Changing TEMPLATE_STRING_IF_INVALID breaks Admin Interface display of model list

[robbie@…]

Model functions (as opposed to fields) used in "list_display" for the Admin interface lack the attribute "class_attrib". If TEMPLATE_STRING_IF_INVALID is changed from the default "" the new string is inserted at the end of the column's <th> tag, causing the model's list table to break on rendering.

Affects Admin interface template:

contrib/admin/templates/admin/change_list_results.html

[Russell Keith-Magee]

(In [3901]) Fixes #2384,#2566 -- Clarify the role that TEMPLATE_STRING_IF_INVALID plays in the template system, and the problems that can occur if it is used on a production site.

[Chris Wilson]

Well, that doesn't actually fix the problem, it just documents it.

In ​https://github.com/django/django/blob/master/django/contrib/admin/templatetags/admin_list.py#L109, result_headers yields a text block without a class_attrib value. But the template expects it to be present. We could just have it send an empty one.

[Luke Plant]

We basically gave up attempting to fix this bug. Because no-one ever uses non-default values of TEMPLATE_STRING_IF_INVALID in production, no-one ever catches bugs caused by non-default values, so the bug always comes back, in the same place or a different place. Attempting to fix it will just waste more time.

[Collin Anderson]

I would be interested in revisiting the policy on TEMPLATE_STRING_IF_INVALID. This is an optional setting that can make the template language much more strict. It has shown me lots of template bugs on my websites that I didn't realize existed, and I trust it enough to even use it in production, despite the recommendation. I would recommend to people to use it more, if only the django admin didn't rely on silent invalid variable errors.

We could use a non-default value in our admin tests to be sure these types of errors do not crop up again.

Is it worth fixing the admin? Yes. As far as I can tell, the admin is actually very close to being 100% free of relying on invalid variables. Our original policy on TEMPLATE_STRING_IF_INVALID was put in place back in the old-admin days, but things are better now. As far as I've seen on live websites it's really just this header.class_attrib problem. I have USE_I18N = False set, so the LANGUAGE_CODE variable is also invalid. On a live production website, those are the only two invalid variables I have run into.

It's easy to fix these errors, just wrap them in {% if header.class_attrib %}.

Here's what I use as my "string" in my settings. Note the workaround for this ticket:

class InvalidVar(object):
    def __mod__(self, s):
        assert s.var in ('LANGUAGE_CODE', 'header.class_attrib'), s.var
        return ''
    def __contains__(self, x):
        return x == '%s'
TEMPLATE_STRING_IF_INVALID = InvalidVar()