code: def pexpire(self, ttl=None):
          
          if ttl is not None:
              self.database.pexpire(self.key, ttl)
          else:
              self.database.persist(self.key)
docs: Expire the given key in the given number of milliseconds.
          If ``ttl`` is ``None``, then any expiry will be cleared
          and key will be persisted.