New rule for tzinfo subclasses handling both standard and daylight time:

When daylight time ends, an hour repeats on the local clock (for example,
in US Eastern, the clock jumps from 1:59 back to 1:00 again).  Times in
the repeated hour are ambiguous.  A tzinfo subclass that wants to play
with astimezone() needs to treat times in the repeated hour as being
standard time.  astimezone() previously required that such times be
treated as daylight time.  There seems no killer argument either way,
but Guido wants the standard-time version, and it does seem easier the
new way to code both American (local-time based) and European (UTC-based)
switch rules, and the astimezone() implementation is simpler.

1 files changed, 27 insertions, 20 deletions

diff --git a/Lib/test/test_datetime.py b/Lib/test/test_datetime.py
index a46e3ca..8a8d315 100644
--- a/Lib/test/test_datetime.py
+++ b/Lib/test/test_datetime.py
@@ -2561,8 +2561,10 @@ DAY = timedelta(days=1)
 # In the US, DST starts at 2am (standard time) on the first Sunday in April.
 DSTSTART = datetime(1, 4, 1, 2)
 # and ends at 2am (DST time; 1am standard time) on the last Sunday of Oct,
-# which is the first Sunday on or after Oct 25.
-DSTEND = datetime(1, 10, 25, 2)
+# which is the first Sunday on or after Oct 25.  Because we view 1:MM as
+# being standard time on that day, there is no spelling in local time of
+# the last hour of DST (that's 1:MM DST, but 1:MM is taken as standard time).
+DSTEND = datetime(1, 10, 25, 1)
 
 class USTimeZone(tzinfo):
 
@@ -2616,9 +2618,9 @@ utc_real = FixedOffset(0, "UTC", 0)
 utc_fake = FixedOffset(-12*60, "UTCfake", 0)
 
 class TestTimezoneConversions(unittest.TestCase):
-    # The DST switch times for 2002, in local time.
+    # The DST switch times for 2002, in std time.
     dston = datetime(2002, 4, 7, 2)
-    dstoff = datetime(2002, 10, 27, 2)
+    dstoff = datetime(2002, 10, 27, 1)
 
     theclass = datetime
 
@@ -2656,25 +2658,25 @@ class TestTimezoneConversions(unittest.TestCase):
             # We're not in the redundant hour.
             self.assertEqual(dt, there_and_back)
 
-        # Because we have a redundant spelling when DST begins,
-        # there is (unforunately) an hour when DST ends that can't
-        # be spelled at all in local time.  When DST ends, the
-        # clock jumps from 1:59:59 back to 1:00:00 again.  The
-        # hour beginning then has no spelling in local time:
-        # 1:MM:SS is taken to be daylight time, and 2:MM:SS as
-        # standard time.  The hour 1:MM:SS standard time ==
-        # 2:MM:SS daylight time can't be expressed in local time.
-        # Nevertheless, we want conversion back from UTC to mimic
-        # the local clock's "repeat an hour" behavior.
+        # Because we have a redundant spelling when DST begins, there is
+        # (unforunately) an hour when DST ends that can't be spelled at all in
+        # local time.  When DST ends, the clock jumps from 1:59 back to 1:00
+        # again.  The hour 1:MM DST has no spelling then:  1:MM is taken to be
+        # standard time.  1:MM DST == 0:MM EST, but 0:MM is taken to be
+        # daylight time.  The hour 1:MM daylight == 0:MM standard can't be
+        # expressed in local time.  Nevertheless, we want conversion back
+        # from UTC to mimic the local clock's "repeat an hour" behavior.
         nexthour_utc = asutc + HOUR
         nexthour_tz = nexthour_utc.astimezone(tz)
-        if dt.date() == dstoff.date() and dt.hour == 1:
-            # We're in the hour before DST ends.  The hour after
+        if dt.date() == dstoff.date() and dt.hour == 0:
+            # We're in the hour before the last DST hour.  The last DST hour
             # is ineffable.  We want the conversion back to repeat 1:MM.
-            expected_diff = ZERO
+            self.assertEqual(nexthour_tz, dt.replace(hour=1))
+            nexthour_utc += HOUR
+            nexthour_tz = nexthour_utc.astimezone(tz)
+            self.assertEqual(nexthour_tz, dt.replace(hour=1))
         else:
-            expected_diff = HOUR
-        self.assertEqual(nexthour_tz - dt, expected_diff)
+            self.assertEqual(nexthour_tz - dt, HOUR)
 
     # Check a time that's outside DST.
     def checkoutside(self, dt, tz, utc):
@@ -2687,6 +2689,11 @@ class TestTimezoneConversions(unittest.TestCase):
 
     def convert_between_tz_and_utc(self, tz, utc):
         dston = self.dston.replace(tzinfo=tz)
+        # Because 1:MM on the day DST ends is taken as being standard time,
+        # there is no spelling in tz for the last hour of daylight time.
+        # For purposes of the test, the last hour of DST is 0:MM, which is
+        # taken as being daylight time (and 1:MM is taken as being standard
+        # time).
         dstoff = self.dstoff.replace(tzinfo=tz)
         for delta in (timedelta(weeks=13),
                       DAY,
@@ -2759,7 +2766,7 @@ class TestTimezoneConversions(unittest.TestCase):
         # wall  0:MM  1:MM  1:MM  2:MM  against these
         for utc in utc_real, utc_fake:
             for tz in Eastern, Pacific:
-                first_std_hour = self.dstoff - timedelta(hours=3) # 23:MM
+                first_std_hour = self.dstoff - timedelta(hours=2) # 23:MM
                 # Convert that to UTC.
                 first_std_hour -= tz.utcoffset(None)
                 # Adjust for possibly fake UTC.