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

According to the django documentation, undefined variables are treated as ''(empty string) by default. While in if for regroup, it's None. If you are going to identify the variable undefined, change TEMPLATE_STRING_IF_INVALID in settings. '%s' makes the invalid variable to be rendered as its variable name, in this way, u can identify easily. how-invalid-variables-are-handled

Answer 2

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]"

Answer 3

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

Answer 4

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.

Answer 5

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

You can print this variable in view.

Answer 6

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.