Enable widget instanciation from front-end.

IPython/html/static/widgets/js/manager.js

@@ -187,12 +187,47 b' define(['
 
     WidgetManager.prototype._handle_comm_open = function (comm, msg) {
         // Handle when a comm is opened.
+        return this.create_model({model_name: msg.content.data.target_name, comm: comm});
+    };
+
+    WidgetManager.prototype.create_model = function (model_name, target_name) {
+        // Create and return a new widget model.
+        //
+        // Parameters
+        // ----------
+        // model_name: string
+        //      Target name of the widget model to create.
+        // target_name: string
+        //      Target name of the widget in the back-end.
+        return this._create_model({model_name: model_name, target_name: target_name});
+    };
+
+    WidgetManager.prototype._create_model = function (options) {
+        // Create and return a new widget model.
+        //
+        // Parameters
+        // ----------
+        // options: dictionary
+        //  Dictionary of options with the following contents:
+        //      model_name: string
+        //          Target name of the widget model to create.
+        //      target_name: (optional) string
+        //          Target name of the widget in the back-end.
+        //      comm: (optional) Comm
+
+        // Create a comm if it wasn't provided.
+        var comm = options.comm;
+        if (!comm) {
+            comm = this.comm_manager.new_comm('ipython.widget', {'target_name': options.target_name});
+        }
+
+        // Create and return a new model that is connected to the comm.
         var that = this;
         
         var instantiate_model = function(ModelType) {
             var model_id = comm.comm_id;
             var widget_model = new ModelType(that, model_id, comm);
-            widget_model.on('comm:close', function () {
+            widget_model.on('comm:close', function () {sss
               delete that._models[model_id];
             });
             that._models[model_id] = widget_model;

IPython/html/widgets/widget.py

@@ -18,6 +18,7 b' import collections'
 from IPython.core.getipython import get_ipython
 from IPython.kernel.comm import Comm
 from IPython.config import LoggingConfigurable
+from IPython.utils.importstring import import_item
 from IPython.utils.traitlets import Unicode, Dict, Instance, Bool, List, \
     CaselessStrEnum, Tuple, CUnicode, Int, Set
 from IPython.utils.py3compat import string_types
@@ -95,6 +96,15 b' class Widget(LoggingConfigurable):'
         if Widget._widget_construction_callback is not None and callable(Widget._widget_construction_callback):
             Widget._widget_construction_callback(widget)
 
+    @staticmethod
+    def handle_comm_opened(comm, msg):
+        """Static method, called when a widget is constructed."""
+        target_name = msg['content']['data']['target_name']
+        widget_class = import_item(target_name)
+        widget = widget_class(open_comm=False)
+        widget.set_comm(comm)
+
+
     #-------------------------------------------------------------------------
     # Traits
     #-------------------------------------------------------------------------
@@ -125,12 +135,13 b' class Widget(LoggingConfigurable):'
     #-------------------------------------------------------------------------
     # (Con/de)structor
     #-------------------------------------------------------------------------
-    def __init__(self, **kwargs):
+    def __init__(self, open_comm=True, **kwargs):
         """Public constructor"""
         self._model_id = kwargs.pop('model_id', None)
         super(Widget, self).__init__(**kwargs)
 
         Widget._call_widget_constructed(self)
+        if open_comm:
         self.open()
 
     def __del__(self):
@@ -149,15 +160,19 b' class Widget(LoggingConfigurable):'
                               'model_module': self._model_module})
             if self._model_id is not None:
                 args['comm_id'] = self._model_id
-            self.comm = Comm(**args)
+            self.set_comm(Comm(**args))
+            
+            # first update
+            self.send_state()
+
+    def set_comm(self, comm):
+        """Set's the comm of the widget."""
+        self.comm = comm
             self._model_id = self.model_id
             
             self.comm.on_msg(self._handle_msg)
             Widget.widgets[self.model_id] = self
 
-            # first update
-            self.send_state()
-
     @property
     def model_id(self):
         """Gets the model id of this widget.

IPython/kernel/comm/manager.py

@@ -107,15 +107,21 b' class CommManager(LoggingConfigurable):'
                     iopub_socket=self.iopub_socket,
                     primary=False,
         )
+        self.register_comm(comm)
         if f is None:
             self.log.error("No such comm target registered: %s", target_name)
-            comm.close()
-            return
+        else:
         try:
             f(comm, msg)
+                return
         except Exception:
             self.log.error("Exception opening comm with target: %s", target_name, exc_info=True)
+        
+        # Failure.
+        try:
             comm.close()
+        except:
+            pass # Eat errors, nomnomnom
     
     def comm_msg(self, stream, ident, msg):
         """Handler for comm_msg messages"""