[1.8.x] Fixed debug view crash during autumn DST change.

This only happens if USE_TZ = False and pytz is installed (perhaps not
the most logical combination, but who am I to jugde?)

Refs #23714 which essentially fixed the same problem when USE_TZ = True.

Thanks Florian and Carl for insisting until I wrote a complete patch.

Backport of 1014ba026e from master

3 files changed, 48 insertions, 6 deletions

diff --git a/django/utils/dateformat.py b/django/utils/dateformat.py
index f4de4e7f7c..ea55eaaf9d 100644
--- a/django/utils/dateformat.py
+++ b/django/utils/dateformat.py
@@ -136,6 +136,8 @@ class TimeFormat(Formatter):
             return ""
 
         seconds = self.Z()
+        if seconds == "":
+            return ""
         sign = '-' if seconds < 0 else '+'
         seconds = abs(seconds)
         return "%s%02d%02d" % (sign, seconds // 3600, (seconds // 60) % 60)
@@ -167,7 +169,14 @@ class TimeFormat(Formatter):
         if not self.timezone:
             return ""
 
-        name = self.timezone.tzname(self.data) if self.timezone else None
+        name = None
+        try:
+            name = self.timezone.tzname(self.data)
+        except Exception:
+            # pytz raises AmbiguousTimeError during the autumn DST change.
+            # This happens mainly when __init__ receives a naive datetime
+            # and sets self.timezone = get_default_timezone().
+            pass
         if name is None:
             name = self.format('O')
         return six.text_type(name)
@@ -188,7 +197,14 @@ class TimeFormat(Formatter):
         if not self.timezone:
             return ""
 
-        offset = self.timezone.utcoffset(self.data)
+        try:
+            offset = self.timezone.utcoffset(self.data)
+        except Exception:
+            # pytz raises AmbiguousTimeError during the autumn DST change.
+            # This happens mainly when __init__ receives a naive datetime
+            # and sets self.timezone = get_default_timezone().
+            return ""
+
         # `offset` is a datetime.timedelta. For negative values (to the west of
         # UTC) only days can be negative (days=-1) and seconds are always
         # positive. e.g. UTC-1 -> timedelta(days=-1, seconds=82800, microseconds=0)
@@ -228,10 +244,16 @@ class DateFormat(TimeFormat):
 
     def I(self):
         "'1' if Daylight Savings Time, '0' otherwise."
-        if self.timezone and self.timezone.dst(self.data):
-            return '1'
-        else:
-            return '0'
+        try:
+            if self.timezone and self.timezone.dst(self.data):
+                return '1'
+            else:
+                return '0'
+        except Exception:
+            # pytz raises AmbiguousTimeError during the autumn DST change.
+            # This happens mainly when __init__ receives a naive datetime
+            # and sets self.timezone = get_default_timezone().
+            return ''
 
     def j(self):
         "Day of the month without leading zeros; i.e. '1' to '31'"
diff --git a/docs/releases/1.8.7.txt b/docs/releases/1.8.7.txt
index 6db055b5fc..9aed739c01 100644
--- a/docs/releases/1.8.7.txt
+++ b/docs/releases/1.8.7.txt
@@ -9,4 +9,7 @@ Django 1.8.7 fixes several bugs in 1.8.6.
 Bugfixes
 ========
 
+* Fixed a crash of the debug view during the autumn DST change when
+  :setting:`USE_TZ` is ``False`` and ``pytz`` is installed.
+
 * ...
diff --git a/tests/utils_tests/test_dateformat.py b/tests/utils_tests/test_dateformat.py
index 1edf01422d..61859fbaec 100644
--- a/tests/utils_tests/test_dateformat.py
+++ b/tests/utils_tests/test_dateformat.py
@@ -10,6 +10,11 @@ from django.utils.timezone import (
     get_default_timezone, get_fixed_timezone, make_aware, utc,
 )
 
+try:
+    import pytz
+except ImportError:
+    pytz = None
+
 
 @override_settings(TIME_ZONE='Europe/Copenhagen')
 class DateFormatTests(TestCase):
@@ -29,6 +34,18 @@ class DateFormatTests(TestCase):
         dt = datetime(2009, 5, 16, 5, 30, 30)
         self.assertEqual(datetime.fromtimestamp(int(format(dt, 'U'))), dt)
 
+    def test_naive_ambiguous_datetime(self):
+        # dt is ambiguous in Europe/Copenhagen. LocalTimezone guesses the
+        # offset (and gets it wrong 50% of the time) while pytz refuses the
+        # temptation to guess. In any case, this shouldn't crash.
+        dt = datetime(2015, 10, 25, 2, 30, 0)
+
+        # Try all formatters that involve self.timezone.
+        self.assertEqual(format(dt, 'I'), '0' if pytz is None else '')
+        self.assertEqual(format(dt, 'O'), '+0100' if pytz is None else '')
+        self.assertEqual(format(dt, 'T'), 'CET' if pytz is None else '')
+        self.assertEqual(format(dt, 'Z'), '3600' if pytz is None else '')
+
     @requires_tz_support
     def test_datetime_with_local_tzinfo(self):
         ltz = get_default_timezone()