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>).

Doc/lib/libdatetime.tex

@@ -924,11 +924,11 @@ implement all of them.
       \code{tz.utcoffset(dt) - tz.dst(dt)}
 
   must return the same result for every \class{datetimetz} \var{dt}
-  in a given year with \code{dt.tzinfo==tz}  For sane \class{tzinfo}
-  subclasses, this expression yields the time zone's "standard offset"
-  within the year, which should be the same across all days in the year.
-  The implementation of \method{datetimetz.astimezone()} relies on this,
-  but cannot detect violations; it's the programmer's responsibility to
+  with \code{dt.tzinfo==tz}  For sane \class{tzinfo} subclasses, this
+  expression yields the time zone's "standard offset", which should not
+  depend on the date or the time, but only on geographic location.  The
+  implementation of \method{datetimetz.astimezone()} relies on this, but
+  cannot detect violations; it's the programmer's responsibility to
   ensure it.
 
 \begin{methoddesc}{tzname}{self, dt}
@@ -970,6 +970,50 @@ Example \class{tzinfo} classes:
 
 \verbatiminput{tzinfo-examples.py}
 
+Note that there are unavoidable subtleties twice per year in a tzinfo
+subclass accounting for both standard and daylight time, at the DST
+transition points.  For concreteness, consider US Eastern (UTC -0500),
+where EDT begins the minute after 1:59 (EST) on the first Sunday in
+April, and ends the minute after 1:59 (EDT) on the last Sunday in October:
+
+\begin{verbatim}
+    UTC   3:MM  4:MM  5:MM  6:MM  7:MM  8:MM
+    EST  22:MM 23:MM  0:MM  1:MM  2:MM  3:MM
+    EDT  23:MM  0:MM  1:MM  2:MM  3:MM  4:MM
+
+  start  22:MM 23:MM  0:MM  1:MM  3:MM  4:MM
+
+    end  23:MM  0:MM  1:MM  1:MM  2:MM  3:MM
+\end{verbatim}
+
+When DST starts (the "start" line), the local wall clock leaps from 1:59
+to 3:00.  A wall time of the form 2:MM doesn't really make sense on that
+day, so astimezone(Eastern) won't deliver a result with hour=2 on the
+day DST begins.  How an Eastern class chooses to interpret 2:MM on
+that day is its business.  The example Eastern class above chose to
+consider it as a time in EDT, simply because it "looks like it's
+after 2:00", and so synonymous with the EST 1:MM times on that day.
+Your Eastern class may wish, for example, to raise an exception instead
+when it sees a 2:MM time on the day Eastern begins.
+
+When DST ends (the "end" line), there's a potentially worse problem:
+there's an hour that can't be spelled at all in local wall time, the
+hour beginning at the moment DST ends.  In this example, that's times of
+the form 6:MM UTC on the day daylight time ends.  The local wall clock
+leaps from 1:59 (daylight time) back to 1:00 (standard time) again.
+1:MM is taken as daylight time (it's "before 2:00"), so maps to 5:MM UTC.
+2:MM is taken as standard time (it's "after 2:00"), so maps to 7:MM UTC.
+There is no local time that maps to 6:MM UTC on this day.
+
+Just as the wall clock does, astimezone(Eastern) maps both UTC hours 5:MM
+and 6:MM to Eastern hour 1:MM on this day.  However, this result is
+ambiguous (there's no way for Eastern to know which repetition of 1:MM
+is intended).  Applications that can't bear such ambiguity even one hour
+per year should avoid using hybrid tzinfo classes; there are no
+ambiguities when using UTC, or any other fixed-offset tzinfo subclass
+(such as a class representing only EST (fixed offset -5 hours), or only
+EDT (fixed offset -4 hours)).
+
 
 \subsection{\class{timetz} Objects \label{datetime-timetz}}
 

Doc/lib/tzinfo-examples.py

-from datetime import tzinfo, timedelta
+from datetime import tzinfo, timedelta, datetime
 
 ZERO = timedelta(0)
+HOUR = timedelta(hours=1)
 
 # A UTC class.
 
@@ -76,3 +77,63 @@ class LocalTimezone(tzinfo):
         return tt.tm_isdst > 0
 
 Local = LocalTimezone()
+
+
+# A complete implementation of current DST rules for major US time zones.
+
+def first_sunday_on_or_after(dt):
+    days_to_go = 6 - dt.weekday()
+    if days_to_go:
+        dt += timedelta(days_to_go)
+    return dt
+
+# In the US, DST starts at 2am (standard time) on the first Sunday in April.
+DSTSTART = datetime(1, 4, 1, 2)
+# and ends at 2am (DST time; 1am standard time) on the last Sunday of Oct.
+# which is the first Sunday on or after Oct 25.
+DSTEND = datetime(1, 10, 25, 2)
+
+class USTimeZone(tzinfo):
+
+    def __init__(self, hours, reprname, stdname, dstname):
+        self.stdoffset = timedelta(hours=hours)
+        self.reprname = reprname
+        self.stdname = stdname
+        self.dstname = dstname
+
+    def __repr__(self):
+        return self.reprname
+
+    def tzname(self, dt):
+        if self.dst(dt):
+            return self.dstname
+        else:
+            return self.stdname
+
+    def utcoffset(self, dt):
+        return self.stdoffset + self.dst(dt)
+
+    def dst(self, dt):
+        if dt is None or dt.tzinfo is None:
+            # An exception may be sensible here, in one or both cases.
+            # It depends on how you want to treat them.  The astimezone()
+            # implementation always passes a datetimetz with
+            # dt.tzinfo == self.
+            return ZERO
+        assert dt.tzinfo is self
+
+        # Find first Sunday in April & the last in October.
+        start = first_sunday_on_or_after(DSTSTART.replace(year=dt.year))
+        end = first_sunday_on_or_after(DSTEND.replace(year=dt.year))
+
+        # Can't compare naive to aware objects, so strip the timezone from
+        # dt first.
+        if start <= dt.replace(tzinfo=None) < end:
+            return HOUR
+        else:
+            return ZERO
+
+Eastern  = USTimeZone(-5, "Eastern",  "EST", "EDT")
+Central  = USTimeZone(-6, "Central",  "CST", "CDT")
+Mountain = USTimeZone(-7, "Mountain", "MST", "MDT")
+Pacific  = USTimeZone(-8, "Pacific",  "PST", "PDT")

Lib/test/test_datetime.py

@@ -2592,7 +2592,7 @@ Pacific  = USTimeZone(-8, "Pacific",  "PST", "PDT")
 utc_real = FixedOffset(0, "UTC", 0)
 # For better test coverage, we want another flavor of UTC that's west of
 # the Eastern and Pacific timezones.
-utc_fake = FixedOffset(-12, "UTCfake", 0)
+utc_fake = FixedOffset(-12*60, "UTCfake", 0)
 
 class TestTimezoneConversions(unittest.TestCase):
     # The DST switch times for 2002, in local time.
@@ -2643,25 +2643,17 @@ class TestTimezoneConversions(unittest.TestCase):
         # 1:MM:SS is taken to be daylight time, and 2:MM:SS as
         # standard time.  The hour 1:MM:SS standard time ==
         # 2:MM:SS daylight time can't be expressed in local time.
+        # Nevertheless, we want conversion back from UTC to mimic
+        # the local clock's "repeat an hour" behavior.
         nexthour_utc = asutc + HOUR
+        nexthour_tz = nexthour_utc.astimezone(tz)
         if dt.date() == dstoff.date() and dt.hour == 1:
             # We're in the hour before DST ends.  The hour after
-            # is ineffable.
-            # For concreteness, picture Eastern.  during is of
-            # the form 1:MM:SS, it's daylight time, so that's
-            # 5:MM:SS UTC.  Adding an hour gives 6:MM:SS UTC.
-            # Daylight time ended at 2+4 == 6:00:00 UTC, so
-            # 6:MM:SS is (correctly) taken to be standard time.
-            # But standard time is at offset -5, and that maps
-            # right back to the 1:MM:SS Eastern we started with.
-            # That's correct, too, *if* 1:MM:SS were taken as
-            # being standard time.  But it's not -- on this day
-            # it's taken as daylight time.
-            self.assertRaises(ValueError,
-                              nexthour_utc.astimezone, tz)
+            # is ineffable.  We want the conversion back to repeat 1:MM.
+            expected_diff = ZERO
         else:
-            nexthour_tz = nexthour_utc.astimezone(utc)
-            self.assertEqual(nexthour_tz - dt, HOUR)
+            expected_diff = HOUR
+        self.assertEqual(nexthour_tz - dt, expected_diff)
 
     # Check a time that's outside DST.
     def checkoutside(self, dt, tz, utc):
@@ -2739,6 +2731,31 @@ class TestTimezoneConversions(unittest.TestCase):
         got = sixutc.astimezone(Eastern).astimezone(None)
         self.assertEqual(expected, got)
 
+        # Now on the day DST ends, we want "repeat an hour" behavior.
+        #  UTC  4:MM  5:MM  6:MM  7:MM  checking these
+        #  EST 23:MM  0:MM  1:MM  2:MM
+        #  EDT  0:MM  1:MM  2:MM  3:MM
+        # wall  0:MM  1:MM  1:MM  2:MM  against these
+        for utc in utc_real, utc_fake:
+            for tz in Eastern, Pacific:
+                first_std_hour = self.dstoff - timedelta(hours=3) # 23:MM
+                # Convert that to UTC.
+                first_std_hour -= tz.utcoffset(None)
+                # Adjust for possibly fake UTC.
+                asutc = first_std_hour + utc.utcoffset(None)
+                # First UTC hour to convert; this is 4:00 when utc=utc_real &
+                # tz=Eastern.
+                asutcbase = asutc.replace(tzinfo=utc)
+                for tzhour in (0, 1, 1, 2):
+                    expectedbase = self.dstoff.replace(hour=tzhour)
+                    for minute in 0, 30, 59:
+                        expected = expectedbase.replace(minute=minute)
+                        asutc = asutcbase.replace(minute=minute)
+                        astz = asutc.astimezone(tz)
+                        self.assertEqual(astz.replace(tzinfo=None), expected)
+                    asutcbase += HOUR
+
+
     def test_bogus_dst(self):
         class ok(tzinfo):
             def utcoffset(self, dt): return HOUR

Misc/NEWS

@@ -26,11 +26,15 @@ Extension modules
 - datetime changes:
 
   today() and now() now round system timestamps to the closest
-  microsecond <http://www.python.org/sf/661086>.
+  microsecond <http://www.python.org/sf/661086>.  This repairs an
+  irritation most likely seen on Windows systems.
 
   In dt.asdatetime(tz), if tz.utcoffset(dt) returns a duration,
   ValueError is raised if tz.dst(dt) returns None (2.3a1 treated it
-  as 0 instead).
+  as 0 instead, but a tzinfo subclass wishing to participate in
+  time zone conversion has to take a stand on whether it supports
+  DST; if you don't care about DST, then code dst() to return 0 minutes,
+  meaning that DST is never in effect).
 
   The tzinfo methods utcoffset() and dst() must return a timedelta object
   (or None) now.  In 2.3a1 they could also return an int or long, but that
@@ -40,6 +44,12 @@ Extension modules
   The example tzinfo class for local time had a bug.  It was replaced
   by a later example coded by Guido.
 
+  datetimetz.astimezone(tz) no longer raises an exception when the
+  input datetime has no UTC equivalent in tz.  For typical "hybrid" time
+  zones (a single tzinfo subclass modeling both standard and daylight
+  time), this case can arise one hour per year, at the hour daylight time
+  ends.  See new docs for details.
+
 Library
 -------
 

Modules/datetimemodule.c

@@ -4754,7 +4754,7 @@ datetimetz_astimezone(PyDateTime_DateTimeTZ *self, PyObject *args,
 
 	PyObject *result;
 	PyObject *temp;
-	int selfoff, resoff, resdst, total_added_to_result;
+	int selfoff, resoff, dst1, dst2;
 	int none;
 	int delta;
 
@@ -4792,19 +4792,24 @@ datetimetz_astimezone(PyDateTime_DateTimeTZ *self, PyObject *args,
 
 	/* 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 follows the proof
+	 * closely, w/ x=self, y=result, z=result, and z'=temp.
 	 */
-	resdst = call_dst(tzinfo, result, &none);
-	if (resdst == -1 && PyErr_Occurred())
+	dst1 = call_dst(tzinfo, result, &none);
+	if (dst1 == -1 && PyErr_Occurred())
 		goto Fail;
 	if (none) {
 		PyErr_SetString(PyExc_ValueError, "astimezone(): utcoffset() "
 		"returned a duration but dst() returned None");
 		goto Fail;
 	}
-	total_added_to_result = resoff - resdst - selfoff;
-	if (total_added_to_result != 0) {
-		mm += total_added_to_result;
+	delta = resoff - dst1 - selfoff;
+	if (delta) {
+		mm += delta;
 		if ((mm < 0 || mm >= 60) &&
 		    normalize_datetime(&y, &m, &d, &hh, &mm, &ss, &us) < 0)
 			goto Fail;
@@ -4814,58 +4819,47 @@ datetimetz_astimezone(PyDateTime_DateTimeTZ *self, PyObject *args,
 		Py_DECREF(result);
 		result = temp;
 
-		resoff = call_utcoffset(tzinfo, result, &none);
-		if (resoff == -1 && PyErr_Occurred())
+		dst1 = call_dst(tzinfo, result, &none);
+		if (dst1 == -1 && PyErr_Occurred())
 			goto Fail;
 		if (none)
 			goto Inconsistent;
 	}
-
-	/* The distance now from self to result is
-	 * self - result == naive(self) - selfoff - (naive(result) - resoff) ==
-	 * naive(self) - selfoff -
-	 *             ((naive(self) + total_added_to_result - resoff) ==
-	 * - selfoff - total_added_to_result + resoff.
-	 */
-	delta = resoff - selfoff - total_added_to_result;
-
-	/* Now self and result are the same UTC time iff delta is 0.
-	 * If it is 0, we're done, although that takes some proving.
-	 */
-	if (delta == 0)
+	if (dst1 == 0)
 		return result;
 
-	total_added_to_result += delta;
-	mm += delta;
+	mm += dst1;
 	if ((mm < 0 || mm >= 60) &&
 	    normalize_datetime(&y, &m, &d, &hh, &mm, &ss, &us) < 0)
 		goto Fail;
-
 	temp = new_datetimetz(y, m, d, hh, mm, ss, us, tzinfo);
 	if (temp == NULL)
 		goto Fail;
-	Py_DECREF(result);
-	result = temp;
 
-	resoff = call_utcoffset(tzinfo, result, &none);
-	if (resoff == -1 && PyErr_Occurred())
+	dst2 = call_dst(tzinfo, temp, &none);
+	if (dst2 == -1 && PyErr_Occurred()) {
+		Py_DECREF(temp);
 		goto Fail;
-	if (none)
+	}
+	if (none) {
+		Py_DECREF(temp);
 		goto Inconsistent;
+	}
 
-	if (resoff - selfoff == total_added_to_result)
-		/* self and result are the same UTC time */
-		return result;
-
-        /* Else there's no way to spell self in zone tzinfo. */
-        PyErr_SetString(PyExc_ValueError, "astimezone(): the source "
-        		"datetimetz can't be expressed in the target "
-        		"timezone's local time");
-        goto Fail;
+	if (dst1 == dst2) {
+		/* The normal case:  we want temp, not result. */
+		Py_DECREF(result);
+		result = temp;
+	}
+	else {
+		/* The "unspellable hour" at the end of DST. */
+		Py_DECREF(temp);
+	}
+	return result;
 
 Inconsistent:
-	PyErr_SetString(PyExc_ValueError, "astimezone(): tz.utcoffset() "
-			"gave inconsistent results; cannot convert");
+	PyErr_SetString(PyExc_ValueError, "astimezone(): tz.dst() gave"
+			"inconsistent results; cannot convert");
 
 	/* fall thru to failure */
 Fail: