A quicker astimezone() implementation, rehabilitating an earlier
suggestion from Guido, along with a formal correctness proof of the
trickiest bit. The intricacy of the proof reveals how delicate this
is, but also how robust the conclusion: correctness doesn't rely on
dst() returning +- one hour (not all real time zones do!), it only
relies on:
1. That dst() returns a (any) non-zero value if and only if daylight
time is in effect.
and
2. That the tzinfo subclass implements a consistent notion of time zone.
The meaning of "consistent" was a hidden assumption, which is now an
explicit requirement in the docs. Alas, it's an unverifiable (by the
datetime implementation) requirement, but so it goes.
diff --git a/Lib/test/test_datetime.py b/Lib/test/test_datetime.py
index 4a7aa60..4fe2ad2 100644
--- a/Lib/test/test_datetime.py
+++ b/Lib/test/test_datetime.py
@@ -2703,6 +2703,31 @@
# self.convert_between_tz_and_utc(Eastern, Central) # can't work
# self.convert_between_tz_and_utc(Central, Eastern) # can't work
+ def test_tricky(self):
+ # 22:00 on day before daylight starts.
+ fourback = self.dston - timedelta(hours=4)
+ ninewest = FixedOffset(-9*60, "-0900", 0)
+ fourback = fourback.astimezone(ninewest)
+ # 22:00-0900 is 7:00 UTC == 2:00 EST == 3:00 DST. Since it's "after
+ # 2", we should get the 3 spelling.
+ # If we plug 22:00 the day before into Eastern, it "looks like std
+ # time", so its offset is returned as -5, and -5 - -9 = 4. Adding 4
+ # to 22:00 lands on 2:00, which makes no sense in local time (the
+ # local clock jumps from 1 to 3). The point here is to make sure we
+ # get the 3 spelling.
+ expected = self.dston.replace(hour=3)
+ got = fourback.astimezone(Eastern).astimezone(None)
+ self.assertEqual(expected, got)
+
+ # Similar, but map to 6:00 UTC == 1:00 EST == 2:00 DST. In that
+ # case we want the 1:00 spelling.
+ sixutc = self.dston.replace(hour=6).astimezone(utc_real)
+ # Now 6:00 "looks like daylight", so the offset wrt Eastern is -4,
+ # and adding -4-0 == -4 gives the 2:00 spelling. We want the 1:00 EST
+ # spelling.
+ expected = self.dston.replace(hour=1)
+ got = sixutc.astimezone(Eastern).astimezone(None)
+ self.assertEqual(expected, got)
def test_suite():
allsuites = [unittest.makeSuite(klass, 'test')