Push templates checks down to backend engine classes

[Adam Johnson]

Currently, the three system checks for template settings are individual functions in ​django.core.checks.templates. This structure leads to some issues:

1. The checks are specific to DTL (Django Template Language), but get run for all backends. DTL-specific code gets loaded and run even when not using it, notably the fairly slow get_template_tag_modules().
2. check_for_template_tags_with_the_same_name is inaccurate because it combines libraries from all template backends, so might report an issue when none exists.
3. The checks are still run when no template backend is configured (settings.TEMPLATES = []).

I propose the checks be restructured to live within the engine classes in django.template.backends, adopting the same pattern used for model and field checks, admin checks, etc.

In practice, this would mean adding a dummy BaseEngine.check() method, then moving the existing checks into methods called by an overridden DjangoTemplates.check(). A single function left in django.core.checks.templates would loop over django.template.engines.all() to combine results from backends’ check() methods.

[Adam Johnson]

The staticfiles finders checks are actually the perfect template:

​https://github.com/django/django/blob/35e63b7f2896959d3f1939346fd467af92f0d80f/django/contrib/staticfiles/checks.py#L11-L21
​https://github.com/django/django/blob/35e63b7f2896959d3f1939346fd467af92f0d80f/django/contrib/staticfiles/finders.py#L23-L27
​https://github.com/django/django/blob/35e63b7f2896959d3f1939346fd467af92f0d80f/django/contrib/staticfiles/finders.py#L74

[Giannis Terzopoulos]

Thank you for the pointers Adam! I finally found the time to look into this and I have some questions.

2. check_for_template_tags_with_the_same_name is inaccurate because it combines libraries from all template backends, so might report an issue when none exists.

Regarding this, am I right in assuming that we want to allow same names for tags across different engines; Should this test pass?

def test_different_backends_may_have_same_tags(self):
    with self.settings(
        TEMPLATES=[
            {"BACKEND": "django.template.backends.django.DjangoTemplates",
             "OPTIONS": {"libraries": {
                 "same_tags": "check_framework.template_test_apps.same_tags_app_1"
                              ".templatetags.same_tags"}}},
            {"BACKEND": "django.template.backends.OtherBackend",
             "OPTIONS": {"libraries": {
                 "same_tags": "check_framework.template_test_apps.same_tags_app_2"
                              ".templatetags.same_tags"}}}
        ]
    ):
        self.assertListEqual(check_for_template_tags_with_the_same_name(None), [])

And one more thing, I am noticing that the E001 logic is duplicated:

• in Engine.__init__ - raises ImproperlyConfigured (template_backends.test_utils.TemplateUtilsTests.test_backend_improperly_configured)
• and in check_setting_app_dirs_loaders (tests under check_framework.test_templates.CheckTemplateSettingsAppDirsTest)

and I am wondering if there are reasons to keep both, or we could remove one of the two? 🤔

[Adam Johnson]

Regarding this, am I right in assuming that we want to allow same names for tags across different engines; Should this test pass?

Yes, I think so. You can use two DjangoTemplates backends and it would still be valid.

And one more thing, I am noticing that the E001 logic is duplicated:

Indeed, that is true. The system check was added in #24922 because the exception would only appear at runtime, because the backend would only be created lazily. But by actioning this ticket, we’ll force backends to all be instantiated during checks, which will make the exception appear then. So we should be able to remove E001 as part of this work.

[Giannis Terzopoulos]

Thank you for the explanation. I opened a PR for this ​PR

[Mariusz Felisiak <felisiak.mariusz@…>]

In d658a31:

Fixed #35233 -- Moved template engine system checks to backend methods.

Thanks Adam Johnson for reviews.