Fix handling of VTIMEZONEs with subcomponents with the same DTSTARTs

For VTIMEZONE subcomponents without a TZNAME we would construct a
TZNAME from their shared TZID and DTSTART, if their DTSTART was equal,
this would lead to the same TZNAME and with that to a broken timezone
object.

Fixes #217.

CHANGES.rst

@@ -17,6 +17,11 @@ Bug fixes:
 - Don't break on parameter values which contain equal signs, e.g. base64 encoded
   binary data [geier]
 
+- Fix handling of VTIMEZONEs with subcomponents with the same DTSTARTs.
+  [geier]
+
+- *add item here*
+
 
 3.11.3 (2017-02-15)
 -------------------

src/icalendar/cal.py

@@ -517,19 +517,11 @@ class Timezone(Component):
     singletons = ('TZID', 'LAST-MODIFIED', 'TZURL',)
 
     @staticmethod
-    def _extract_offsets(component, zone):
+    def _extract_offsets(component, tzname):
         """extract offsets and transition times from a VTIMEZONE component
         :param component: a STANDARD or DAYLIGHT component
-        :param zone: the name of the zone, used for constructing a TZNAME if
-                     this component has none
+        :param tzname: the name of the zone
         """
-        try:
-            tzname = str(component['TZNAME'])
-        except KeyError:
-            tzname = '{0}_{1}'.format(
-                zone,
-                component['DTSTART'].to_ical().decode('utf-8')
-            )
         offsetfrom = component['TZOFFSETFROM'].td
         offsetto = component['TZOFFSETTO'].td
         dtstart = component['DTSTART'].dt
@@ -586,12 +578,14 @@ class Timezone(Component):
             try:
                 tzname = str(component['TZNAME'])
             except KeyError:
-                tzname = '{0}_{1}'.format(
+                tzname = '{0}_{1}_{2}_{3}'.format(
                     zone,
-                    component['DTSTART'].to_ical().decode('utf-8')
+                    component['DTSTART'].to_ical().decode('utf-8'),
+                    component['TZOFFSETFROM'].to_ical(),  # for whatever reason this is str/unicode
+                    component['TZOFFSETTO'].to_ical(),  # for whatever reason this is str/unicode
                 )
             dst[tzname], component_transitions = self._extract_offsets(
-                component, zone
+                component, tzname
             )
             transitions.extend(component_transitions)
 
@@ -622,6 +616,7 @@ class Timezone(Component):
                         if not dst[transitions[index][3]]:  # [3] is the name
                             dst_offset = osto - transitions[index][2]  # [2] is osto  # noqa
                             break
+            assert dst_offset is not False
             transition_info.append((osto, dst_offset, name))
 
         cls = type(zone, (DstTzInfo,), {

src/icalendar/tests/test_timezoned.py

@@ -266,39 +266,39 @@ class TestTimezoneCreation(unittest.TestCase):
             [(
                 datetime.timedelta(0, 43200),
                 datetime.timedelta(0),
-                'custom_Pacific/Fiji_19151026T000000'
+                'custom_Pacific/Fiji_19151026T000000_+115544_+1200'
             )] +
             3 * [(
                 datetime.timedelta(0, 46800),
                 datetime.timedelta(0, 3600),
-                'custom_Pacific/Fiji_19981101T020000'
+                'custom_Pacific/Fiji_19981101T020000_+1200_+1300'
             ), (
                 datetime.timedelta(0, 43200),
                 datetime.timedelta(0),
-                'custom_Pacific/Fiji_19990228T030000')
+                'custom_Pacific/Fiji_19990228T030000_+1300_+1200')
             ] +
             3 * [(
                 datetime.timedelta(0, 46800),
                 datetime.timedelta(0, 3600),
-                'custom_Pacific/Fiji_20101024T020000'
+                'custom_Pacific/Fiji_20101024T020000_+1200_+1300'
             ), (
                 datetime.timedelta(0, 43200),
                 datetime.timedelta(0),
-                'custom_Pacific/Fiji_19990228T030000'
+                'custom_Pacific/Fiji_19990228T030000_+1300_+1200'
             )] +
             25 * [(
                 datetime.timedelta(0, 46800),
                 datetime.timedelta(0, 3600),
-                'custom_Pacific/Fiji_20101024T020000'
+                'custom_Pacific/Fiji_20101024T020000_+1200_+1300'
             ), (
                 datetime.timedelta(0, 43200),
                 datetime.timedelta(0),
-                'custom_Pacific/Fiji_20140119T020000'
+                'custom_Pacific/Fiji_20140119T020000_+1300_+1200'
             )] +
             [(
                 datetime.timedelta(0, 46800),
                 datetime.timedelta(0, 3600),
-                'custom_Pacific/Fiji_20101024T020000'
+                'custom_Pacific/Fiji_20101024T020000_+1200_+1300'
             )]
         )
 
@@ -306,7 +306,7 @@ class TestTimezoneCreation(unittest.TestCase):
             (
                 datetime.timedelta(0, 46800),
                 datetime.timedelta(0, 3600),
-                'custom_Pacific/Fiji_19981101T020000'
+                'custom_Pacific/Fiji_19981101T020000_+1200_+1300'
             ),
             tz._tzinfos.keys()
         )
@@ -314,7 +314,7 @@ class TestTimezoneCreation(unittest.TestCase):
             (
                 datetime.timedelta(0, 43200),
                 datetime.timedelta(0),
-                'custom_Pacific/Fiji_19990228T030000'
+                'custom_Pacific/Fiji_19990228T030000_+1300_+1200'
             ),
             tz._tzinfos.keys()
         )