How to use the bidict._miss._MISS function in bidict

assert oldval != val
            # We have to collapse nodefwd and nodeinv into a single node, i.e. drop one of them.
            # Drop nodeinv, so that the item with the same key is the one overwritten in place.
            nodeinv.prv.nxt = nodeinv.nxt
            nodeinv.nxt.prv = nodeinv.prv
            # Don't remove nodeinv's references to its neighbors since
            # if the update fails, we'll need them to undo this write.
            # Update fwdm and invm.
            tmp = fwdm.pop(oldkey)
            assert tmp is nodeinv
            tmp = invm.pop(oldval)
            assert tmp is nodefwd
            fwdm[key] = invm[val] = nodefwd
        elif isdupkey:
            oldval = invm.inverse[nodefwd]  # pylint: disable=no-member
            oldkey = _MISS
            oldnodeinv = invm.pop(oldval)
            assert oldnodeinv is nodefwd
            invm[val] = nodefwd
        else:  # isdupval
            oldkey = fwdm.inverse[nodeinv]  # pylint: disable=no-member
            oldval = _MISS
            oldnodefwd = fwdm.pop(oldkey)
            assert oldnodefwd is nodeinv
            fwdm[key] = nodeinv
        return _WriteResult(key, val, oldkey, oldval)

    def pop(self, key, default=_MISS):
        """*x.pop(k[, d]) → v*

        Remove specified key and return the corresponding value.

        :raises KeyError: if *key* is not found and no *default* is provided.
        """
        try:
            return self._pop(key)
        except KeyError:
            if default is _MISS:
                raise
            return default

def pop(self, key, default=_MISS):
        """*x.pop(k[, d]) → v*

        Remove specified key and return the corresponding value.

        :raises KeyError: if *key* is not found and no *default* is provided.
        """
        try:
            return self._pop(key)
        except KeyError:
            if default is _MISS:
                raise
            return default

# if the update fails, we'll need them to undo this write.
            # Update fwdm and invm.
            tmp = fwdm.pop(oldkey)
            assert tmp is nodeinv
            tmp = invm.pop(oldval)
            assert tmp is nodefwd
            fwdm[key] = invm[val] = nodefwd
        elif isdupkey:
            oldval = invm.inverse[nodefwd]  # pylint: disable=no-member
            oldkey = _MISS
            oldnodeinv = invm.pop(oldval)
            assert oldnodeinv is nodefwd
            invm[val] = nodefwd
        else:  # isdupval
            oldkey = fwdm.inverse[nodeinv]  # pylint: disable=no-member
            oldval = _MISS
            oldnodefwd = fwdm.pop(oldkey)
            assert oldnodefwd is nodeinv
            fwdm[key] = nodeinv
        return _WriteResult(key, val, oldkey, oldval)

def _write_item(self, key, val, dedup_result):  # pylint: disable=too-many-locals
        fwdm = self._fwdm  # bidict mapping keys to nodes
        invm = self._invm  # bidict mapping vals to nodes
        isdupkey, isdupval, nodeinv, nodefwd = dedup_result
        if not isdupkey and not isdupval:
            # No key or value duplication -> create and append a new node.
            sntl = self._sntl
            last = sntl.prv
            node = _Node(last, sntl)
            last.nxt = sntl.prv = fwdm[key] = invm[val] = node
            oldkey = oldval = _MISS
        elif isdupkey and isdupval:
            # Key and value duplication across two different nodes.
            assert nodefwd is not nodeinv
            oldval = invm.inverse[nodefwd]  # pylint: disable=no-member
            oldkey = fwdm.inverse[nodeinv]  # pylint: disable=no-member
            assert oldkey != key
            assert oldval != val
            # We have to collapse nodefwd and nodeinv into a single node, i.e. drop one of them.
            # Drop nodeinv, so that the item with the same key is the one overwritten in place.
            nodeinv.prv.nxt = nodeinv.nxt
            nodeinv.nxt.prv = nodeinv.prv
            # Don't remove nodeinv's references to its neighbors since
            # if the update fails, we'll need them to undo this write.
            # Update fwdm and invm.
            tmp = fwdm.pop(oldkey)
            assert tmp is nodeinv