docs: def initial_update(self, mapping):
          '''Initially fills the underlying dictionary with keys, values and
          timestamps.
          '''
          for key, val in mapping.items():
              _, timestamp = val
              if not self.TTL or (datetime.utcnow() -
                  datetime.utcfromtimestamp(timestamp) < self.TTL):
                  self.__setitem__(key, val, raw=True)