Warn user about offset-naive object in last_modified property (aio-li…

…bs#5304)

CHANGES/5304.bugfix

@@ -0,0 +1 @@
+Warn user about offset-naive object in last_modified property.

aiohttp/web_response.py

@@ -262,6 +262,14 @@ def last_modified(
                 "%a, %d %b %Y %H:%M:%S GMT", time.gmtime(math.floor(value))
             )
         elif isinstance(value, datetime.datetime):
+            if not value.tzinfo:
+                warnings.warn(
+                    RuntimeWarning(
+                        "'last_modified' property received a offset-naive datetime. "
+                        "Please specify a timezone for your datetime object."
+                    )
+                )
+
             self._headers[hdrs.LAST_MODIFIED] = time.strftime(
                 "%a, %d %b %Y %H:%M:%S GMT", value.utctimetuple()
             )

tests/test_web_response.py

@@ -269,6 +269,13 @@ def test_last_modified_round_consistency() -> None:
     assert datetime_last_modified == float_last_modified
 
 
+def test_last_modified_tz_unaware_warning() -> None:
+    resp = StreamResponse()
+
+    with pytest.warns(RuntimeWarning):
+        resp.last_modified = datetime.datetime.now()
+
+
 async def test_start() -> None:
     req = make_request("GET", "/")
     resp = StreamResponse()