Handle empty argument list for callbacks

Fixes #26

socketio/server.py

@@ -401,7 +401,9 @@ class Server(object):
         if id is not None:
             # send ACK packet with the response returned by the handler
             # tuples are expanded as multiple arguments
-            if isinstance(r, tuple):
+            if r is None:
+                data = []
+            elif isinstance(r, tuple):
                 data = list(r)
             else:
                 data = [r]

tests/test_server.py

@@ -306,6 +306,15 @@ class TestServer(unittest.TestCase):
         s.eio.send.assert_called_once_with('123', '31000["foo"]',
                                            binary=False)
 
+    def test_handle_event_with_ack_none(self, eio):
+        s = server.Server()
+        handler = mock.MagicMock(return_value=None)
+        s.on('my message', handler)
+        s._handle_eio_message('123', '21000["my message","foo"]')
+        handler.assert_called_once_with('123', 'foo')
+        s.eio.send.assert_called_once_with('123', '31000[]',
+                                           binary=False)
+
     def test_handle_event_with_ack_tuple(self, eio):
         mgr = mock.MagicMock()
         s = server.Server(client_manager=mgr)