code: def expire(self, secs):
          
          self.add_field('exp',
                  lambda req: int(time.time() + secs))
docs: Adds the standard 'exp' field, used to prevent replay attacks.
  
          Adds the 'exp' field to the payload.  When a request is made,
          the field says that it should expire at now + `secs` seconds.
  
          Of course, this provides no protection unless the server reads
          and interprets this field.