[Python-checkins] python/dist/src/Lib/test test_datetime.py,1.18,1.19

[tim_one@users.sourceforge.net]

Update of /cvsroot/python/python/dist/src/Lib/test
In directory sc8-pr-cvs1:/tmp/cvs-serv23673/python/Lib/test

Modified Files:
	test_datetime.py 
Log Message:
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.


Index: test_datetime.py
===================================================================
RCS file: /cvsroot/python/python/dist/src/Lib/test/test_datetime.py,v
retrieving revision 1.18
retrieving revision 1.19
diff -C2 -d -r1.18 -r1.19
*** test_datetime.py	1 Jan 2003 04:18:51 -0000	1.18
--- test_datetime.py	1 Jan 2003 21:51:36 -0000	1.19
***************
*** 2704,2707 ****
--- 2704,2732 ----
          # 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():