workaround for absence of comm_list_[request/reply]

Author: SylvainCorlay

ipywidgets/__init__.py

@@ -5,3 +5,13 @@
 ip = get_ipython()
 if ip is not None and hasattr(ip, 'kernel') and hasattr(ip.kernel, 'comm_manager'):
     ip.kernel.comm_manager.register_target('ipython.widget', Widget.handle_comm_opened)
+
+# Workaround for the absence ofa comm_list_[request/reply] shell message
+class CommList(Widget):
+    """CommList widgets are is typically instantiated by the front-end. As soon as it is instantiated, it sends the list of valid comms, and kills itself. It is a *very dumb* workaround to the absence of comm_list shell message.""" 
+    
+    def __init__(self, **kwargs):
+        super(CommList, self).__init__(**kwargs)
+        self.send(dict(comm_list=list(self.comm.kernel.comm_manager.comms.keys())))
+        self.close()
+

ipywidgets/static/widgets/js/manager.js

@@ -495,11 +495,28 @@ define([
         /**
          * Gets a promise for the list of comms open in the backend
          */
-        var that = this;
+
+        // Version using the comm_list_[request/reply] shell message.
+        /*var that = this;
         return new Promise(function(resolve, reject) {
              that.comm_manager.kernel.comm_list(function(msg) {
                  resolve(msg['content']['comm_list']);
              });
+        });*/
+
+        // Workaround for absence of comm_list_[request/reply] shell message.
+        // Create a new widget that gives the comm list and commits suicide.
+        var that = this;
+        return new Promise(function(resolve, reject) {
+            comm = that.comm_manager.new_comm('ipython.widget',
+                                {'widget_class': 'ipywidgets.CommList'},
+                                'comm_list');
+            comm.on_msg(function(msg) {
+                var data = msg.content.data;
+                if (data.content && data.method === 'custom') {
+                    resolve(data.content.comm_list);
+                }
+            });
         });
     };