code: def modify_snapshot_attribute(self, snapshot_id,
                                    attribute='createVolumePermission',
                                    operation='add', user_ids=None, groups=None):
          
          params = {'SnapshotId' : snapshot_id,
                    'Attribute' : attribute,
                    'OperationType' : operation}
          if user_ids:
              self.build_list_params(params, user_ids, 'UserId')
          if groups:
              self.build_list_params(params, groups, 'UserGroup')
          return self.get_status('ModifySnapshotAttribute', params, verb='POST')
docs: Changes an attribute of an image.
  
          :type snapshot_id: string
          :param snapshot_id: The snapshot id you wish to change
  
          :type attribute: string
          :param attribute: The attribute you wish to change.  Valid values are:
                            createVolumePermission
  
          :type operation: string
          :param operation: Either add or remove (this is required for changing
                            snapshot ermissions)
  
          :type user_ids: list
          :param user_ids: The Amazon IDs of users to add/remove attributes
  
          :type groups: list
          :param groups: The groups to add/remove attributes.  The only valid
                         value at this time is 'all'.