modify existing transient - set some argument "active" by default

Question

I'm trying to modify a transient defined in python-pytest and add some suffix for pytest-django extension. I would like one of my arguments to be "active". It is similar to https://emacs.stackexchange.com/a/66462/13945, but I cannot modify the original transient's :value.

The following image should explain what I want (my stuff is in the group "Options for pytest-django"):

I define a variable that the user can customize and that will be the default for my suffix (and an helper function). Then I define the new argument that I would like to be set active by default.

My code is as follows:

(defcustom DJANGO_SETTINGS_MODULE nil
  "django settings module used by pytest."
  :type '(string)
  )

(defun python-pytest-django:--ds__init ()
  "Init value of --ds suffix"
  (concat "--ds " DJANGO_SETTINGS_MODULE))

(transient-define-argument python-pytest-django:--ds ()
  :description "Set DJANGO_SETTINGS_MODULE."
  :class 'transient-option
  :argument (python-pytest-django:--ds__init)
  ;; :init-value '(python-pytest-django:--ds__init)  ;; 🠮 transient-setup: Invalid function:...
  ;; :value '(python-pytest-django:--ds__init)  ;; 🠮 invalid-slot-name
  :allow-empty nil
  :key "--ds"
  :reader 'python-pytest--read-quoted-argument-for-short-flag)

(defun append-pytest-django-suffixes ()
  "Append options related to pytest-django to python-pytest transient."
  (transient-append-suffix 'python-pytest-dispatch '(-2)  ;; just before the last group
    ["Options for pytest-django"
     ("--rd" "reuse DB" "--reuse-db")
     ("--cd" "create DB" "--create-db")
     (python-pytest-django:--ds)
     ]))

(append-pytest-django-suffixes)

Answer 1

I was using the macro transient-define-argument wrong. As explained in the Developer Quick Start Guide, I can use :init-value to achieve my goal:

(transient-define-argument python-pytest-django:--ds ()
  :description "Set DJANGO_SETTINGS_MODULE."
  :class 'transient-option
  :argument "--ds "
  :init-value (lambda (obj)
                (setf
                 (slot-value obj 'value)
                 DJANGO_SETTINGS_MODULE))
  :allow-empty nil
  :key "--ds"
  )