Add a new context manager, Widget.hold_sync(), which holds any syncing until the manager exits

Sometimes for consistency or efficiency purposes, it makes sense to update
a group of properties all at once.  This context manager makes this possible.

We also fix a bug where send_state was not passing the key on to get_state.

IPython/html/widgets/widget.py

@@ -13,11 +13,12 b' in the IPython notebook front-end.'
 # Imports
 #-----------------------------------------------------------------------------
 from contextlib import contextmanager
+import collections
 
 from IPython.core.getipython import get_ipython
 from IPython.kernel.comm import Comm
 from IPython.config import LoggingConfigurable
-from IPython.utils.traitlets import Unicode, Dict, Instance, Bool, List, Tuple, Int
+from IPython.utils.traitlets import Unicode, Dict, Instance, Bool, List, Tuple, Int, Set
 from IPython.utils.py3compat import string_types
 
 #-----------------------------------------------------------------------------
@@ -110,7 +111,8 b' class Widget(LoggingConfigurable):'
         return [name for name in self.traits(sync=True)]
     
     _property_lock = Tuple((None, None))
-    
+    _send_state_lock = Int(0)
+    _states_to_send = Set(allow_none=False)
     _display_callbacks = Instance(CallbackDispatcher, ())
     _msg_callbacks = Instance(CallbackDispatcher, ())
     
@@ -174,12 +176,12 b' class Widget(LoggingConfigurable):'
 
         Parameters
         ----------
-        key : unicode (optional)
-            A single property's name to sync with the front-end.
+        key : unicode, or iterable (optional)
+            A single property's name or iterable of property names to sync with the front-end.
         """
         self._send({
             "method" : "update",
-            "state"  : self.get_state()
+            "state"  : self.get_state(key=key)
         })
 
     def get_state(self, key=None):
@@ -187,10 +189,17 b' class Widget(LoggingConfigurable):'
 
         Parameters
         ----------
-        key : unicode (optional)
-            A single property's name to get.
+        key : unicode or iterable (optional)
+            A single property's name or iterable of property names to get.
         """
-        keys = self.keys if key is None else [key]
+        if key is None:
+            keys = self.keys
+        elif isinstance(key, string_types):
+            keys = [key]
+        elif isinstance(key, collections.Iterable):
+            keys = key
+        else:
+            raise ValueError("key must be a string, an iterable of keys, or None")
         state = {}
         for k in keys:
             f = self.trait_metadata(k, 'to_json')
@@ -255,10 +264,26 b' class Widget(LoggingConfigurable):'
         finally:
             self._property_lock = (None, None)
 
+    @contextmanager
+    def hold_sync(self):
+        """Hold syncing any state until the context manager is released"""
+        # We increment a value so that this can be nested.  Syncing will happen when
+        # all levels have been released.
+        self._send_state_lock += 1
+        try:
+            yield
+        finally:
+            self._send_state_lock -=1
+            if self._send_state_lock == 0:
+                self.send_state(self._states_to_send)
+                self._states_to_send.clear()
+
     def _should_send_property(self, key, value):
         """Check the property lock (property_lock)"""
-        return key != self._property_lock[0] or \
-        value != self._property_lock[1]
+        if self._send_state_lock > 0:
+            self._states_to_send.add(key)
+            return False
+        return key != self._property_lock[0] or value != self._property_lock[1]
     
     # Event handlers
     @_show_traceback