Feature Request: get_default_backend() in django.template.Engine

[Peter Thomassen]

Hi,

We're trying to create a template from string using 
DjangoTemplates.from_string() of the default template backend, which wraps the 
result of Engine.from_string() to construct an instance of the Template class 
defined in the backend definition file: django/template/backends/django.py#30

This Template class differs from the Engine-delivered Template in that it 
accepts a dictionary as the context, see 
https://docs.djangoproject.com/en/4.0/topics/templates/#django.template.backends.base.Template.render.

The default backend's engine is easily retrieved via 
django.template.Engine.get_default(). This function identifies the first 
configured DjangoTemplates backend, and then returns its .engine attribute. As 
a result, when from_string() is called, it will be called on the engine and not 
on the backend, thus the wrapping Template() call is missing.

I would like to propose adding a get_default_backend() function to 
django.template.Engine, which is like get_default(), but returns the backend 
itself (and not its .engine attribute). get_default() can then be adjusted to 
simply return get_default_backend().engine.

While trying to solve the issue (context argument type mismatch: dict vs 
Context), I found the docs not very helpful, as they are not very accurate in 
the distinction between a backend and its engine. I've also added a commit that 
clarifies that language.

I have submitted a patch here: https://github.com/django/django/pull/15944


FWIW, the problem can also be solved on the application layer, like:

def get_default_template_backend():
# Ad-hoc implementation of https://github.com/django/django/pull/15944
from django.template import engines
for backend in engines.all():
if isinstance(backend, DjangoTemplates):
return backend
raise ImproperlyConfigured("No DjangoTemplates backend is configured.")

However and IMHO, this needlessly duplicates the logic from get_default() and 
adds undue complication to the app (like having to know about and looping over 
the .all() iterable; raising the exception), whereas that complexity already is 
in the core.


Please let me know what you think!

Thanks,
Peter

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Re: Feature Request: get_default_backend() in django.template.Engine

[Peter Thomassen]

Hi Carlton,

On Mon, Aug 22, 2022 at 6:21 AM Carlton Gibson  wrote:
The current get_default() behaviour was only added to allow folks manually creating `Template("My String")` instances to continue to operate with multiple Django backends defined (#27359) — the initial assumption there was that anonymous backend usage wouldn't be supported, i.e. that folks would do `engines["django"]` to get a specific backend.  Could you use that? 


Yeah, that works! Thank you for the hint.

The docs mention this functionality with the wrong wording, which is why I was 
not aware of this functionality:


Template engines are available in django.template.engines:

from django.template import engines

django_engine = engines['django']

template = django_engine.from_string("Hello {{ name }}!")

-- 
https://docs.djangoproject.com/en/4.0/topics/templates/#django.template.loader.engines

This is not accurate: `engines` ironically holds the template backends, not the 
engines, and each engine can be retrieved by calling .engine (such as 
engines['django'].engine).

In general, the docs are very inconsistent in this regard and use the terms "backend" and 
"engine" pretty much interchangeably. The code is naturally more strict, but the iterable 
holding the backends nevertheless is called `engines`.

If people feel this is a correct assessment, I can try improving on the wording.

Thanks,
Peter

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