Improve error reports for exceptions in models.loading.load_app

[Henrik Vendelbo]

If you old attributes in your model fields or anything else that will raise an exception when trying to load the application models you may get very little error information. In my case all I got was the following:

$ ./manage.py test
Error: cannot import name settings

The exception message is correct, but it didn't really tell me anything useful. If I tried importing settings in a Python shell it worked fine. And if I tried to run any manage.py command, it would fail with the same error message. What was worse, was that I didn't know where the error was caught, so I was really stuck. The real problem is much more clear with a stack trace:

Traceback (most recent call last):
  File ".../django/core/management/base.py", line 88, in execute
    translation.activate('en-us')
  File ".../django/utils/translation/__init__.py", line 74, in activate
    return real_activate(language)
  File ".../django/utils/translation/__init__.py", line 44, in delayed_loader
    return g['real_%s' % caller](*args, **kwargs)
  File ".../django/utils/translation/trans_real.py", line 209, in activate
    _active[currentThread()] = translation(language)
  File ".../django/utils/translation/trans_real.py", line 198, in translation
    default_translation = _fetch(settings.LANGUAGE_CODE)
  File ".../django/utils/translation/trans_real.py", line 183, in _fetch
    app = __import__(appname, {}, {}, [])
  File ".../mailer/__init__.py", line 1, in <module>
    import mailer.admin
  File ".../mailer/admin.py", line 13, in <module>
    admin.site.register(Message, MessageAdmin)
  File ".../django/contrib/admin/sites.py", line 76, in register
    validate(admin_class, model)
  File ".../django/contrib/admin/validation.py", line 22, in validate
    models.get_apps()
  File ".../django/db/models/loading.py", line 103, in get_apps
    self._populate()
  File ".../django/db/models/loading.py", line 57, in _populate
    self.load_app(app_name, True)
  File ".../django/db/models/loading.py", line 73, in load_app
    mod = __import__(app_name, {}, {}, ['models'])
  File ".../retail/models.py", line 10, in <module>
    import tagging
  File ".../tagging/__init__.py", line 3, in <module>
    from tagging.managers import ModelTaggedItemManager, TagDescriptor
  File ".../tagging/managers.py", line 8, in <module>
    from tagging.models import Tag, TaggedItem
  File ".../tagging/models.py", line 16, in <module>
    from tagging import settings
ImportError: cannot import name settings

Outputting a stack trace is really too much information. The reason all commands fail is that the translation is changed to en-us when executing a command, so settings must be imported which in turn loads models.

I think that a more general improvement could be made, but I think it's more important to minimise the pain for people switching to 1.0

I propose a try-catch around the import call in load_app, which instead would have given me this output:

$ ./manage.py test
Error: Failed to load models for retail, cannot import name settings in ../tagging/models.py line 16

[Henrik Vendelbo]

load_app method with better error reporting

[Jacob]

Can you please upload a real patch (see the contributing doc for guidelines) so we can figure out what you're changing here?

[Luc Saffre]

hvendelbo, why do you say that "Outputting a stack trace is really too much information"?
Here is a patch that helped me to solve the same problem (import error caused by my code,
occuring during translation.activate('en-us') where import errors are captured).
I don't see why outputting the stack trace is really too much information.

[Aymeric Augustin]

Change UI/UX from NULL to False.

[Anssi Kääriäinen]

This seems to be fixed in master, you get an informative exception if your models.py contains invalid code.