Our project also suffers extensively with mutating objects assigned from 
setUp, preventing us from moving most of our tests to setUpTestData. I'll 
likely begin using your pypi package right away, thanks Simon!

Backward compat issues are probably likely - but they'd be in test cases 
exclusively, making them extremely easy to find during an upgrade. That 
said, a deprecation warning is probably the most sensible path forward to 
prevent the need for immediate action.

Is there anyway to determine the pickle-ability of something without just 
trying to pickle it? I wouldn't be keen on that overhead. Could you just 
capture any copy exceptions, raise a deprecation warning, and abandon the 
copy for that attribute?

On Saturday, 24 November 2018 14:29:33 UTC+11, charettes wrote:
> Dear developers,
> Django 1.8 introduced the `TestCase.setUpTestData()` class method as a 
> mean to
> speed up test fixtures initialization as compared to using `setUp()`[0].
> As I've come to use this feature and review changes from peers using it in
> different projects the fact that test data assigned during its execution
> couldn't be safely altered by test methods without compromising test 
> isolation
> has often be the source of confusion and frustration.
> While the `setUpTestData` documentation mentions this limitation[1] and 
> ways to
> work around it by using `refresh_from_db()` in `setUp()` I believe it 
> defeats
> the whole purpose of the feature; avoiding unnecessary roundtrips to the
> database to speed up execution. Given `TestCase` goes through great 
> lengths to
> ensure database level data isolation I believe it should do the same with 
> class
> level in-memory data assigned during `setUpTestData`.
> In order to get rid of this caveat of the feature I'd like to propose an
> adjustment to ensure such in-memory test data isolation.
> What I suggest doing is wrapping all attributes assigned during 
> `setUpTestData`
> in descriptors that lazily return `copy.deepcopy()`ed values on instance
> attribute accesses. By attaching the `deepcopy()`'s memo on test instances 
> we
> can ensure that the reference graph between objects is preserved and thus
> backward compatible.
> In other words, the following test would pass even if `self.book` is a deep
> copy of `cls.book`.
> class BookTests(TestCase):
>     @classmethod
>     def setUpTestData(cls):
>         cls.author = Author.objects.create()
>         cls.book = cls.author.books.create()
>     def test_relationship_preserved(self):
>         self.assertIs(self.book.author, self.author)
> Lazily returning `deepcopy'ies and caching returned values in `__dict__` à 
> la
> `cached_property` should also make sure the slight performance overhead 
> this
> incurs is minimized.
> From a check against a few projects and Django's test suite[2] I have only
> identified a single issue which is that attributes assigned during
> `setUpTestData` would now have to be `deepcopy()`able but it shouldn't be
> a blocker given `Model` instance are.
> In order to allow other possible issues from being identified against 
> existing
> projects I packaged the proposed feature[3] and made it available on 
> pypi[4]. It
> requires decorating `setUpTestData` methods but it shouldn't be too hard to
> apply to your projects if you want to give it a try.
> Given this reaches consensus that this could be a great addition I'd file
> a ticket and finalize what I have so far[2].
> Thank your for your time,
> Simon
> [0] 
> https://docs.djangoproject.com/en/1.8/releases/1.8/#testcase-data-setup
> [1] 
> https://docs.djangoproject.com/en/2.1/topics/testing/tools/#django.test.TestCase.setUpTestData
> [2] 
> https://github.com/charettes/django/compare/setuptestdata...charettes:testdata
> [3] https://github.com/charettes/django-testdata
> [4] https://pypi.org/project/django-testdata/

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