Fixed #19708 -- Exception in timezone.override(None).

Thanks rafales.

django/utils/timezone.py

@@ -111,6 +111,7 @@ def get_default_timezone():
         if isinstance(settings.TIME_ZONE, six.string_types) and pytz is not None:
             _localtime = pytz.timezone(settings.TIME_ZONE)
         else:
+            # This relies on os.environ['TZ'] being set to settings.TIME_ZONE.
             _localtime = LocalTimezone()
     return _localtime
 
@@ -198,10 +199,10 @@ class override(object):
             activate(self.timezone)
 
     def __exit__(self, exc_type, exc_value, traceback):
-        if self.old_timezone is not None:
+        if self.old_timezone is None:
-            _active.value = self.old_timezone
+            deactivate()
         else:
-            del _active.value
+            _active.value = self.old_timezone
 
 
 # Templates

tests/regressiontests/utils/timezone.py

@@ -3,9 +3,14 @@ import datetime
 import pickle
 from django.test.utils import override_settings
 from django.utils import timezone
+from django.utils.tzinfo import FixedOffset
 from django.utils import unittest
 
 
+EAT = FixedOffset(180)      # Africa/Nairobi
+ICT = FixedOffset(420)      # Asia/Bangkok
+
+
 class TimezoneTests(unittest.TestCase):
 
     def test_localtime(self):
@@ -20,6 +25,31 @@ class TimezoneTests(unittest.TestCase):
         with override_settings(USE_TZ=False):
             self.assertTrue(timezone.is_naive(timezone.now()))
 
+    def test_override(self):
+        default = timezone.get_default_timezone()
+        try:
+            timezone.activate(ICT)
+
+            with timezone.override(EAT):
+                self.assertIs(EAT, timezone.get_current_timezone())
+            self.assertIs(ICT, timezone.get_current_timezone())
+
+            with timezone.override(None):
+                self.assertIs(default, timezone.get_current_timezone())
+            self.assertIs(ICT, timezone.get_current_timezone())
+
+            timezone.deactivate()
+
+            with timezone.override(EAT):
+                self.assertIs(EAT, timezone.get_current_timezone())
+            self.assertIs(default, timezone.get_current_timezone())
+
+            with timezone.override(None):
+                self.assertIs(default, timezone.get_current_timezone())
+            self.assertIs(default, timezone.get_current_timezone())
+        finally:
+            timezone.deactivate()
+
     def test_copy(self):
         self.assertIsInstance(copy.copy(timezone.UTC()), timezone.UTC)
         self.assertIsInstance(copy.copy(timezone.LocalTimezone()), timezone.LocalTimezone)