Append item to the list at key. Creates the list at key if it
        doesn't exist.

def append(self, key, item):
        """Append item to the list at key. Creates the list at key if it
        doesn't exist.
        """
        with self._lock:
            if key in self._dict:
                self._dict[key].append(item)
            else:
                self._dict[key] = [item]