check assertion

see collective#321

src/icalendar/tests/issue_321_assert_dst_offset_is_not_false.ics

@@ -0,0 +1,22 @@
+BEGIN:VCALENDAR
+VERSION:2.0
+PRODID:-//PIMUTILS.ORG//NONSGML khal / icalendar //EN
+BEGIN:VTIMEZONE
+TZID:Europe/Berlin
+BEGIN:DAYLIGHT
+DTSTART;VALUE=DATE-TIME:20200329T030000
+TZNAME:CEST
+TZOFFSETFROM:+0100
+TZOFFSETTO:+0200
+END:DAYLIGHT
+END:VTIMEZONE
+BEGIN:VEVENT
+SUMMARY:Reifenwechsel
+DTSTART;TZID=Europe/Berlin;VALUE=DATE-TIME:20200403T102000
+DTEND;TZID=Europe/Berlin;VALUE=DATE-TIME:20200403T113000
+DTSTAMP;VALUE=DATE-TIME:20200326T165608Z
+UID:TCK6EMQBQHQ8KB4PDQPT04Q7HBYNS2MEPZMO
+SEQUENCE:0
+LOCATION:Musterhausen
+END:VEVENT
+END:VCALENDAR

src/icalendar/tests/test_fixed_issues.py

@@ -479,6 +479,17 @@ def test_issue_237(self):
         self.assertEqual(dtstart.tzinfo.zone, expected_zone)
         self.assertEqual(dtstart.tzname(), expected_tzname)
 
+    def test_issue_321_assert_dst_offset_is_not_false(self):
+        """This tests the assertion hitting for a calendar.
+
+        See https://github.com/collective/icalendar/issues/321
+        """
+        directory = os.path.dirname(__file__)
+        path = os.path.join(directory,
+                            'issue_321_assert_dst_offset_is_not_false.ics')
+        with open(path, 'rb') as ics:
+            cal = icalendar.Calendar.from_ical(ics.read()) # assertion should fail
+
     def test_issue_345(self):
         """Issue #345 - Why is tools.UIDGenerator a class (that must be instantiated) instead of a module? """
         uid1 = icalendar.tools.UIDGenerator.uid()