Test suite fails if pytz is installed

[mgorny]

When running the test suite in an environment with pytz installed, I'm getting the following test failure:

$ python -m pytest -vv tests/test_fields.py::TestPytzNaiveDayLightSavingTimeTimeZoneDateTimeField::test_invalid_inputs
/tmp/django-rest-framework/.venv/lib/python3.10/site-packages/coreapi/utils.py:5: DeprecationWarning: pkg_resources is deprecated as an API. See https://setuptools.pypa.io/en/latest/pkg_resources.html
  import pkg_resources
max_value in DecimalField should be Decimal type.
min_value in DecimalField should be Decimal type.
========================================================= test session starts =========================================================
platform linux -- Python 3.10.14, pytest-7.4.4, pluggy-1.4.0 -- /tmp/django-rest-framework/.venv/bin/python
cachedir: .pytest_cache
django: version: 5.0.3
rootdir: /tmp/django-rest-framework
configfile: setup.cfg
plugins: django-4.8.0, cov-4.1.0
collected 1 item                                                                                                                      

tests/test_fields.py::TestPytzNaiveDayLightSavingTimeTimeZoneDateTimeField::test_invalid_inputs FAILED                          [100%]

============================================================== FAILURES ===============================================================
______________________________ TestPytzNaiveDayLightSavingTimeTimeZoneDateTimeField.test_invalid_inputs _______________________________
tests/test_fields.py:681: in test_invalid_inputs
    self.field.run_validation(input_value)
rest_framework/fields.py:540: in run_validation
    value = self.to_internal_value(data)
rest_framework/fields.py:1192: in to_internal_value
    return self.enforce_timezone(parsed)
rest_framework/fields.py:1170: in enforce_timezone
    raise e
rest_framework/fields.py:1164: in enforce_timezone
    if not valid_datetime(dt):
rest_framework/utils/timezone.py:23: in valid_datetime
    if isinstance(dt.tzinfo, tzinfo) and not datetime_ambiguous(dt):
rest_framework/utils/timezone.py:16: in datetime_ambiguous
    return datetime_exists(dt) and (
rest_framework/utils/timezone.py:9: in datetime_exists
    return dt.astimezone(timezone.utc) == dt
E   NotImplementedError: a tzinfo subclass must implement utcoffset()
========================================================== warnings summary ===========================================================
.venv/lib/python3.10/site-packages/_pytest/config/__init__.py:1373
  /tmp/django-rest-framework/.venv/lib/python3.10/site-packages/_pytest/config/__init__.py:1373: PytestConfigWarning: Unknown config option: testspath
  
    self._warn_or_fail_if_strict(f"Unknown config option: {key}\n")

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
======================================================= short test summary info =======================================================
FAILED tests/test_fields.py::TestPytzNaiveDayLightSavingTimeTimeZoneDateTimeField::test_invalid_inputs - NotImplementedError: a tzinfo subclass must implement utcoffset()
==================================================== 1 failed, 1 warning in 0.27s =====================================================

This is with 3.15.1 and with f4194c4.

pip list:

Package             Version
------------------- --------
asgiref             3.8.1
attrs               22.1.0
certifi             2024.2.2
charset-normalizer  3.3.2
coreapi             2.3.1
coreschema          0.0.4
coverage            7.4.4
Django              5.0.3
django-filter       24.1
django-guardian     2.4.0
djangorestframework 3.15.1
exceptiongroup      1.2.0
idna                3.6
importlib-metadata  4.13.0
inflection          0.5.1
iniconfig           2.0.0
itypes              1.2.0
Jinja2              3.1.3
Markdown            3.6
MarkupSafe          2.1.5
packaging           24.0
pip                 24.0
pluggy              1.4.0
psycopg2-binary     2.9.9
Pygments            2.13.0
pytest              7.4.4
pytest-cov          4.1.0
pytest-django       4.8.0
pytz                2024.1
PyYAML              5.3.1
requests            2.31.0
setuptools          69.2.0
sqlparse            0.4.4
tomli               2.0.1
typing_extensions   4.10.0
uritemplate         4.1.1
urllib3             2.2.1
zipp                3.18.1

[cclauss]

Yes, see the test results on Py311 and Py312 at:

• xfail pytz test_invalid_inputs() on Django >= v5.0  #9344

Dj5 drops support for pytz so we should skip this test on an invalid configuration.

https://docs.djangoproject.com/en/5.0/releases/5.0/#features-removed-in-5-0

• The USE_DEPRECATED_PYTZ transitional setting is removed.
• Support for pytz timezones is removed.

[tomchristie]

Dj5 drops support for pytz so we should skip this test on an invalid configuration.

Rather than dropping test cases, should we be ensuring that this function...

django-rest-framework/rest_framework/utils/timezone.py

Lines 4 to 9 in 085b7e1

	def datetime_exists(dt):
	"""Check if a datetime exists. Taken from: https://pytz-deprecation-shim.readthedocs.io/en/latest/migration.html"""
	# There are no non-existent times in UTC, and comparisons between
	# aware time zones always compare absolute times; if a datetime is
	# not equal to the same datetime represented in UTC, it is imaginary.
	return dt.astimezone(timezone.utc) == dt

Is handled in the compat module, and has behaviours that are correct with or without pytz installed?