TEMPLATE_STRING_IF_INVALID settable per template rendering

[Klaas van Schelven]

It would be nice if TEMPLATE_STRING_IF_INVALID is settable per template rendering.

I have created a version of this here:
​http://bitbucket.org/vanschelven/django/changeset/7cfe30a771b7

In this particular prototype the TEMPLATE_STRING_IF_INVALID is looked up in the context, as well as in the settings (in that order).
A greater reworking of the templating system would allow for
template.render(......, string_if_invalid="YOUR_STRING %s")

In some cases you want to be absolutely sure that whatever you tell the template to render either shows up, or generates an error.
This can also be used in combination with Exceptions on rendering by using a tool like this:

​http://bitbucket.org/vanschelven/django_invalid_template_exception/src/2c5772c3f21d/invalid_template_exception/__init__.py

In some cases (notably the admin) it is unfortunately currently impossible to use the TEMPLATE_STRING_IF_INVALID at all.
Having a per-template approach allows for more flexibility.

[mariarchi]

I think it's arguable whether this feature is a good idea. I would recommend raising a discussion on django-developers

[Luke Plant]

I don't think this is a good idea. TEMPLATE_STRING_IF_INVALID is clearly marked as being for debugging purposes only, and that is very unlikely to change. Re-usable apps often make the same assumption that the admin does.

The workaround for your use case is to make a unicode subclass like this:

class TemplateInvalidString(unicode):
    pass

TEMPLATE_STRING_IF_INVALID = TemplateInvalidString(u"")

You can then implement some template filter that detects TemplateInvalidString and raises an error or something. This allows for a much more fine-grained approach - you put a filter just on the variables that must be present, and not others.