Show undefined variable errors in Django templates?

Question

How can I ask Django to tell me when it encounters, for example, an undefined variable error while it\'s rendering templates?

I\'ve tried the obvious DEBUG = T

Answer 1

How to log a warning on undefined variable in a template

It seems that Django relies on undefined variables being a simple empty string. So instead of changing this behaviour or making it throw an exception, let's keep it the same but have it log a warning instead!

In your settings.py file:

TEMPLATES = [
    {
        'BACKEND': 'django.template.backends.django.DjangoTemplates',
        # ...
        'OPTIONS': {
            # ...
            'string_if_invalid': InvalidStringShowWarning("%s"),
        },
    }
]

(string_if_invalid replaces TEMPLATE_STRING_IF_INVALID in newer Django versions.)

And further up, you'll need to define the InvalidStringShowWarning class, making it behave while logging a warning:

class InvalidStringShowWarning(str):
    def __mod__(self, other):
        import logging
        logger = logging.getLogger(__name__)
        logger.warning("In template, undefined variable or unknown value for: '%s'" % (other,))
        return ""

You should be able to see the warning in the output of python manage.py runserver.