docs: def _settings_checker(self, required_settings=None, accept_none=True):
          """
          Take a list of required _settings dictionary keys
          and make sure they are set. This can be added to a custom
          constructor in a subclass and tested to see if it returns ``True``.
  
          :arg list required_settings: A list of required keys to look for.
          :arg bool accept_none: Boolean set to True if None is an acceptable
                                 setting. Set to False if None is not an
                                 acceptable setting.
  
          :returns: * bool ``True`` if all required settings exist, OR
                    * str <key name> for the first key missing from _settings.
          """
          if required_settings is not None:
              for keyname in required_settings:
                  if keyname not in self._settings:
                      return keyname
                  if accept_none is False and self._settings[keyname] is None:
                      return keyname
          return True