TECHNICAL_404_TEMPLATE logs an "Exception while resolving variable" warning

[None]

There is template:

    <div style="{{ style }}"></div>

and code:

    render(request, 'template.html', context={'style': "background-image: url('image.png');"})

After run it I get some strange console output:

    Exception while resolving variable 'name' in template 'unknown'.
    Traceback (most recent call last):
    File ".../.virtualenvs/project/local/lib/python2.7/site-packages/django/template/base.py", line 905, in _resolve_lookup
    (bit, current))  # missing attribute
    VariableDoesNotExist: Failed lookup for key [name] in u'<RegexURLResolver <RegexURLPattern list> (admin:admin) ^admin/>'
    Exception while resolving variable 'name' in template 'unknown'.
    Traceback (most recent call last):
    File ".../.virtualenvs/project/local/lib/python2.7/site-packages/django/template/base.py", line 905, in _resolve_lookup
    (bit, current))  # missing attribute
    VariableDoesNotExist: Failed lookup for key [name] in u"<RegexURLResolver <module 'django.contrib.admindocs.urls' from '.../.virtualenvs/project/local/lib/python2.7/site-packages/django/contrib/admindocs/urls.pyc'> (None:None) ^admin/doc/>"
    ....
    etc.

[Tim Graham]

Could you please provide a sample project to reproduce the issue? That minimal view and template doesn't create that error if I add it to the tutorial's project so we're probably missing some steps to reproduce.

[nrogers64]

I wasn't the one to report this bug, but I ran into the same thing. I made a fresh Django 1.10.1 project and added this to settings.py:

LOGGING = {
    'version': 1,
    'disable_existing_loggers': False,
    'handlers': {
        'console': {
            'class': 'logging.StreamHandler',
        },
    },
    'loggers': {
        'django.template': {
            'handlers': ['console'],
            'level': 'DEBUG',
            'propagate': False,
        },
    }
}

That is the only code I added. I then did the initial migrate and started runserver. Then, in another Terminal tab, I ran curl http://localhost:8001/favicon.ico, which resulted in a "Page not found at /favicon.ico", as expected. But this was in the output of runserver:

Exception while resolving variable 'name' in template 'unknown'.
Traceback (most recent call last):
  File ".../.virtualenvs/project/local/lib/python2.7/site-packages/django/template/base.py", line 907, in _resolve_lookup
    (bit, current))  # missing attribute
VariableDoesNotExist: Failed lookup for key [name] in u'<RegexURLResolver <RegexURLPattern list> (admin:admin) ^admin/>'

[Tim Graham]

The error comes from the ​TECHNICAL_404_TEMPLATE: {% if forloop.last and pat.name %}[name='{{ pat.name }}']{% endif %}

There isn't a problem besides the confusing logging. The error occurs because RegexURLResolver doesn't have a name attribute so the exception is expected. Only a RegexURLPattern has a name in that loop. I'm not sure if there's a simple solution to avoid that exception logging (added in #18773).

[Tim Graham]

Accepting for further thought.

[Tim Abbott]

We were seeing this a lot in the Zulip development environment and found it pretty confusing for new contributors, so we worked around it using ​https://github.com/zulip/zulip/pull/4076.

[Tim Graham]

I started a thread on the ​django-developers mailing list to see if there might be consensus to remove logging of undefined variables.

[Sharmila G Sivakumar]

Hi,

I am another developer who has encountered the same issue. I perused the comments on the mailing list also to see how this would be handled. I agree with the sentiment that errors need to be raised as they are good indicators of underlying issues that need to be fixed. That is true even if, overall, the system works. On the other hand it would be great if the error can be identified for what it is - An known error raised by django internals and handled accordingly. Like the contributer Antony King suggested, a single line capturing the essence of the error would suffice

some_app/home.html:32:24: Undefined variable: missing_variable

The reason I am suggesting this is, I am using Django 1.11 and the Template_404's error messages are drowning out all other errors. The error is basically caused by favicon.ico resulting in a 404. In the process of development, favicons are the last priority. So, the developers might have to endure the error messages for a long time. Adding a temporary favicon is a possibility, but I would rather prefer a "GET /favicon.ico HTTP/1.1" 404 4296 message on the console to remind that favicon is not set :)

I am providing an example of an exception trace from a single favicon 404 error. May be I am doing something wrong too. If so please suggest an appropriate correction.

Exception while resolving variable 'name' in template 'unknown'.
Traceback (most recent call last):
  File "/home/code/product_recco/reddit_env/lib/python3.6/site-packages/django/core/handlers/exception.py", line 41, in inner
    response = get_response(request)
  File "/home/code/product_recco/reddit_env/lib/python3.6/site-packages/django/core/handlers/base.py", line 172, in _get_response
    resolver_match = resolver.resolve(request.path_info)
  File "/home/code/product_recco/reddit_env/lib/python3.6/site-packages/django/urls/resolvers.py", line 392, in resolve
    raise Resolver404({'tried': tried, 'path': new_path})
django.urls.exceptions.Resolver404: {'tried': [[<RegexURLResolver <module 'debug_toolbar.toolbar' from '/home/code/product_recco/reddit_env/lib/python3.6/site-packages/debug_toolbar/toolbar.py'> (djdt:djdt) ^__debug__/>], [<RegexURLResolver <RegexURLPattern list> (admin:admin) ^admin/>], [<RegexURLResolver <module 'userssay.urls' from '/home/code/product_recco/camerasforpros/src/recoapp/userssay/urls.py'> (None:None) ^>, <RegexURLPattern index ^$>], [<RegexURLResolver <module 'userssay.urls' from '/home/code/product_recco/camerasforpros/src/recoapp/userssay/urls.py'> (None:None) ^>, <RegexURLPattern subscribe ^subscribe$>], [<Re
gexURLResolver <module 'userssay.urls' from '/home/code/product_recco/camerasforpros/src/recoapp/userssay/
urls.py'> (None:None) ^>, <RegexURLPattern search ^search$>], [<RegexURLResolver <module 'userssay.urls' f
rom '/home/code/product_recco/camerasforpros/src/recoapp/userssay/urls.py'> (None:None) ^>, <RegexURLPatte
rn category_most_voted ^categories/(?P<cat_slug>[a-zA-Z0-9-]+)/most-voted/$>], [<RegexURLResolver <module 
'userssay.urls' from '/home/code/product_recco/camerasforpros/src/recoapp/userssay/urls.py'> (None:None) ^>, <RegexURLPattern category_most_recent ^categories/(?P<cat_slug>[a-zA-Z0-9-]+)/most-recent/$>], [<RegexU
RLResolver <module 'userssay.urls' from '/home/code/product_recco/camerasforpros/src/recoapp/userssay/urls
.py'> (None:None) ^>, <RegexURLPattern tag_most_voted ^tags/(?P<tag_slug>[a-zA-Z0-9-]+)/most-voted/$>], [<
RegexURLResolver <module 'userssay.urls' from '/home/code/product_recco/camerasforpros/src/recoapp/userssa
y/urls.py'> (None:None) ^>, <RegexURLPattern tag_most_recent ^tags/(?P<tag_slug>[a-zA-Z0-9-]+)/most-recent
/$>], [<RegexURLResolver <module 'userssay.urls' from '/home/code/product_recco/camerasforpros/src/recoapp
/userssay/urls.py'> (None:None) ^>, <RegexURLPattern get_product_info ^product/(?P<id_>[0-9]+)/(?P<slug>[a
-zA-Z0-9-_]+)/$>], [<RegexURLResolver <module 'userssay.urls' from '/home/code/product_recco/camerasforpros/src/recoapp/userssay/urls.py'> (None:None) ^>, <RegexURLPattern get_accessories ^accessories/(?P<slug>[a
-zA-Z0-9-_]+)/$>], [<RegexURLResolver <module 'userssay.urls' from '/home/code/product_recco/camerasforpro
s/src/recoapp/userssay/urls.py'> (None:None) ^>, <RegexURLPattern django.contrib.sitemaps.views.sitemap ^s
itemap\.xml$>], [<RegexURLPattern None ^static\/(?P<path>.*)$>]], 'path': 'favicon.ico'}

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/code/product_recco/reddit_env/lib/python3.6/site-packages/django/template/base.py", line 882
, in _resolve_lookup
    current = current[bit]
TypeError: 'RegexURLResolver' object is not subscriptable

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/code/product_recco/reddit_env/lib/python3.6/site-packages/django/template/base.py", line 890
, in _resolve_lookup
    current = getattr(current, bit)
AttributeError: 'RegexURLResolver' object has no attribute 'name'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/code/product_recco/reddit_env/lib/python3.6/site-packages/django/template/base.py", line 896, in _resolve_lookup
    current = current[int(bit)]
ValueError: invalid literal for int() with base 10: 'name'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/code/product_recco/reddit_env/lib/python3.6/site-packages/django/template/base.py", line 903, in _resolve_lookup
    (bit, current))  # missing attribute
django.template.base.VariableDoesNotExist: Failed lookup for key [name] in "<RegexURLResolver <module 'debug_toolbar.toolbar' from '/home/code/product_recco/reddit_env/lib/python3.6/site-packages/debug_toolbar/toolbar.py'> (djdt:djdt) ^__debug__/>"
Exception while resolving variable 'name' in template 'unknown'.
Traceback (most recent call last):
  File "/home/code/product_recco/reddit_env/lib/python3.6/site-packages/django/core/handlers/exception.py", line 41, in inner
    response = get_response(request)
  File "/home/code/product_recco/reddit_env/lib/python3.6/site-packages/django/core/handlers/base.py", line 172, in _get_response
    resolver_match = resolver.resolve(request.path_info)
  File "/home/code/product_recco/reddit_env/lib/python3.6/site-packages/django/urls/resolvers.py", line 392, in resolve
    raise Resolver404({'tried': tried, 'path': new_path})
django.urls.exceptions.Resolver404: {'tried': [.....................huge..list..of..urls..removed.........................], 'path': 'favicon.ico'}

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/code/product_recco/reddit_env/lib/python3.6/site-packages/django/template/base.py", line 882, in _resolve_lookup
    current = current[bit]
TypeError: 'RegexURLResolver' object is not subscriptable

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/code/product_recco/reddit_env/lib/python3.6/site-packages/django/template/base.py", line 890, in _resolve_lookup
    current = getattr(current, bit)
AttributeError: 'RegexURLResolver' object has no attribute 'name'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/code/product_recco/reddit_env/lib/python3.6/site-packages/django/template/base.py", line 896, in _resolve_lookup
    current = current[int(bit)]
ValueError: invalid literal for int() with base 10: 'name'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/code/product_recco/reddit_env/lib/python3.6/site-packages/django/template/base.py", line 903, in _resolve_lookup
    (bit, current))  # missing attribute
django.template.base.VariableDoesNotExist: Failed lookup for key [name] in '<RegexURLResolver <RegexURLPattern list> (admin:admin) ^admin/>'
Not Found: /favicon.ico
"GET /favicon.ico HTTP/1.1" 404 4296

[Tim Graham]

I created #28526 to address the problem of verbose, often unhelpful logging of undefined variables.