Allow database backends to skip tests and mark expected failures

[Tim Graham]

Currently Django's built-in database backends have special privileges in the test suite when it comes to test skipping such as:
tests/model_fields/test_integerfield.py: @unittest.skipIf(connection.vendor == 'sqlite', "SQLite doesn't have a constraint.")

While DatabaseFeatures attributes are generally preferred to connection.vendor checking, I think allowing backends to skip certain tests has its place. Adding features flags that are likely to apply to only a single database is a bit cumbersome, especially for third-party backends.

Django should provide more generic way to skip tests and mark expected failures. For example, see ​django-cockroachdb's logic which was inspired by ​django-mssql-backend. In a more refined version, expected_failures and skip_classes would be attributes of DatabaseFeatures.

That logic uses a RUNNING_COCKROACH_BACKEND_TESTS environment variable to decide whether or not the Django test suite is running. There might be a better way to determine that. At least the variable name would be more generic.

[Hasan Ramezani]

In a more refined version, expected_failures and skip_classes would be attributes of DatabaseFeatures.

What can we do to skip a single test? I checked the django-cockroachdb and there you have a skip_tests which can be used to skip a single test(If I got it right).

[Tim Graham]

Yes, skip_tests should also be a database feature attribute.

[Hasan Ramezani]

Hi Tim,

I created a ​PR based on django-cockroachdb example. Please check the PR if you have time and if you confirm it, I will continue with moving skip tests to database creation files.

I added expected_failures_tests and skip_tests(which can be used for skipping both test class and function) and added a mark_tests method similar to the one that you did for django-cockroachdb.

[Tim Graham]

I'll mark this as RFC when I'm done iterating with Hasan. (Meanwhile, let's leave "needs improvement" checked so it doesn't appear in the review queue for others.)

[GitHub <noreply@…>]

In 275dd4eb:

Fixed #32178 -- Allowed database backends to skip tests and mark expected failures.

Co-authored-by: Tim Graham <timograham@…>

[GitHub <noreply@…>]

In 3f140dde:

Refs #32178 -- Changed BaseDatabaseFeatures.django_test_expected_failures to set().

[GitHub <noreply@…>]

In 1e765311:

Refs #32178 -- Fixed test_mark_expected_failures_and_skips_call teardown.

Test isolation failure observed on CockroachDB and PostgreSQL.

[Chris Jerdonek]

I have a comment / question about this feature. I was analyzing a test by inspecting the code, and I couldn't figure out why it was passing on SQLite because it used to have "skip" code which is no longer there. And then I came across this ticket via the history.

Would it be possible to have at least a code convention of including a code comment at the test site when the test is included in django_test_skips? Otherwise, the average reader wouldn't know to check there for the test's name. (I would call this behavior "magical" because it's not explicit at the code site.) Also, even for people that know about it, it's not something that's easy to check while e.g. browsing the code on the web.

[Tim Graham]

I sympathize but would you give third-party backends the same privilege? If so, it may be better to scrap this feature and allow third-parties to contribute their skips directly in the test suite. If not, this feature could at least be documented in the contributing guide.

[Hasan Ramezani]

I've created a ​PR to add them to the contributing guide.

[GitHub <noreply@…>]

In ca34db46:

Refs #32178 -- Doc'd DatabaseFeatures.django_test_skips/django_test_expected_failures in contributing guide.

[Mariusz Felisiak <felisiak.mariusz@…>]

In ce130749:

[3.2.x] Refs #32178 -- Doc'd DatabaseFeatures.django_test_skips/django_test_expected_failures in contributing guide.

Backport of ca34db46504fca1221e27f6ab13734dfdfde6e1c from main