Handle non-DST->non-DST transitions better, from jfs per https://bugs.launchpad.net/pytz/+bug/1378150

2 files changed, 27 insertions, 10 deletions

diff --git a/src/pytz/tests/test_tzinfo.py b/src/pytz/tests/test_tzinfo.py
index adf622a..677649d 100644
--- a/src/pytz/tests/test_tzinfo.py
+++ b/src/pytz/tests/test_tzinfo.py
@@ -659,6 +659,23 @@ class LocalTestCase(unittest.TestCase):
         loc_time = loc_tz.localize(datetime(1945, 9, 30, 1, 0, 0), is_dst=0)
         self.assertEqual(loc_time.strftime('%Z%z'), 'EST-0500')
 
+        # Weird changes - ambiguous time (end-of-DST like) but is_dst==False
+        for zonename, ambiguous_naive, expected in [
+                ('Europe/Warsaw', datetime(1915, 8, 4, 23, 59, 59),
+                 ['1915-08-04 23:59:59 WMT+0124',
+                  '1915-08-04 23:59:59 CET+0100']),
+                ('Europe/Moscow', datetime(2014, 10, 26, 1, 30),
+                 ['2014-10-26 01:30:00 MSK+0400',
+                  '2014-10-26 01:30:00 MSK+0300'])]:
+            loc_tz = pytz.timezone(zonename)
+            self.assertRaises(pytz.AmbiguousTimeError,
+                loc_tz.localize, ambiguous_naive, is_dst=None
+                )
+            # Also test non-boolean is_dst in the weird case
+            for dst in [True, timedelta(1), False, timedelta(0)]:
+                loc_time = loc_tz.localize(ambiguous_naive, is_dst=dst)
+                self.assertEqual(loc_time.strftime(fmt), expected[not dst])
+
     def testNormalize(self):
         tz = pytz.timezone('US/Eastern')
         dt = datetime(2004, 4, 4, 7, 0, 0, tzinfo=UTC).astimezone(tz)
@@ -677,7 +694,7 @@ class LocalTestCase(unittest.TestCase):
     def testPartialMinuteOffsets(self):
         # utcoffset in Amsterdam was not a whole minute until 1937
         # However, we fudge this by rounding them, as the Python
-        # datetime library 
+        # datetime library
         tz = pytz.timezone('Europe/Amsterdam')
         utc_dt = datetime(1914, 1, 1, 13, 40, 28, tzinfo=UTC) # correct
         utc_dt = utc_dt.replace(second=0) # But we need to fudge it
@@ -817,4 +834,3 @@ def test_suite():
 if __name__ == '__main__':
     warnings.simplefilter("error") # Warnings should be fatal in tests.
     unittest.main(defaultTest='test_suite')
-
diff --git a/src/pytz/tzinfo.py b/src/pytz/tzinfo.py
index d53e9ff..1318872 100644
--- a/src/pytz/tzinfo.py
+++ b/src/pytz/tzinfo.py
@@ -142,7 +142,7 @@ class StaticTzInfo(BaseTzInfo):
 
     def __reduce__(self):
         # Special pickle to zone remains a singleton and to cope with
-        # database changes. 
+        # database changes.
         return pytz._p, (self.zone,)
 
 
@@ -369,13 +369,15 @@ class DstTzInfo(BaseTzInfo):
         # hints to be passed in (such as the UTC offset or abbreviation),
         # but that is just getting silly.
         #
-        # Choose the earliest (by UTC) applicable timezone.
-        sorting_keys = {}
+        # Choose the earliest (by UTC) applicable timezone if is_dst=True
+        # Choose the latest (by UTC) applicable timezone if is_dst=False
+        # i.e., behave like end-of-DST transition
+        dates = {} # utc -> local
         for local_dt in filtered_possible_loc_dt:
-            key = local_dt.replace(tzinfo=None) - local_dt.tzinfo._utcoffset
-            sorting_keys[key] = local_dt
-        first_key = sorted(sorting_keys)[0]
-        return sorting_keys[first_key]
+            utc_time = local_dt.replace(tzinfo=None) - local_dt.tzinfo._utcoffset
+            assert utc_time not in dates
+            dates[utc_time] = local_dt
+        return dates[[min, max][not is_dst](dates)]
 
     def utcoffset(self, dt, is_dst=None):
         '''See datetime.tzinfo.utcoffset
@@ -560,4 +562,3 @@ def unpickler(zone, utcoffset=None, dstoffset=None, tzname=None):
     inf = (utcoffset, dstoffset, tzname)
     tz._tzinfos[inf] = tz.__class__(inf, tz._tzinfos)
     return tz._tzinfos[inf]
-