bug#22703: 24.5; PROPOSAL: Is there a way to update available choices for defcustom variable?

[Oleksandr Gavenko <gavenkoa@gmail.com>]

Recently I ask question:

  http://emacs.stackexchange.com/questions/20350/defcustom-values-for-selecting-from-function-call

because I didn't understand show to build code from that I read in docs.

While answer to question help me to build my first "customize" code it miss
main point and I think because the way Emacs customize API works, so nobody
even think about doing as I asked.

I have values:

  (defvar my/python-checker-alist
    '((pylint . (my/python-pylint-command my/python-pylint-args))
      (pep8 . (my/python-pep8-command my/python-pep8-args))
      (pyflakes . (my/python-pyflakes-command my/python-pyflakes-args)))
    "Known Python source code checkers.")

and want to build `defcustom' definition based on that values:

  (choice (const . pylint) (const . pep8) (const . pyflakes))

  (defcustom my/python-default-checker 'pyflakes
    "Default Python source code checker. See `my/python-checker-alist' for full alist."
    :group 'my/python
    :type (cons 'choice (mapcar (lambda (e) (cons 'const (car e))) my/python-checker-alist)) )

I am worrying is there a mechanic that **automatically** update `:type' value when
user visit:

  M-x customize-group
  M-x customize-variable

with my variable?

Official info states:

  The argument of ‘:type’ is evaluated, but only once when the
  ‘defcustom’ is executed, so it isn’t useful for the value to vary.

If someone potentially extend my/python-checker-alist his addition would be
invisible in my/python-default-checker.

I thing it is wrong to ask someone not only to update possible values:

  (with-eval-after-load 'my/python
    (add-to-list 'my/python-checker-alist ...))

but also somehow to update default values (for my/python-default-checker)
synchronously.

Are there standard convention for such situation?

I see ":options" and "custom-add-frequent-value" but this is not useful when
":type (choice ...)" used. Also it cumbersome even in case of :options ask any
extension developers to call "custom-add-frequent-value" for their additions.

":set" function called after user see menu with pre-filled values while
desired situation when used see updated list of available options.

Solution to this request is to allow value of ":type" be a function which is
evaluated on   M-x customize-group /  M-x customize-variable.

-- 
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