code: def is_valid(self):
          
          if not isinstance(self._expiration, datetime.datetime):
              raise InvalidArgumentError('Expiration datetime must be specified.')
  
          if 'key' not in self.form_data:
              raise InvalidArgumentError('object key must be specified.')
  
          if 'bucket' not in self.form_data:
              raise InvalidArgumentError('bucket name must be specified.')
docs: Validate for required parameters.