[Django]-Show undefined variable errors in Django templates?

Put this in your debug settings:

class InvalidString(str):
    def __mod__(self, other):
        from django.template.base import TemplateSyntaxError
        raise TemplateSyntaxError(
            "Undefined variable or unknown value for: \"%s\"" % other)

TEMPLATE_STRING_IF_INVALID = InvalidString("%s")

This should raise an error when the template engine sees or finds an undefined value.

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 ""

    def __bool__(self): # if using Python 2, use __nonzero__ instead
        # make the template tag `default` use its fallback value
        return False

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

Finding template variables that didn’t exist in the context was important to me as several times bugs made it into production because views had changed but templates had not.

I used this technique, implemented in manage.py, to achieve the effect of breaking tests when template variables not found in the context were used. Note that this technique works with for loops and if statements and not just {{ variables }}.

import sys

# sometimes it's OK if a variable is undefined:
allowed_undefined_variables = [
    'variable_1',
    'variable_2',
]

if 'test' in sys.argv:
    import django.template.base as template_base

    old_resolve = template_base.Variable.resolve

    def new_resolve(self, context):
        try:
            value = old_resolve(self, context)
        except template_base.VariableDoesNotExist as e:
            # if it's not a variable that's allowed to not exist then raise a
            # base Exception so Nodes can't catch it (which will make the test
            # fail)
            if self.var not in allowed_undefined_variables:
                raise Exception(e)

            # re-raise the original and let the individual Nodes deal with it
            # however they'd like
            raise e

        return value

    template_base.Variable.resolve = new_resolve

I believe that’s a major oversight on Django’s part and the primary reason I prefer not to use their default template engine. The sad truth is that, at least for now (Django 1.9), you can’t achieve this effect reliably.

• You can make Django raise an exception when {{ undefined_variable }} is encountered – by using “the hack” described in slacy’s answer.

• You can’t make Django raise the same exception on {% if undefined_variable %} or {% for x in undefined_variable %} etc. “The hack” doesn’t work in such cases.

• Even in cases in which you can, it is strongly discouraged by Django authors to use this technique in production environment. Unless you’re sure you don’t use Django’s built-in templates in your app, you should use “the hack” only in DEBUG mode.

However, if you’re stuck with Django’s templates for now, I would recommend to use slacy’s answer, just make sure you’re in DEBUG mode.

Read up on how invalid variable are handled in templates. Basically, just set TEMPLATE_STRING_IF_INVALID to something in your settings.py.

TEMPLATE_STRING_IF_INVALID = "He's dead Jim! [%s]"

I am use next:

import logging

from django.utils.html import format_html
from django.utils.safestring import mark_safe


class InvalidTemplateVariable(str):
    """
    Class for override output that the Django template system
    determinated as invalid (e.g. misspelled) variables.
    """

    # styles for display message in HTML`s pages
    styles = mark_safe('style="color: red; font-weight: bold;"')

    def __mod__(self, variable):
        """Overide a standart output here."""

        # access to current settings
        from django.conf import settings

        # display the message on page in make log it only on stage development
        if settings.DEBUG is True:

            # format message with captured variable
            msg = 'Attention! A variable "{}" does not exists.'.format(variable)

            # get logger and make
            logger = self.get_logger()
            logger.warning(msg)

            # mark text as non-escaped in HTML
            return format_html('<i {}>{}</i>', self.styles, msg)

        # on production it will be not displayed
        return ''

    def get_logger(self):
        """Create own logger with advanced error`s details."""

        logger = logging.getLogger(self.__class__.__name__)

        logger.setLevel(logging.DEBUG)

        handler = logging.StreamHandler()
        handler.setLevel(logging.DEBUG)

        formatter = logging.Formatter('%(asctime)s - %(name)s - %(levelname)s - %(message)s')

        handler.setFormatter(formatter)

        logger.addHandler(handler)

        return logger

Usage in settings file (by default it settings.py):

TEMPLATES = [
    {
        ......
        'OPTIONS': {
            .....................
            'string_if_invalid': InvalidTemplateVariable('%s'),
            .....................
        },

    },
]

or directly

TEMPLATES[0]['OPTIONS']['string_if_invalid'] = InvalidTemplateVariable('%s')

A result if DEBUG = True:

On page

In console

> System check identified 1 issue (0 silenced). October 03, 2016 -
> 12:21:40 Django version 1.10.1, using settings 'settings.development'
> Starting development server at http://127.0.0.1:8000/ Quit the server
> with CONTROL-C. 2016-10-03 12:21:44,472 - InvalidTemplateVariable -
> WARNING - Attention! A variable "form.media" does not exists.

You can use the pytest-django setting FAIL_INVALID_TEMPLATE_VARS

The invaild vars get checked if pytest executes the code.

[pytest]
DJANGO_SETTINGS_MODULE = mysite.settings
FAIL_INVALID_TEMPLATE_VARS = True

If there is a undefined variable in templates, django won’t tell you.

You can print this variable in view.