docs: def _retrieve_cached_source_variable(self, source_id, name):
          """
          Retrieves the cache state of the named variable for a particular
          source. If the variable has not been cached then the UncachedVariable
          exception is raised.
          """
          try:
              s = self._source_state[source_id][name.lower()]
              logger.debug("Source Cache retrieve S[%d].%s = %s",
                           source_id, name, s)
              return s
          except KeyError:
              raise UncachedVariable