New built-in tag to disable invoking callable variables during template variable resolution

[Alex Epshteyn]

To improve upon the sub-optimal workaround introduced by #15791, I propose the following non-breaking change:

Proposal for new built-in tag callables [on|off]

Introduce a new built-in tag similar to ​autoescape, that would allow disabling the implicit invocation of callable variables in a particular template block. For example:

{% callables off %}
  <div>The class name is {{ foo.bar|type_name }}</div>
{% endcallables %}

Any variable resolutions performed inside this block would not implicitly invoke a callable during variable resolution unless the previous "bit" in the "lookups" path for this variable was an instance and the current "bit" is a method defined for the instance (see ​django.template.base.Variable._resolve_lookup).

My justification for this feature request is detailed in #30197 (specifically, this comment).

The introduction of this new callables [on|off] tag would be a non-breaking way to:

1. solve the frequently-reported problems described in #30197, #15791, #29382, and #29306
2. adhere to the fundamental ​Django design philosophies of ​Loose coupling (by moving template-specific concerns from the application layer to the template layer), ​Less code, and ​Explicit is better than implicit.

[Tim Graham]

As with #30197, please make your proposal on the DevelopersMailingList. I can't remember the last time a new template tag was added. I'm skeptical since "the template system is meant to express presentation, not program logic". Something like {% callables off %} doesn't seem aligned with that philosophy.

[Carlton Gibson]

...sub-optimal workaround...

You're always welcome to add such a tag in your own project, but I'm not convinced that do_not_call_in_templates is sub-optimal at all. It's simple, explicit, effective, etc. Having two ways to do things isn't necessary.

I'm inclined to close this as wontfix unless there is a consensus to change it. (Rather than having it open forever as a Someday/Maybe.)

[Tim Graham]

I bumped to someday/maybe merely so as not to appear overly aggressive following #30197 also closed as wontfix. If there's no follow up on django-developers within a week or two, I would close this ticket as wontfix rather than keep it open forever.

I haven't look into implementing the proposed tag, but my gut says it may not be possible to implement as a third-party tag without some hook in Django (i.e. how could a tag change the behavior of Variable._resolve_lookup()?)

[Carlton Gibson]

There’s always a way… But fine, OK, let’s leave it for now. As ever, the DevelopersMailingList is the place to go. Thanks Tim.

[Alex Epshteyn]

Replying to Tim Graham:

As with #30197, please make your proposal on the DevelopersMailingList. I can't remember the last time a new template tag was added. I'm skeptical since "the template system is meant to express presentation, not program logic". Something like {% callables off %} doesn't seem aligned with that philosophy.

That sounds reasonable. I just posted a new topic about this on the django-developers mailing list. Might need approval from a moderator on the Google Group for the post to go through.