code: def swap(self, key, items):
          
          if not isinstance(items, list):
              raise ValueError("items must be a list")
          return_value = []
          with self._lock:
              if key in self._dict:
                  return_value = self._dict[key]
              # Make a copy since we don't want users keeping a reference that is
              # outside the lock
              self._dict[key] = items.copy()
          return return_value
docs: Set key to a copy of items and return the list that was previously
          stored if the key was set. If not key was set, returns an empty list.