code: def get_cached_source_variable(self, source_id, variable, default=None):
          
  
          source_id = int(source_id)
          try:
              return self._retrieve_cached_source_variable(
                      source_id, variable)
          except UncachedVariable:
              return default
docs: Get the cached value of a source variable. If the variable is not
          cached return the default value.