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.

def _FindFileContainingSymbolInDb(self, symbol):
    """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.
    """
    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)