docs: def setlistdefault(self, key, default_list=None):
          # type: (Hashable, List[Any]) -> List[Any]
          """
          Like `setdefault` but sets multiple values.  The list returned
          is not a copy, but the list that is actually used internally.  This
          means that you can put new values into the dict by appending items
          to the list:
          >>> d = MultiValueDict({"foo": 1})
          >>> d.setlistdefault("foo").extend([2, 3])
          >>> d.getlist("foo")
          [1, 2, 3]
  
          :param key: The key to be looked up.
          :param default_list: An iterable of default values.  It is either copied
                               (in case it was a list) or converted into a list
                               before returned.
          :return: a :class:`list`
  
          """
          if key not in self:
              default_list = list(default_list or ())
              dict.__setitem__(self, key, default_list)
          else:
              default_list = dict.__getitem__(self, key)
          return default_list