[Zope3-checkins] CVS: Zope3/src/datetime - _datetime.py:1.24

[Tim Peters]

Update of /cvs-repository/Zope3/src/datetime
In directory cvs.zope.org:/tmp/cvs-serv11591/src/datetime

Modified Files:
	_datetime.py 
Log Message:
Refactoring of, and new rules for, dt.astimezone(tz).

dt must be aware now, and tz.utcoffset() and tz.dst() must not return None.
The old dt.astimezone(None) no longer works to change an aware datetime
into a naive datetime; use dt.replace(tzinfo=None) instead.

The tzinfo base class now supplies a new fromutc(self, dt) method, and
datetime.astimezone(tz) invokes tz.fromutc().  The default implementation
of fromutc() reproduces the same results as the old astimezone()
implementation, but tzinfo subclasses can override fromutc() if the
default implementation isn't strong enough to get the correct results
in all cases (for example, this may be necessary if a tzinfo subclass
models a time zone whose "standard offset" (wrt UTC) changed in some
year(s), or in some variations of double-daylight time -- the creativity
of time zone politics can't be captured in a single default implementation).


=== Zope3/src/datetime/_datetime.py 1.23 => 1.24 ===
--- Zope3/src/datetime/_datetime.py:1.23	Mon Jan 20 17:13:02 2003
+++ Zope3/src/datetime/_datetime.py	Tue Jan 21 16:30:14 2003
@@ -877,6 +877,38 @@
         """
         raise NotImplementedError("tzinfo subclass must override dst()")
 
+    def _inconsistent_dst(self):
+        raise ValueError("astimezone():  tz.dst() gave "
+                         "inconsistent results; cannot convert")
+
+    def fromutc(self, dt):
+        "datetime in UTC -> datetime in local time."
+
+        if dt.tzinfo is not self:
+            raise ValueError("dt.tzinfo is not self")
+
+        dtoff = dt.utcoffset()
+        if dtoff is None:
+            raise ValueError("fromutc() requires a non-None utcoffset() "
+                             "result")
+
+        # See the long comment block at the end of this file for an
+        # explanation of this algorithm.
+        dtdst = dt.dst()
+        if dtdst is None:
+            raise ValueError("fromutc() requires a non_none dst() result")
+        delta = dtoff - dtdst
+        if delta:
+            dt += delta
+            dtdst = dt.dst()
+            if dtdst is None:
+                raise ValueError("fromutc(): dt.dst gave inconsistent "
+                                 "results; cannot convert")
+        if dtdst:
+            return dt + dtdst
+        else:
+            return dt
+
     # pickle support
 
     __safe_for_unpickling__ = True
@@ -1273,64 +1305,25 @@
         return datetime(year, month, day, hour, minute, second,
                           microsecond, tzinfo)
 
-    def _inconsistent_dst(self):
-        raise ValueError("astimezone():  tz.dst() gave "
-                         "inconsistent results; cannot convert")
-
     def astimezone(self, tz):
-        _check_tzinfo_arg(tz)
-        other = self.replace(tzinfo=tz)
+        if not isinstance(tz, tzinfo):
+            raise TypeError("tz argument must be an instance of tzinfo")
 
-        # Don't call utcoffset unless necessary.  First check trivial cases.
-        if tz is None or self._tzinfo is None or self._tzinfo is tz:
-            return other
-
-        # Get the offsets.  If either object turns out to be naive, again
-        # there's no conversion of date or time fields.
-        myoff = self.utcoffset()
-        if myoff is None:
-            return other
-        otoff = other.utcoffset()
-        if otoff is None:
-            return other
+        mytz = self.tzinfo
+        if mytz is None:
+            raise ValueError("astimezone() requires an aware datetime")
+
+        if tz is mytz:
+            return self
+
+        # Convert self to UTC, and attach the new time zone object.
+        myoffset = self.utcoffset()
+        if myoffset is None:
+            raise ValuError("astimezone() requires an aware datetime")
+        utc = (self - myoffset).replace(tzinfo=tz)
 
-        # 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.  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 "
-                             "but dst() returned None")
-        delta = otoff - otdst - myoff
-        if delta:
-            other += delta
-            otdst = other.dst()
-            if otdst is None:
-                self._inconsistent_dst()
-        if not otdst:
-            return other
-
-        another = other + otdst
-        return another
-
-        # XXX Leaving this unreachable code here for a while.  It may be
-        # XXX needed again real soon <wink>.
-        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
+        # Convert from UTC to tz's local time.
+        return tz.fromutc(utc)
 
     # Ways to produce a string.