Set org-babel source block parameters dynamically

Question

I am trying to set the default header parameters for org source blocks in plantuml-mode.

My goal is to generate the result file in the org attachment directory with a generic name by default, and display it as an inline image in the result drawer.

This is my first attempt:

(setq org-babel-default-header-args:plantuml
         `((:results . "file link") (:output-dir . ,(org-attach-dir-get-create)) (:file-ext . "png") (:file . "diagram.png") (:exports . "results")))

This works fine the first time, but then, the org-attach-dir string is prepended to the file path string at every code block execution.

So for example, if org-attach-dir-get-create returns "mydir/", the first time I execute the block, the :file value is mydir/diagram.png as intended. The second time I execute the block, I get mydir/mydir/diagram.png, the third time, mydir/mydir/mydir/diagram.png, and so on.

Please note that I am keeping file-ext, file, and output-dir as separated parameters in the hope of being able to override them individually if needed in specific cases.

I understood from that failure that the header-defaults are set at each block execution, so I tried to pass them all as a function:

(defun my/org-babel-plantuml-header-args ()
  "Set dynamic header arguments for PlantUML source blocks."
  (let ((output-dir (org-attach-dir-get-create)))
    `((:results . "file link")
      (:output-dir . ,output-dir)
      (:file-ext . "png")
      (:file . "diagram.png")
      (:exports . "results"))))

  (setq org-babel-default-header-args:plantuml
         (my/org-babel-plantuml-header-args))

This has exactly the same outcome. It does not make sense to me, but my tests are very stubborn at proving me wrong.

I would appreciate some help in gettng it to work, because I used some of my most advanced Elisp tricks for that one :)

=== EDIT ===

After further testing, I jumped to the wrong conclusion when blaming this buggy behavior on the dynamic setting of the output directory.

I can reproduce it every time I set both output-dir and file within the org-babel-default-header-args variable even with a static string.

Regarding my initial goal, which was to have the diagram files created in the attachment directory, and displayed in the buffer, it is not possible to achieve using the header default arguments.

As @NickD commented, any function used there to set an option is only called once upon setting the org-babel-default-header-args:plantuml variable. This approach even had other adverse effects as I am using an org config file.

I managed to achieve the same results by using a custom function in a Yasnippet snippet to build the output-dir string. It works fine, just has a bit of a cluttered header.

Sorry if this thread is a bit all over the place. If future readers are looking to achieve the same kind of thing, I would gladly share the snippet code, and the custom hook function to set the attachment tag to the appropriate org node. I am not posting it because it uses Org-roam which is a niche use case I think, and I think it is off-topic considering how the thread started.

Answer 1

Instead of setting org-babel-header-args:plantuml once-and-for-all, you can use an infile property:

:PROPERTIES:
:header-args:plantuml: :results file link :file diagram.png :output-dir (org-attach-dir-get-create) :file-ext png
:END:

* plantuml diagram

#+BEGIN_SRC plantuml
@startuml
(*) --> "First Activity"
-->[You can put also labels] "Second Activity"
--> (*)
@enduml
#+END_SRC

* Another plantuml diagram

#+BEGIN_SRC plantuml :file diagram1.png
@startuml
(*) --> "Start"
-->[You can put also labels] "Continue"
--> (*)
@enduml
#+END_SRC

Evaluating each block evaluates the call to org-attach-dir-get-create which adds an ID property to the section where the block is located and does not cause any infinite concatenation problems, even though both output-dir and file header args are defined. Note also that the second block overrides the file header arg from the property with its own.

I don't know how to do the same thing with the variable yet.

UPDATE: As mentioned in the other answer, it was not possible to "do the same thing with the variable" because of a bug in Org mode. See the EDIT in my other answer for the patch, which has now (2024-10-10) been committed to the Org mode git repo. If you install Org mode from git, it should be possible to do what you wanted by modifying the variable as you did originally.

Answer 2

Try this setting:

#+begin_src elisp :results drawer
   (setq org-babel-default-header-args:plantuml
            `((:results . "file link")
              (:file-ext . "png")
              (:file . ,(concat (org-attach-dir-get-create) "/diagram.png"))
              (:exports . "results")))
#+end_src

For some reason, it seems that org-babel-get-src-block-info concatenates the value of the :output-dir property and the value of the :file property, modifying the value of the :file property in the process. So if both are set, you get the infinite concatenation you observe.

This feels like a bug to me, but I have not investigated enough to convince me one way or the other yet.

EDIT: I submitted a bug report and patch to the Org mode ML. UPDATE: The patch has now been committed to the Org mode git repo. It should be available in an update shortly.