From f605e86aeab7ac045c21e2eac0d677916031022e Mon Sep 17 00:00:00 2001
From: Aymeric Augustin <aymeric.augustin@m4x.org>
Date: Sun, 20 Feb 2022 21:28:28 +0100
Subject: [PATCH] Restore backwards-compatibility for partial handlers.

Fix #1095.
---
 docs/project/changelog.rst         |  3 +++
 src/websockets/legacy/server.py    | 28 +++++++++++++++++++---------
 tests/legacy/test_client_server.py | 17 +++++++++++++++++
 3 files changed, 39 insertions(+), 9 deletions(-)

diff --git a/docs/project/changelog.rst b/docs/project/changelog.rst
index 3c264a31b..b47ae2341 100644
--- a/docs/project/changelog.rst
+++ b/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
diff --git a/src/websockets/legacy/server.py b/src/websockets/legacy/server.py
index 8bc466a38..ea5b0d1fa 100644
--- a/src/websockets/legacy/server.py
+++ b/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]],
diff --git a/tests/legacy/test_client_server.py b/tests/legacy/test_client_server.py
index e6fb05d57..acc231d76 100644
--- a/tests/legacy/test_client_server.py
+++ b/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"