How to use the multidict._multidict_py.MultiDictProxy function in multidict

def _extend(self, args, kwargs, name, method):
        if len(args) > 1:
            raise TypeError(
                "{} takes at most 1 positional argument"
                " ({} given)".format(name, len(args))
            )
        if args:
            arg = args[0]
            if isinstance(args[0], (MultiDict, MultiDictProxy)) and not kwargs:
                items = arg._impl._items
            else:
                if hasattr(arg, "items"):
                    arg = arg.items()
                if kwargs:
                    arg = list(arg)
                    arg.extend(list(kwargs.items()))
                items = []
                for item in arg:
                    if not len(item) == 2:
                        raise TypeError(
                            "{} takes either dict or list of (key, value) "
                            "tuples".format(name)
                        )
                    items.append((self._title(item[0]), self._key(item[0]), item[1]))

raise TypeError(
                "ctor requires MultiDict or MultiDictProxy instance"
                ", not {}".format(type(arg))
            )

        self._impl = arg._impl

    def __reduce__(self):
        raise TypeError("can't pickle {} objects".format(self.__class__.__name__))

    def copy(self):
        """Return a copy of itself."""
        return MultiDict(self.items())


class CIMultiDictProxy(MultiDictProxy):
    """Read-only proxy for CIMultiDict instance."""

    def __init__(self, arg):
        if not isinstance(arg, (CIMultiDict, CIMultiDictProxy)):
            raise TypeError(
                "ctor requires CIMultiDict or CIMultiDictProxy instance"
                ", not {}".format(type(arg))
            )

        self._impl = arg._impl

    def _title(self, key):
        return key.title()

    def copy(self):
        """Return a copy of itself."""