[Zope3-checkins] CVS: Zope3/src/datetime - _datetime.py:1.19

[Tim Peters]

Update of /cvs-repository/Zope3/src/datetime
In directory cvs.zope.org:/tmp/cvs-serv6639/src/datetime

Modified Files:
	_datetime.py 
Log Message:
A new implementation of astimezone() that does what we agreed on in all
cases, plus even tougher tests of that.  This implementation follows
the correctness proof very closely, and should also be quicker (yes,
I wrote the proof before the code, and the code proves the proof <wink>).


=== Zope3/src/datetime/_datetime.py 1.18 => 1.19 ===
--- Zope3/src/datetime/_datetime.py:1.18	Fri Jan  3 19:18:26 2003
+++ Zope3/src/datetime/_datetime.py	Fri Jan  3 23:52:47 2003
@@ -1610,8 +1610,8 @@
         return datetimetz(year, month, day, hour, minute, second,
                           microsecond, tzinfo)
 
-    def _inconsistent_utcoffset_error(self):
-        raise ValueError("astimezone():  tz.utcoffset() gave "
+    def _inconsistent_dst(self):
+        raise ValueError("astimezone():  tz.dst() gave "
                          "inconsistent results; cannot convert")
 
     def astimezone(self, tz):
@@ -1633,38 +1633,37 @@
 
         # See the long comment block at the end of this file for an
         # explanation of this algorithm.  That it always works requires a
-        # pretty intricate proof.
+        # pretty intricate proof.  There are many equivalent ways to code
+        # up the proof as an algorithm.  This way favors calling dst() over
+        # calling utcoffset(), because "the usual" utcoffset() calls dst()
+        # itself, and calling the latter instead saves a Python-level
+        # function call.  This way of coding it also follow the proof
+        # closely, w/ x=self, y=other, z=other, and z'=another.
         otdst = other.dst()
         if otdst is None:
-            raise ValueError("astimezone():  utcoffset() returned a duration "
+            raise ValueError("astimezone(): utcoffset() returned a duration "
                              "but dst() returned None")
-        total_added_to_other = otoff - otdst - myoff
-        if total_added_to_other:
-            other += total_added_to_other
-            otoff = other.utcoffset()
-            if otoff is None:
-                self._inconsistent_utcoffset_error()
-        # The distance now from self to other is
-        # self - other == naive(self) - myoff - (naive(other) - otoff) ==
-        # naive(self) - myoff -
-        #             ((naive(self) + total_added_to_other - otoff) ==
-        # - myoff - total_added_to_other + otoff
-        delta = otoff - myoff - total_added_to_other
-        ##assert (other == self) == (not delta) # expensive
-        if not delta:
+        delta = otoff - otdst - myoff
+        if delta:
+            other += delta
+            otdst = other.dst()
+            if otdst is None:
+                self._inconsistent_dst()
+        if not otdst:
             return other
 
-        # Must have crossed a DST switch point.
-        total_added_to_other += delta
-        other += delta
-        otoff = other.utcoffset()
-        if otoff is None:
-            self._inconsistent_utcoffset_error()
-        ##assert (other == self) == (otoff - myoff == total_added_to_other)
-        if otoff - myoff == total_added_to_other:
-            return other
-        raise ValueError("astimezone():  the source datetimetz can't be "
-                         "expressed in the target timezone's local time")
+        another = other + otdst
+        anotherdst = another.dst()
+        if anotherdst is None:
+            self._inconsistent_dst()
+
+        if otdst == anotherdst:
+            other = another
+        else:
+            # This is the "unspellable hour" case, and we *don't* want
+            # the DST spelling here.
+            pass
+        return other
 
     def isoformat(self, sep='T'):
         s = super(datetimetz, self).isoformat(sep)