Save method of custom WagtailAdminPageForm called multiple times, even on page load: How to trigger custom save action only once on save?

I want to automatically generate field content based on another, editor-filled field for a wagtail page model on page save.

I followed the wagtail docs and I am able to populate and save a field programmatically on page save/publish:

from wagtail.models import Page
from wagtail.admin.forms import WagtailAdminPageForm

class TestPageForm(WagtailAdminPageForm):
    def clean(self):
        cleaned_data = super().clean()
        print("-> called TestPageForm.clean()")
        return cleaned_data

    def save(self, commit=True):
        print(f"1 {commit=}")
        page = super().save(commit=False)
        print(f"2 {commit=}")
        print("-> called TestPageForm.save()")
        # process something, save result to instance field, e.g.:
        # page.pdf_cover_image = get_cover_image(self.cleaned_data["pdf"])

        if commit:
            print(f"{commit=}: calling page.save()")
            page.save()

        return page


class TestPage(Page):
    base_form_class = TestPageForm

But the forms clean() and save() methods are called multiple times, even when I did not expect them to be called at all:

Edit page

Requesting the page instance via wagtail backend ("Edit page"):

http://127.0.0.1:8000/admin/pages/139/edit/
⇒

[10/May/2023 10:35:37] "GET /admin/pages/139/edit/ HTTP/1.1" 200 71082
[...assets...]
-> called TestPageForm.clean()
1 commit=False
2 commit=False
-> called TestPageForm.save()
[10/May/2023 10:35:37] "GET /admin/pages/139/edit/preview/?in_preview_panel=true&mode= HTTP/1.1" 200 0
[10/May/2023 10:35:37] "DELETE /admin/pages/139/edit/preview/ HTTP/1.1" 200 17
-> called TestPageForm.clean()
[10/May/2023 10:35:37] "POST /admin/pages/139/edit/preview/ HTTP/1.1" 200 40
-> called TestPageForm.clean()
1 commit=False
-> called TestPageForm.clean()
2 commit=False
-> called TestPageForm.save()
1 commit=False
2 commit=False
-> called TestPageForm.save()
[10/May/2023 10:35:37] "GET /admin/pages/139/edit/preview/?in_preview_panel=true&mode= HTTP/1.1" 200 0

Save page

[10/May/2023 10:35:37] "GET /admin/pages/139/edit/preview/?in_preview_panel=true&mode= HTTP/1.1" 200 0
-> called TestPageForm.clean()
1 commit=False
2 commit=False
-> called TestPageForm.save()
[10/May/2023 10:38:10] "POST /admin/pages/139/edit/ HTTP/1.1" 302 0
[10/May/2023 10:38:10] "GET /admin/pages/8/ HTTP/1.1" 200 45356

So on page save the forms clean() and save() methods are called only once - but as commit is always False I'm not able to differentiate between all these clean-and-save-calls...

So how do I trigger a custom save()-action which will only be called once and only on page save()?

If you're on Wagtail 4+ then the clean/save is being called by the preview panel regardless of whether it's visible (this shouldn't happen IMO but that's how it is for now).

I have some heavy field calculating code I needed to shift out of clean when I upgraded due to this. I went with the after create/edit hooks:

@hooks.register("after_edit_page")
@hooks.register("after_create_page")
def get_something(request, page):
    if page.specific_class == SomePage:
        try:
            page.something = calculate_something()
            if page.has_unpublished_changes:
                page.save_revision()
            else:
                page.save()
        except Exception as e:
            print(f"{type(e).__name__} at line {e.__traceback__.tb_lineno} of {__file__}: {e}")       
            messages.error(request, _('There was a problem generating the something'))

This will only get triggered when the editor clicks on save/save draft, and they will need to do this to see any rendered effect of changes made affecting this calculation in the live preview.