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A quicker astimezone() implementation, rehabilitating an earlier

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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.
This commit is contained in:
Tim Peters 2003-01-01 21:51:37 +00:00
parent 0233bd9c7d
commit f36151556f
3 changed files with 201 additions and 73 deletions
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25
Lib/test/test_datetime.py
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@ -2703,6 +2703,31 @@ class TestTimezoneConversions(unittest.TestCase):
# 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')