Fixed DST behavior for CronTrigger

Fixes #120.

2 files changed, 43 insertions, 4 deletions

diff --git a/apscheduler/triggers/cron/__init__.py b/apscheduler/triggers/cron/__init__.py
index ce675dd..c3e40fc 100644
--- a/apscheduler/triggers/cron/__init__.py
+++ b/apscheduler/triggers/cron/__init__.py
@@ -1,5 +1,6 @@
 from datetime import datetime, timedelta
 
+from pytz import NonExistentTimeError, AmbiguousTimeError
 from tzlocal import get_localzone
 import six
 
@@ -145,7 +146,7 @@ class CronTrigger(BaseTrigger):
         difference = datetime(**values) - dateval.replace(tzinfo=None)
         return self.timezone.normalize(dateval + difference), fieldnum
 
-    def _set_field_value(self, dateval, fieldnum, new_value):
+    def _set_field_value(self, dateval, fieldnum, new_value, is_dst=None):
         values = {}
         for i, field in enumerate(self.fields):
             if field.REAL:
@@ -156,7 +157,7 @@ class CronTrigger(BaseTrigger):
                 else:
                     values[field.name] = new_value
 
-        return self.timezone.localize(datetime(**values))
+        return self.timezone.localize(datetime(**values), is_dst=is_dst)
 
     def get_next_fire_time(self, previous_fire_time, now):
         if previous_fire_time:
@@ -179,8 +180,21 @@ class CronTrigger(BaseTrigger):
             elif next_value > curr_value:
                 # A valid, but higher than the starting value, was found
                 if field.REAL:
-                    next_date = self._set_field_value(next_date, fieldnum, next_value)
-                    fieldnum += 1
+                    try:
+                        next_date = self._set_field_value(next_date, fieldnum, next_value)
+                        fieldnum += 1
+                    except NonExistentTimeError:
+                        # Skip this field value
+                        next_date, fieldnum = self._increment_field_value(next_date, fieldnum)
+                    except AmbiguousTimeError:
+                        # Try this datetime with DST set unless it's earlier than
+                        # previous_run_time, in which case don't set DST
+                        next_date = self._set_field_value(next_date, fieldnum, next_value,
+                                                          is_dst=True)
+                        if previous_fire_time and next_date <= previous_fire_time:
+                            next_date = self._set_field_value(next_date, fieldnum, next_value,
+                                                              is_dst=False)
+                        fieldnum += 1
                 else:
                     next_date, fieldnum = self._increment_field_value(next_date, fieldnum)
             else:
diff --git a/tests/test_triggers.py b/tests/test_triggers.py
index dffc71d..0779ab7 100644
--- a/tests/test_triggers.py
+++ b/tests/test_triggers.py
@@ -410,6 +410,31 @@ class TestCronTrigger(object):
         trigger = CronTrigger.from_crontab(expr, timezone)
         assert repr(trigger) == expected_repr
 
+    @pytest.mark.parametrize('args, now, expected', [
+        ({'minute': '*/5'}, datetime(2016, 3, 27, 1, 59), datetime(2016, 3, 27, 3)),
+        ({'hour': 2, 'minute': 30}, datetime(2016, 3, 26, 2, 31), datetime(2016, 3, 28, 2, 30))
+    ], ids=['jump_forward', 'skip_day'])
+    def test_dst_forward(self, args, now, expected, timezone):
+        trigger = CronTrigger(timezone=timezone, **args)
+        now = timezone.localize(now)
+        expected = timezone.localize(expected)
+        assert trigger.get_next_fire_time(None, now) == expected
+
+    @pytest.mark.parametrize('args, previous, previous_dst, expected, expected_dst', [
+        ({'minute': '*/5'}, datetime(2016, 10, 30, 2, 59), True, datetime(2016, 10, 30, 2), False),
+        ({'hour': 2, 'minute': 30}, datetime(2016, 10, 30, 2, 30), True,
+         datetime(2016, 10, 30, 2, 30), False),
+        ({'hour': 2, 'minute': 30}, datetime(2016, 10, 30, 1), True,
+         datetime(2016, 10, 30, 2, 30), True)
+    ], ids=['backward1', 'backward2', 'forward'])
+    def test_dst_backward(self, args, previous: datetime, previous_dst, expected, expected_dst,
+                          timezone):
+        trigger = CronTrigger(timezone=timezone, **args)
+        now = timezone.localize(datetime(2016, 10, 30, 4))
+        previous = timezone.localize(previous, is_dst=previous_dst)
+        expected = timezone.localize(expected, is_dst=expected_dst)
+        assert trigger.get_next_fire_time(previous, now) == expected
+
 
 class TestDateTrigger(object):
     @pytest.mark.parametrize('run_date,alter_tz,previous,now,expected', [