Fix widget initialization

src/ypywidgets/comm.py

@@ -54,6 +54,8 @@ def __init__(
         msg = create_sync_message(ydoc)
         self._comm.send(buffers=[msg])
         self._comm.on_msg(self._receive)
+        ydoc.observe(self._bufferize)
+        self._updates: list[bytes] | None = []
 
     @property
     def awareness(self) -> Awareness:
@@ -67,15 +69,23 @@ def _receive(self, msg):
                 if reply is not None:
                     self._comm.send(buffers=[reply])
                 if message[1] == YSyncMessageType.SYNC_STEP2:
+                    assert self._updates is not None
+                    for update in self._updates:
+                        message = create_update_message(update)
+                        self._comm.send(buffers=[message])
+                    self._updates = None
                     self._ydoc.observe(self._send)
             case YMessageType.AWARENESS:
                 # Same as pycrdt.websocket.yroom: strip Y message kind, decode body.
                 update = read_message(message[1:])
                 self._awareness.apply_awareness_update(update, None)
 
+    def _bufferize(self, event: TransactionEvent):
+        if self._updates is not None:
+            self._updates.append(event.update)
+
     def _send(self, event: TransactionEvent):
-        update = event.update
-        message = create_update_message(update)
+        message = create_update_message(event.update)
         self._comm.send(buffers=[message])
 
 

tests/test_attributes.py

@@ -45,6 +45,7 @@ async def test_create_ydoc(synced_widgets, context):
 async def test_sync_attribute(widget_factories, synced_widgets, context):
     async with context:
         local_widget = await synced_widgets.get_local_widget()
+        local_widget.foo = "foo3"
         remote_widget = await synced_widgets.get_remote_widget()
 
         with pytest.raises(AttributeError):
@@ -53,8 +54,10 @@ async def test_sync_attribute(widget_factories, synced_widgets, context):
         with pytest.raises(AttributeError):
             assert remote_widget.wrong_attr2
 
+        await sleep(0.01)
+        assert remote_widget.foo == "foo3"
         local_widget.foo = "foo2"
-        assert remote_widget.foo is None  # not synced yet
+        assert remote_widget.foo == "foo3"  # not synced yet
         await sleep(0.01)  # wait for sync
         assert remote_widget.foo == "foo2"