Merge lp:~cjwatson/pytz/smarter-utcoffset into lp:pytz

[Colin Watson]

Use the most recent appropriate transition for DstTzInfo._utcoffset and friends, rather than the first transition which may be very old.

This fixes nonsensical _utcoffset values for various timezones that were constructed based on extremely old timezone transitions. The replacement algorithm here is much closer to what localtime() does. I've confirmed that this fixes the test failures from http://lpbuildbot.canonical.com/builders/lp-devel-precise/builds/277/steps/shell_9/logs/summary that aren't just due to renamed zones.

Unmerged revisions

357. By Colin Watson on 2014-12-30

Use the most recent appropriate transition for DstTzInfo._utcoffset and friends, rather than the first transition which may be very old.

=== modified file 'src/pytz/tzinfo.py'
--- src/pytz/tzinfo.py	2014-11-03 08:24:30 +0000
+++ src/pytz/tzinfo.py	2014-12-31 13:19:05 +0000
@@ -164,15 +164,32 @@
     _dst = None # DST offset
 
     def __init__(self, _inf=None, _tzinfos=None):
+        '''Construct a new DstTzInfo object.
+
+        >>> from pytz import timezone
+        >>> london = timezone('Europe/London')
+        >>> dt = datetime(2014, 1, 1, 0, 0, 0, tzinfo=london)
+        >>> london.utcoffset(dt)
+        datetime.timedelta(0)
+        '''
         if _inf:
             self._tzinfos = _tzinfos
             self._utcoffset, self._dst, self._tzname = _inf
         else:
             _tzinfos = {}
             self._tzinfos = _tzinfos
-            self._utcoffset, self._dst, self._tzname = self._transition_info[0]
-            _tzinfos[self._transition_info[0]] = self
-            for inf in self._transition_info[1:]:
+            if len(self._transition_info) == 1:
+                # Use the first rule (which should also be the only one).
+                self._utcoffset, self._dst, self._tzname = (
+                    self._transition_info[0])
+                _tzinfos[self._transition_info[0]] = self
+            else:
+                for inf in reversed(self._transition_info):
+                    if not inf[1]:
+                        self._utcoffset, self._dst, self._tzname = inf
+                        _tzinfos[inf] = self
+                        break
+            for inf in self._transition_info:
                 if inf not in _tzinfos:
                     _tzinfos[inf] = self.__class__(inf, _tzinfos)