More explicit error message when redirected to a non websocket uri

src/websockets/asyncio/client.py

@@ -18,6 +18,7 @@
     InvalidProxyMessage,
     InvalidProxyStatus,
     InvalidStatus,
+    InvalidURI,
     ProxyError,
     SecurityError,
 )
@@ -493,7 +494,10 @@ def process_redirect(self, exc: Exception) -> Exception | str:
 
         old_ws_uri = parse_uri(self.uri)
         new_uri = urllib.parse.urljoin(self.uri, exc.response.headers["Location"])
-        new_ws_uri = parse_uri(new_uri)
+        try:
+            new_ws_uri = parse_uri(new_uri)
+        except InvalidURI as uri_exception:
+            raise InvalidURI("Redirection URI is invalid", uri_exception)
 
         # If connect() received a socket, it is closed and cannot be reused.
         if self.connection_kwargs.get("sock") is not None:

tests/asyncio/test_client.py

@@ -354,6 +354,25 @@ def redirect(connection, request):
             "cannot follow redirect to ws://invalid/ with a preexisting socket",
         )
 
+    async def test_not_a_websocket_redirect(self):
+        """Client raises an explicit error when redirected to an absolute uri that isn't using websocket protocole."""
+
+        def redirect(connection, request):
+            response = connection.respond(http.HTTPStatus.FOUND, "")
+            response.headers["Location"] = "https://not-a-websocket.com"
+            return response
+
+        async with serve(*args, process_request=redirect) as server:
+            host, port = get_host_port(server)
+            with self.assertRaises(InvalidURI) as raised:
+                async with connect("ws://overridden/", host=host, port=port):
+                    self.fail("did not raise")
+
+        self.assertEqual(
+            str(raised.exception),
+            "Redirection URI is invalid isn't a valid URI: https://not-a-websocket.com isn't a valid URI: scheme isn't ws or wss",
+        )
+
     async def test_invalid_uri(self):
         """Client receives an invalid URI."""
         with self.assertRaises(InvalidURI):