Restore backwards-compatibility for partial handlers.

Fix #1095.

docs/project/changelog.rst

@@ -38,6 +38,9 @@ Improvements
 Bug fixes
 .........
 
+* Fixed backwards-incompatibility in 10.1 for connection handlers created with
+  :func:`functools.partial`.
+
 * Avoided leaking open sockets when :func:`~client.connect` is canceled.
 
 10.1

src/websockets/legacy/server.py

@@ -1126,17 +1126,27 @@ def remove_path_argument(
         Callable[[WebSocketServerProtocol, str], Awaitable[Any]],
     ]
 ) -> Callable[[WebSocketServerProtocol], Awaitable[Any]]:
-    if len(inspect.signature(ws_handler).parameters) == 2:
-        # Enable deprecation warning and announce deprecation in 11.0.
-        # warnings.warn("remove second argument of ws_handler", DeprecationWarning)
+    try:
+        inspect.signature(ws_handler).bind(None)
+    except TypeError:
+        try:
+            inspect.signature(ws_handler).bind(None, "")
+        except TypeError:  # pragma: no cover
+            # ws_handler accepts neither one nor two arguments; leave it alone.
+            pass
+        else:
+            # ws_handler accepts two arguments; activate backwards compatibility.
+
+            # Enable deprecation warning and announce deprecation in 11.0.
+            # warnings.warn("remove second argument of ws_handler", DeprecationWarning)
 
-        async def _ws_handler(websocket: WebSocketServerProtocol) -> Any:
-            return await cast(
-                Callable[[WebSocketServerProtocol, str], Awaitable[Any]],
-                ws_handler,
-            )(websocket, websocket.path)
+            async def _ws_handler(websocket: WebSocketServerProtocol) -> Any:
+                return await cast(
+                    Callable[[WebSocketServerProtocol, str], Awaitable[Any]],
+                    ws_handler,
+                )(websocket, websocket.path)
 
-        return _ws_handler
+            return _ws_handler
 
     return cast(
         Callable[[WebSocketServerProtocol], Awaitable[Any]],

tests/legacy/test_client_server.py

@@ -497,6 +497,23 @@ async def handler_with_path(ws, path):
                     "/path",
                 )
 
+    def test_ws_handler_argument_backwards_compatibility_partial(self):
+        async def handler_with_path(ws, path, extra):
+            await ws.send(path)
+
+        bound_handler_with_path = functools.partial(handler_with_path, extra=None)
+
+        with self.temp_server(
+            handler=bound_handler_with_path,
+            # Enable deprecation warning and announce deprecation in 11.0.
+            # deprecation_warnings=["remove second argument of ws_handler"],
+        ):
+            with self.temp_client("/path"):
+                self.assertEqual(
+                    self.loop.run_until_complete(self.client.recv()),
+                    "/path",
+                )
+
     async def process_request_OK(path, request_headers):
         return http.HTTPStatus.OK, [], b"OK\n"