Fix DateTimeField TZ handling (#5435)

* Add failing TZ tests for DateTimeField

- tests "current" timezone activation
- tests output for non-UTC timezones

* Update DateTimeField TZ aware/naive test output

* Fix DateTimeField TZ handling

* Add Release Note for BC change

docs/topics/release-notes.md

@@ -44,7 +44,11 @@ You can determine your currently installed version using `pip freeze`:
 
 * Fix `DjangoModelPermissions` to ensure user authentication before calling the view's `get_queryset()` method. As a side effect, this changes the order of the HTTP method permissions and authentication checks, and 405 responses will only be returned when authenticated. If you want to replicate the old behavior, see the PR for details. [#5376][gh5376]
 * Deprecated `exclude_from_schema` on `APIView` and `api_view` decorator. Set `schema = None` or `@schema(None)` as appropriate. [#5422][gh5422]
+* Timezone-aware `DateTimeField`s now respect active or default) `timezone` during serialization, instead of always using UTC.
 
+    Resolves inconsistency whereby instances were serialised with supplied datetime for `create` but UTC for `retrieve`. [#3732][gh3732]
+
+    **Possible backwards compatibility break** if you were relying on datetime strings being UTC. Have client interpret datetimes or [set default or active timezone (docs)][djangodocs-set-timezone] to UTC if needed.
 
 ### 3.6.4
 
@@ -1426,3 +1430,6 @@ For older release notes, [please see the version 2.x documentation][old-release-
 <!-- 3.6.5 -->
 [gh5376]: https://github.com/encode/django-rest-framework/issues/5376
 [gh5422]: https://github.com/encode/django-rest-framework/issues/5422
+[gh5408]: https://github.com/encode/django-rest-framework/issues/5408
+[gh3732]: https://github.com/encode/django-rest-framework/issues/3732
+[djangodocs-set-timezone]: https://docs.djangoproject.com/en/1.11/topics/i18n/timezones/#default-time-zone-and-current-time-zone

requirements/requirements-optionals.txt

 # Optional packages which may be used with REST framework.
+pytz==2017.2
 markdown==2.6.4
 django-guardian==1.4.8
 django-filter==1.0.4

rest_framework/fields.py

@@ -1125,7 +1125,9 @@ class DateTimeField(Field):
         """
         field_timezone = getattr(self, 'timezone', self.default_timezone())
 
-        if (field_timezone is not None) and not timezone.is_aware(value):
+        if field_timezone is not None:
+            if timezone.is_aware(value):
+                return value.astimezone(field_timezone)
             try:
                 return timezone.make_aware(value, field_timezone)
             except InvalidTimeError:
@@ -1135,7 +1137,7 @@ class DateTimeField(Field):
         return value
 
     def default_timezone(self):
-        return timezone.get_default_timezone() if settings.USE_TZ else None
+        return timezone.get_current_timezone() if settings.USE_TZ else None
 
     def to_internal_value(self, value):
         input_formats = getattr(self, 'input_formats', api_settings.DATETIME_INPUT_FORMATS)
@@ -1174,6 +1176,7 @@ class DateTimeField(Field):
             return value
 
         if output_format.lower() == ISO_8601:
+            value = self.enforce_timezone(value)
             value = value.isoformat()
             if value.endswith('+00:00'):
                 value = value[:-6] + 'Z'

tests/test_fields.py

@@ -10,13 +10,18 @@ import pytest
 from django.http import QueryDict
 from django.test import TestCase, override_settings
 from django.utils import six
-from django.utils.timezone import utc
+from django.utils.timezone import activate, deactivate, utc
 
 import rest_framework
 from rest_framework import compat, serializers
 from rest_framework.fields import is_simple_callable
 
 try:
+    import pytz
+except ImportError:
+    pytz = None
+
+try:
     import typings
 except ImportError:
     typings = False
@@ -1168,7 +1173,7 @@ class TestDateTimeField(FieldValues):
         datetime.date(2001, 1, 1): ['Expected a datetime but got a date.'],
     }
     outputs = {
-        datetime.datetime(2001, 1, 1, 13, 00): '2001-01-01T13:00:00',
+        datetime.datetime(2001, 1, 1, 13, 00): '2001-01-01T13:00:00Z',
         datetime.datetime(2001, 1, 1, 13, 00, tzinfo=utc): '2001-01-01T13:00:00Z',
         '2001-01-01T00:00:00': '2001-01-01T00:00:00',
         six.text_type('2016-01-10T00:00:00'): '2016-01-10T00:00:00',
@@ -1230,10 +1235,59 @@ class TestNaiveDateTimeField(FieldValues):
         '2001-01-01 13:00': datetime.datetime(2001, 1, 1, 13, 00),
     }
     invalid_inputs = {}
-    outputs = {}
+    outputs = {
+        datetime.datetime(2001, 1, 1, 13, 00): '2001-01-01T13:00:00',
+        datetime.datetime(2001, 1, 1, 13, 00, tzinfo=utc): '2001-01-01T13:00:00',
+    }
     field = serializers.DateTimeField(default_timezone=None)
 
 
+@pytest.mark.skipif(pytz is None, reason='pytz not installed')
+class TestTZWithDateTimeField(FieldValues):
+    """
+    Valid and invalid values for `DateTimeField` when not using UTC as the timezone.
+    """
+    @classmethod
+    def setup_class(cls):
+        # use class setup method, as class-level attribute will still be evaluated even if test is skipped
+        kolkata = pytz.timezone('Asia/Kolkata')
+
+        cls.valid_inputs = {
+            '2016-12-19T10:00:00': kolkata.localize(datetime.datetime(2016, 12, 19, 10)),
+            '2016-12-19T10:00:00+05:30': kolkata.localize(datetime.datetime(2016, 12, 19, 10)),
+            datetime.datetime(2016, 12, 19, 10): kolkata.localize(datetime.datetime(2016, 12, 19, 10)),
+        }
+        cls.invalid_inputs = {}
+        cls.outputs = {
+            datetime.datetime(2016, 12, 19, 10): '2016-12-19T10:00:00+05:30',
+            datetime.datetime(2016, 12, 19, 4, 30, tzinfo=utc): '2016-12-19T10:00:00+05:30',
+        }
+        cls.field = serializers.DateTimeField(default_timezone=kolkata)
+
+
+@pytest.mark.skipif(pytz is None, reason='pytz not installed')
+@override_settings(TIME_ZONE='UTC', USE_TZ=True)
+class TestDefaultTZDateTimeField(TestCase):
+    """
+    Test the current/default timezone handling in `DateTimeField`.
+    """
+
+    @classmethod
+    def setup_class(cls):
+        cls.field = serializers.DateTimeField()
+        cls.kolkata = pytz.timezone('Asia/Kolkata')
+
+    def test_default_timezone(self):
+        assert self.field.default_timezone() == utc
+
+    def test_current_timezone(self):
+        assert self.field.default_timezone() == utc
+        activate(self.kolkata)
+        assert self.field.default_timezone() == self.kolkata
+        deactivate()
+        assert self.field.default_timezone() == utc
+
+
 class TestNaiveDayLightSavingTimeTimeZoneDateTimeField(FieldValues):
     """
     Invalid values for `DateTimeField` with datetime in DST shift (non-existing or ambiguous) and timezone with DST.