Rendering fails with non-empty TEMPLATE_STRING_IF_INVALID

[jazzband]

Rendering fails with non-empty TEMPLATE_STRING_IF_INVALID

[gregmuellegger]

The reason seems to be a bug (or at least an unexpected behaviour on my side) in django. It is handling invalid values differently when they are assigend to a new value via "with", which makes them truthy if TEMPLATE_STRING_IF_INVALID is set:

In [16]: Template('{% with not_exist as value %}{% if value %}Hello {{ value }}{% endif %}{% endwith %}').render(Context())
Out[16]: u'Hello <INVALID>'

You can fix this in your project with a quick workaround. Put this in your settings.py:

class Invalid(unicode):
    def __nonzero__(self):
        return False

TEMPLATE_STRING_IF_INVALID = Invalid('<INVALID>')

However this will still break the textarea widget with floppyforms (as a quick test showed for me). But this should have been the same behaviour in older floppyforms versions. To fix this, create in your project a floppyforms/textarea.html file with the following content:

<textarea name="{{ name }}"{% if required %} required{% endif %}{% include "floppyforms/attrs.html" %}>{{ value|default:"" }}</textarea>

[gregmuellegger]

Django's behaviour is even documented here:
https://docs.djangoproject.com/en/dev/ref/templates/api/#invalid-template-variables

We provide documentation on that in floppyforms and the fix for the textarea.html template shortly.

[leon-matthews]

Thanks very much Gregor. Your help is very much appreciated!

I also found the behaviour of the 'with' tag in your explanation surprising -- although the question of what the 'with' tag should do in light of missing values is a difficult one when exceptions are verboten...

I had read the advice in the Django Docs about not using a value for TEMPLATE_STRING_IF_INVALID as a development default, but had understood it just as a backwards compatibility issue -- recent versions of the admin templates no longer require an empty value to work.