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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _group(self, group_data):
 """Return previously stored group or new group. Args: group_data (dict|obj):
 An Group dict or instance of Group object. Returns: dict|obj: The new Group dict/object or the previously stored dict/object. """

        if isinstance(group_data, dict):
            # get xid from dict
            xid = group_data.get('xid')
        else:
            # get xid from object
            xid = group_data.xid

        if self.groups.get(xid) is not None:
            # return existing group from memory
            group_data = self.groups.get(xid)
        elif self.groups_shelf.get(xid) is not None:
            # return existing group from shelf
            group_data = self.groups_shelf.get(xid)
        else:
            # store new group
            self.groups[xid] = group_data
        return group_data