def group_key(self, tup):
return None<EOL>
Return the group key used to group Tuples within a batch.

        By default, returns None, which put all Tuples in a single
        batch, effectively just time-based batching. Override this to create
        multiple batches based on a key.

        :param tup: the Tuple used to extract a group key
        :type tup: :class:`pystorm.component.Tuple`
        :returns: Any ``hashable`` value.