code: def defconfig_filename(self):
          
          if self.defconfig_list:
              for filename, cond in self.defconfig_list.defaults:
                  if expr_value(cond):
                      try:
                          with self._open_config(filename.str_value) as f:
                              return f.name
                      except IOError:
                          continue
  
          return None
docs: See the class documentation.