code: def get_viewer(self, v_id, viewer_class=None, width=512, height=512,
                     force_new=False):
          
          if not force_new:
              try:
                  return self.viewers[v_id]
              except KeyError:
                  pass
  
          #  create top level window
          window = self.app.make_window("Viewer %s" % v_id, wid=v_id)
  
          # We get back a record with information about the viewer
          v_info = self.make_viewer(window, viewer_class=viewer_class,
                                    width=width, height=height)
  
          # Save it under this viewer id
          self.viewers[v_id] = v_info
          return v_info
docs: Get an existing viewer by viewer id.  If the viewer does not yet
          exist, make a new one.