code: def restore_default(self, key):
          
          default = self.default_values[key]
          dict.__setitem__(self, key, default)
          if key not in self.defaults:
              self.defaults.append(key)
          return default
docs: Restore (and return) default value for the specified key.
  
          This method will only work for a ConfigObj that was created
          with a configspec and has been validated.
  
          If there is no default value for this key, ``KeyError`` is raised.