Enable optional logging of invalid variable lookups

[wrigley]

Adding something like

# django/template/base.py
if getattr(settings,'TEMPLATE_LOG_IF_INVALID', False):
    logging.getLogger(settings.TEMPLATE_LOG_IF_INVALID).debug(
        "Failed lookup for key in [%s] in %r" %(bit,current))

around line 763, or whenever the silent_variable_failure is tested or something, would be awesome! That way there would be an easy way to check for invalid variables in the templates without compromising neither adminpages, layout or page reliability.

Then you could use something like:

# settings.py
TEMPLATE_LOG_IF_INVALID = 'invalid_log'

LOGGING = {
    'version': 1,
    'handlers': {
        'console': {
            'class': 'logger.StreamHandler',
            'level': 'DEBUG'
        }
    },
    'loggers': {
        'invalid_log': {
            'handlers': 'console',
            'level': 'DEBUG'
        }
    }
}

a loglevel of WARNING might be more appropriate, but I'll leave that decision up to you guys, seeing as I am new to django and don't know neither the code base nor the conventions too well yet.

Cheers,
/J

[Caroline Simpson]

I've submitted a pull request for this issue: ​https://github.com/django/django/pull/2552

Added a new logger, and log when there is an exception in the resolve function. The logger uses the null handler by default.

[Caroline Simpson]

The pull request has been updated to include the template name in the output.

[Tim Graham]

I left comments for improvement on the PR. Please uncheck "Patch needs improvement" when you update it, thanks.

[Caroline Simpson]

I have submitted a new pull request with the updates requested for the previous pull request.

[Caroline Simpson]

I've updated the pull request with fixes from review feedback.

[Tim Graham]

Reviewed PR & needs a rebase.

[Caroline Simpson]

I've updated the patch and done a rebase.

[Tim Graham]

Looks good, pending some minor cosmetic changes.

[Caroline Simpson]

The latest review changes are now incorporated into the patch as well.

[Tim Graham <timograham@…>]

In dc5b01a:

Fixed #18773 -- Added logging for template variable resolving

Added a django.template logger without a default handler. Added
logging if there is an exception while resolving variables in a
template.

[Tim Graham <timograham@…>]

In 85c52743:

Refs #18773 -- Improved template variable exception logging message.