Allow pickling dynamically created Model exception classes (DoesNotExist, MultipleObjectsReturned, RelatedObjectDoesNotExist)

[Rachel Tobin]

Example:

>>> import django
>>> import pickle
>>> pickle.dumps(django.contrib.auth.models.User.DoesNotExist)
>>> PicklingError: Can't pickle <class 'django.contrib.auth.models.DoesNotExist'>: it's not found as django.contrib.auth.models.DoesNotExist

*Instances* of Model.DoesNotExist are pickleable as fixed in #17776, however, the class itself is not. This is particularly problematic when using the Django test runner with the --parallel option enabled when a Model.DoesNotExist exception is raised. The result of this is the test runner bailing with a pickle.PicklingError, making it exceptionally difficult to debug the cause of the Model.DoesNotExist.

[Rachel Tobin]

I've created a patch for this: ​https://github.com/django/django/pull/9076

[Tim Graham <timograham@…>]

In 6c92f711:

Refs #28575 -- Allowed pickling Model.DoesNotExist and MultipleObjectsReturned classes.

[Tim Graham]

The ticket remains open to allow pickling RelatedObjectDoesNotExist which according to Simon, "will require a lot more work."

[Simon Charette]

​https://github.com/django/django/pull/9239

[Tim Graham <timograham@…>]

In 216eda1:

Refs #28575 -- Removed unnecessary code for model exception pickling.

Setting qualname is sufficient for pickling of DoesNotExist and
and MultipleObjectsReturned to work correctly.

[Tim Graham <timograham@…>]

In d4fb7420:

Refs #28575 -- Made RelatedObjectDoesNotExist classes pickable.

Thanks to Rachel Tobin for the initial qualname work and tests.

[Frankie Dintino]

Are there any plans to backport this patch to Django 1.11? Edit: never mind, I see that the fix involved qualname, so it will only work in python 3.