code: def expires_at(self):
          
          try:
              expires = min(sig.key_expiration for sig in itertools.chain(iter(uid.selfsig for uid in self.userids), self.self_signatures)
                            if sig.key_expiration is not None)
  
          except ValueError:
              return None
  
          else:
              return (self.created + expires)
docs: A :py:obj:`~datetime.datetime` object of when this key is to be considered expired, if any. Otherwise, ``None``