def METHOD_NAME(self, tgtkey):
    """Generate the group columns."""
    try:
        while self.currkey == tgtkey:
            value = self.currvalue[self.group_column]
            if value is not None:
                yield value
            self.currvalue = next(self.it)
            self.currkey = self.keyfunc(self.currvalue)
    except StopIteration:
        pass