Python function not callable from tavern script for saving

[ShreyanshAyanger-Nykaa]

Hi, I am calling a save function after getting response from my api, now in the response received i need to format string and save only few elements present,

       response:
            status_code: 200
            save:
                headers:
                    res_key:
                        $ext:
                            function: testing_utils:extract_sessid
                            extra_kwargs:
                                head: headers

however, my tavern yaml is unable to call extract_string method in testing_utils file,
But other functions written in testing_utils are working fine with following syntax

           verify_response_with:
               -   function: testing_utils:check_jsonpath_value

Please help, basically in above way mentioned, the testing_utils file is not accessible ( inside save function) but in same tavern script existing test cases are able to access the same ( with verify_response_with).