code: def _configure_from_mapping(self, item, whitelist_keys=False,
                                  whitelist=None):
          
          if whitelist is None:
              whitelist = self.config.keys()
  
          if whitelist_keys:
              item = {k: v for k, v in item.items() if k in whitelist}
  
          self.config.from_mapping(item)
  
          return self
docs: Configure from a mapping, or dict, like object.
  
          Args:
              item (dict):
                  A dict-like object that we can pluck values from.
  
          Keyword Args:
              whitelist_keys (bool):
                  Should we whitelist the keys before adding them to the
                  configuration? If no whitelist is provided, we use the
                  pre-existing config keys as a whitelist.
              whitelist (list[str]):
                  An explicit list of keys that should be allowed. If provided
                  and ``whitelist_keys`` is true, we will use that as our
                  whitelist instead of pre-existing app config keys.
  
          Returns:
              fleaker.App:
                  Returns itself.