value
+        else:
+            msg = f"Unsupported type for last_modified: {type(value).__name__}"  # type: ignore[unreachable]
+            raise TypeError(msg)
 
     @property
     def etag(self) -> Optional[ETag]:
diff --git a/tests/test_web_response.py b/tests/test_web_response.py
index f4acf23f61b..2a3e09ae5c6 100644
--- a/tests/test_web_response.py
+++ b/tests/test_web_response.py
@@ -255,6 +255,13 @@ def test_last_modified_reset() -> None:
     assert resp.last_modified is None
 
 
+def test_last_modified_invalid_type() -> None:
+    resp = web.StreamResponse()
+
+    with pytest.raises(TypeError, match="Unsupported type for last_modified: object"):
+        resp.last_modified = object()  # type: ignore[assignment]
+
+
 @pytest.mark.parametrize(
     ["header_val", "expected"],
     [