code: def get_viewer_names(self, dataobj):
          
          res = []
          for bnch in self.viewer_db.values():
              for vtype in bnch.vtypes:
                  if isinstance(dataobj, vtype):
                      res.append(bnch.vname)
          return res
docs: Returns a list of viewer names that are registered that
          can view `dataobj`.