code: def schema_key_for(self, seq_no: int) -> SchemaKey:
          
  
          LOGGER.debug('SchemaCache.schema_key_for >>> seq_no: %s', seq_no)
  
          rv = self._seq_no2schema_key.get(seq_no, None)
  
          LOGGER.debug('SchemaCache.schema_key_for <<< %s', rv)
          return rv
docs: Get schema key for schema by sequence number if known, None for no such schema in cache.
  
          :param seq_no: sequence number
          :return: corresponding schema key or None