def process(self, tup):
<EOL>group_key = self.group_key(tup)<EOL>self._batches[group_key].append(tup)<EOL>
Group non-tick Tuples into batches by ``group_key``.

        .. warning::
            This method should **not** be overriden.  If you want to tweak
            how Tuples are grouped into batches, override ``group_key``.