test_empty_not_allowed_if_allow_empty_is_set_to_false emits deprecation warning in Python 3.9

[tirkarthi]

Checklist

• I have verified that that issue exists against the master branch of Django REST framework.
• I have searched for similar issues in both open and closed tickets and cannot find a duplicate.
• This is not a usage question. (Those should be directed to the discussion group instead.)
• This cannot be dealt with as a third party library. (We prefer new functionality to be in the form of third party libraries where possible.)
• I have reduced the issue to the simplest possible case.
• I have included a failing test as a pull request. (If you are unable to do so we can still accept the issue.)

Steps to reproduce

test_empty_not_allowed_if_allow_empty_is_set_to_false emits deprecation warning in Python 3.9 due to NotImplemented being used in boolean context which always evaluates to True

tests/test_serializer_nested.py::TestNestedSerializerWithMany::test_empty_not_allowed_if_allow_empty_is_set_to_false
  /root/django-rest-framework/rest_framework/exceptions.py:77: DeprecationWarning: NotImplemented should not be used in a boolean context
    return r and self.code == other.code

Expected behavior

Actual behavior

[tomchristie]

Believe this to be closed via #7531

[allanlewis]

I'm getting a similar error from __ne__ on 3.12.4, which includes the fix from #7531:

Traceback (most recent call last):
  File "/usr/local/lib/python3.9/unittest/mock.py", line 1336, in patched
    return func(*newargs, **newkeywargs)
  File "<my-code>", line <redacted>, in <redacted>
    assert_that(exc.exception.detail).is_equal_to(
  File "<redacted>/assertpy/base.py", line 125, in is_equal_to
    if self.val != other:
  File "<redacted>/rest_framework/exceptions.py", line 84, in __ne__
    return not self.__eq__(other)
DeprecationWarning: NotImplemented should not be used in a boolean context

So does the fix applied to __eq__ need to be applied to __ne__ as well? If we could backport any change to 3.12.x that would be very helpful 🙂

[tomchristie]

Seems valid to me.

Looks like we need to change from this…

    def __ne__(self, other):
        return not self.__eq__(other)

to this…

    def __ne__(self, other):
        r =  self.__eq__(other)
        if r is NotImplemented:
            return NotImplemented
        return not r

Would you be interested in submitting a pull request for this change?

[allanlewis]

@tomchristie sure, see #8538. I'm not sure how best to add a test to cover that case but I'd be happy to do so given some guidance.