Tell ligolw_sqlite to dump the contents of the database to a file.

def set_xml_output(self, xml_file):
    """
    Tell ligolw_sqlite to dump the contents of the database to a file.
    """
    if self.get_database() is None:
      raise ValueError, "no database specified"
    self.add_file_opt('extract', xml_file)
    self.__xml_output = xml_file