code: def get_source_variable(self, source_id, variable):
          
  
          source_id = int(source_id)
          try:
              return self._retrieve_cached_source_variable(
                      source_id, variable)
          except UncachedVariable:
              return (yield from self._send_cmd("GET S[%d].%s" % (
                  source_id, variable)))
docs: Get the current value of a source variable. If the variable is not
          in the cache it will be retrieved from the controller.