code: def _FindFileContainingSymbolInDb(self, symbol):
      
      try:
        file_proto = self._internal_db.FindFileContainingSymbol(symbol)
      except KeyError as error:
        if self._descriptor_db:
          file_proto = self._descriptor_db.FindFileContainingSymbol(symbol)
        else:
          raise error
      if not file_proto:
        raise KeyError('Cannot find a file containing %s' % symbol)
      return self._ConvertFileProtoToFileDescriptor(file_proto)
docs: Finds the file in descriptor DB containing the specified symbol.
  
      Args:
        symbol: The name of the symbol to search for.
  
      Returns:
        A FileDescriptor that contains the specified symbol.
  
      Raises:
        KeyError: if the file cannot be found in the descriptor database.