django-template-partials and the templates panel do not work well together it seems

[matthiask]

Loading template sources crashes:

[19/Mar/2025 11:06:46] "GET /__debug__/render_panel/?store_id=f32af068303746c588911011cbf7da3f&panel_id=TemplatesPanel HTTP/1.1" 200 249515
Internal Server Error: /__debug__/template_source/
Traceback (most recent call last):
  File ".../.venv/lib/python3.12/site-packages/django/core/handlers/exception.py", line 55, in inner
    response = get_response(request)
               ^^^^^^^^^^^^^^^^^^^^^
  File ".../.venv/lib/python3.12/site-packages/django/core/handlers/base.py", line 197, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ".../projects/django-debug-toolbar/debug_toolbar/decorators.py", line 34, in inner
    return view(request, *args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ".../projects/django-debug-toolbar/debug_toolbar/decorators.py", line 46, in inner
    return view(request, *args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ".../projects/django-debug-toolbar/debug_toolbar/panels/templates/views.py", line 45, in template_source
    source = loader.get_contents(origin)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ".../.venv/lib/python3.12/site-packages/django/template/loaders/cached.py", line 27, in get_contents
    return origin.loader.get_contents(origin)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'get_contents'

(The strange debug toolbar paths are there because I use an editable install)

[tim-schilling]

I wonder if this is related to how django-cotton and django-template-partials don't play nicely together and require some extra setup. https://django-cotton.com/docs/django-template-partials

[matthiask]

We have some code in here to handle cached loaders:

django-debug-toolbar/debug_toolbar/panels/templates/views.py

Lines 32 to 40 in 0d7b859

	for loader in loaders:
	if loader is not None:
	# When the loader has loaders associated with it,
	# append those loaders to the list. This occurs with
	# django.template.loaders.cached.Loader
	if hasattr(loader, "loaders"):
	final_loaders += loader.loaders
	else:
	final_loaders.append(loader)

Template partials does more wrapping of loaders:
https://github.com/carltongibson/django-template-partials/blob/b0c0879b0cde74f152df85256b2619d937a4cfd8/src/template_partials/apps.py#L37

I suspect it has something to do with loaders not being determined correctly or something, but I don't really know. I spent some time on it and didn't find a quick fix (I instead switched to a different project to test the template panel which doesn't use template partials.)

[matthiask]

I have now worked on this a bit because it has bitten me again tonight.

The change works but lacks tests and docs.