Instances of callable classes with __slots__ attributes in settings causes a failure to draw the 500 template

[colons]

In a fresh django project, adding the following snippet to your settings files prevents the 500 page from rendering:

class Class(object):
    __slots__ = ['attr']

    def __init__(self, attr):
        self.attr = attr

    def __call__(self):
        pass

INSTANCE = Class(1)

The error that prevents the 500 error page from rendering is:

Traceback (most recent call last):
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/wsgiref/handlers.py", line 85, in run
    self.result = application(self.environ, self.start_response)
  File "venv/lib/python2.7/site-packages/django/contrib/staticfiles/handlers.py", line 64, in __call__
    return self.application(environ, start_response)
  File "venv/lib/python2.7/site-packages/django/core/handlers/wsgi.py", line 191, in __call__
    response = self.get_response(request)
  File "venv/lib/python2.7/site-packages/django/core/handlers/base.py", line 198, in get_response
    response = self.handle_uncaught_exception(request, resolver, sys.exc_info())
  File "venv/lib/python2.7/site-packages/django/core/handlers/base.py", line 241, in handle_uncaught_exception
    return debug.technical_500_response(request, *exc_info)
  File "venv/lib/python2.7/site-packages/django/views/debug.py", line 79, in technical_500_response
    html = reporter.get_traceback_html()
  File "venv/lib/python2.7/site-packages/django/views/debug.py", line 337, in get_traceback_html
    c = Context(self.get_traceback_data(), use_l10n=False)
  File "venv/lib/python2.7/site-packages/django/views/debug.py", line 315, in get_traceback_data
    'settings': get_safe_settings(),
  File "venv/lib/python2.7/site-packages/django/views/debug.py", line 65, in get_safe_settings
    settings_dict[k] = cleanse_setting(k, getattr(settings, k))
  File "venv/lib/python2.7/site-packages/django/views/debug.py", line 55, in cleanse_setting
    cleansed.do_not_call_in_templates = True
AttributeError: 'Class' object has no attribute 'do_not_call_in_templates

For me, this is a problem because I'm upgrading a project that stores what time of the week certain things happen with instances of dateutil's weekday object in the settings. Reproduced in 1.7c1 and current master, never happened in 1.6. I can probably work around it, but it'll be upsettingly inelegant.

[colons]

A project that exhibits this bug.

[Tim Graham]

Could you bisect Django's history to the commit that introduced the change?

[anonymous]

​https://github.com/django/django/commit/3c5cdaf47aae7e4f21398be1a5eaa07f7c5ce31c

Would it be unreasonable to have cleanse_setting return a repr() or a str() (or unicode()) of the settings rather than the things themselves? That way it has direct control over whether or not they are called and doesn't have to rely on attempting to set attributes on objects that could be a type for all it knows.

[anonymous]

Any object that cannot have the 'do_not_call_in_templates' attribute set on being set in settings will cause this issue, by the way; you can also cause it by using types or exceptions::

SCORE_NUMBER_TYPE = int
PASSABLE_EXCEPTION = ValueError

[Tim Graham]

CCing Baptiste since he committed the change. Perhaps he can offer guidance.

[Hiroki Kiyohara]

trying

[Hiroki Kiyohara]

screen shot with py2

[Hiroki Kiyohara]

screen shot with py3

[Hiroki Kiyohara]

I added patch to solve it.
On this change it solve by using force_text instead of adding do_not_call_in_template.

And also I added screen shots when the patch is applied.

[Baptiste Mispelon]

This means that the debug page gives the wrong information (ADDED_CALLABLE is a function object but the page says it's a string).

We need to find a different approach.

[Hiroki Kiyohara]

@bmispelon Thanks for the review
I fixed it and sent pull-request ​https://github.com/django/django/pull/2967.

Now the setting value appearing the debug page is valid.

[James Bennett <james@…>]

In 63058786882f6a32649a0fdafae9cb425c95cd4e:

Merge pull request #2967 from hirokiky/fix23070

Fixed #23070 -- not to break the debug page by callable setting with slots