Allow rendering of admin forms with Jinja2

[Joey Wilhelm]

As previously ​posted to the django-developers mailing list:

According to the documentation[1], "...django.contrib.admin doesn’t include Jinja2 templates for its widgets due to their usage of Django template tags."

After some digging, though, it appears that the "spaceless" tag is the only one in use which is not readily available in Jinja2. And this is only used in django/contrib/admin/templates/admin/widgets/related_widget_wrapper.html.

So, I was able to reimplement this template in Jinja2, minus the spaceless tag. On a few admin pages with large numbers of inlines, the load time was cut in half or better.

But, of course, there was one more small catch. I had to use the "TemplateSetting" form renderer rather than the "Jinja2" form renderer, because I had no way to alter the Jinja2 environment for the form renderer. The environment I refer to in my settings, however, is pulled almost verbatim from the documentation for the Jinja2 backend[2]. I only had to add one thing: i18n. Which turns out to be fairly simple to do with Jinja2 + Django. I was even able to use the "gettext" and "ngettext" from django.utils.translation.

So what I'm proposing here boils down to a few pieces:

1. Create jinja2 templates for the admin widgets. I already have one of these done, and the others look like it may be possible to simply copy them.
2. Document how to add i18n to the Jinja2 environment
3. Perhaps provide a default environment to Jinja2, providing both static and url, as currently documented in the example Jinja2 environment, along with i18n. This would provide a better out-of-the-box experience for users, including being able to set the FORM_RENDERER to Jinja2, and have the admin Just Work.

[Joey Wilhelm]

​PR here

[David Smith]

When reviewing ​PR14819 it was noticed that django.forms.utils.flatatt() is now only used once by AdminReadonlyField.label_tag() and could be removed as part of the fix for this ticket, see ​comment