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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 attach_intf_router(self, tenant_id, tenant_name, router_id):
 """Routine to attach the interface to the router. """

        in_sub = self.get_in_subnet_id(tenant_id)
        out_sub = self.get_out_subnet_id(tenant_id)
        # Modify Hard coded Name fixme
        subnet_lst = set()
        subnet_lst.add(in_sub)
        subnet_lst.add(out_sub)
        ret = self.os_helper.add_intf_router(router_id, tenant_id, subnet_lst)
        return ret, in_sub, out_sub