Generate VEVENTs in canonical order.

The default sorted order was extremely irritating:
DTEND before DTSTART, etc.
Now the important properties are listed first:
        'SUMMARY', 'DTSTART', 'DTEND', 'DURATION', 'DTSTAMP',
        'UID', 'RECURRENCE-ID', 'SEQUENCE',
        'RRULE' 'EXRULE', 'RDATE', 'EXDATE',

Also, fixed spelling of RECURRENCE-ID and LAST-MODIFIED.

src/icalendar/cal.py

@@ -325,8 +325,7 @@ class Component(CaselessDict):
         """
         vText = types_factory['text']
         properties = [('BEGIN', vText(self.name).ical())]
-        property_names = self.keys()
-        property_names.sort()
+        property_names = self.sorted_keys()
         for name in property_names:
             values = self[name]
             if type(values) == ListType:
@@ -436,11 +435,17 @@ class Event(Component):
 
     name = 'VEVENT'
 
+    canonical_order = (
+        'SUMMARY', 'DTSTART', 'DTEND', 'DURATION', 'DTSTAMP',
+        'UID', 'RECURRENCE-ID', 'SEQUENCE',
+        'RRULE' 'EXRULE', 'RDATE', 'EXDATE',
+    )
+
     required = ('UID',)
     singletons = (
         'CLASS', 'CREATED', 'DESCRIPTION', 'DTSTART', 'GEO',
-        'LAST-MOD', 'LOCATION', 'ORGANIZER', 'PRIORITY', 'DTSTAMP', 'SEQUENCE',
-        'STATUS', 'SUMMARY', 'TRANSP', 'URL', 'RECURID', 'DTEND', 'DURATION',
+        'LAST-MODIFIED', 'LOCATION', 'ORGANIZER', 'PRIORITY', 'DTSTAMP', 'SEQUENCE',
+        'STATUS', 'SUMMARY', 'TRANSP', 'URL', 'RECURRENCE-ID', 'DTEND', 'DURATION',
         'DTSTART',
     )
     exclusive = ('DTEND', 'DURATION', )
@@ -458,8 +463,8 @@ class Todo(Component):
     required = ('UID',)
     singletons = (
         'CLASS', 'COMPLETED', 'CREATED', 'DESCRIPTION', 'DTSTAMP', 'DTSTART',
-        'GEO', 'LAST-MOD', 'LOCATION', 'ORGANIZER', 'PERCENT', 'PRIORITY',
-        'RECURID', 'SEQUENCE', 'STATUS', 'SUMMARY', 'UID', 'URL', 'DUE', 'DURATION',
+        'GEO', 'LAST-MODIFIED', 'LOCATION', 'ORGANIZER', 'PERCENT', 'PRIORITY',
+        'RECURRENCE-ID', 'SEQUENCE', 'STATUS', 'SUMMARY', 'UID', 'URL', 'DUE', 'DURATION',
     )
     exclusive = ('DUE', 'DURATION',)
     multiple = (
@@ -475,8 +480,8 @@ class Journal(Component):
 
     required = ('UID',)
     singletons = (
-        'CLASS', 'CREATED', 'DESCRIPTION', 'DTSTART', 'DTSTAMP', 'LAST-MOD',
-        'ORGANIZER', 'RECURID', 'SEQUENCE', 'STATUS', 'SUMMARY', 'UID', 'URL',
+        'CLASS', 'CREATED', 'DESCRIPTION', 'DTSTART', 'DTSTAMP', 'LAST-MODIFIED',
+        'ORGANIZER', 'RECURRENCE-ID', 'SEQUENCE', 'STATUS', 'SUMMARY', 'UID', 'URL',
     )
     multiple = (
         'ATTACH', 'ATTENDEE', 'CATEGORIES', 'COMMENT', 'CONTACT', 'EXDATE',
@@ -499,12 +504,13 @@ class FreeBusy(Component):
 class Timezone(Component):
 
     name = 'VTIMEZONE'
+    canonical_order = ('TZID', 'STANDARDC', 'DAYLIGHTC',)
 
     required = (
         'TZID', 'STANDARDC', 'DAYLIGHTC', 'DTSTART', 'TZOFFSETTO',
         'TZOFFSETFROM'
         )
-    singletons = ('LAST-MOD', 'TZURL', 'TZID',)
+    singletons = ('LAST-MODIFIED', 'TZURL', 'TZID',)
     multiple = ('COMMENT', 'RDATE', 'RRULE', 'TZNAME',)
 
 
@@ -537,7 +543,7 @@ class Calendar(Component):
     >>> event.set('dtstart', datetime(2005,4,4,8,0,0))
     >>> cal.add_component(event)
     >>> cal.subcomponents[0].as_string()
-    'BEGIN:VEVENT\\r\\nDTSTART;VALUE=DATE-TIME:20050404T080000\\r\\nSUMMARY:Python meeting about calendaring\\r\\nUID:42\\r\\nEND:VEVENT\\r\\n'
+    'BEGIN:VEVENT\\r\\nSUMMARY:Python meeting about calendaring\\r\\nDTSTART;VALUE=DATE-TIME:20050404T080000\\r\\nUID:42\\r\\nEND:VEVENT\\r\\n'
 
     Write to disc
     >>> import tempfile, os
@@ -572,6 +578,7 @@ class Calendar(Component):
     """
 
     name = 'VCALENDAR'
+    canonical_order = ('PRODID', 'VERSION', 'CALSCALE', 'METHOD',)
     required = ('prodid', 'version', )
     singletons = ('prodid', 'version', )
     multiple = ('calscale', 'method', )

src/icalendar/caselessdict.py

@@ -91,3 +91,13 @@ class CaselessDict(dict):
 
     def __repr__(self):
         return 'CaselessDict(' + dict.__repr__(self) + ')'
+
+    canonical_order = None
+
+    def sorted_keys(self):
+        """
+        Sorts keys according to the canonical_order for the derived class.
+        Keys not specified in canonical_order will appear at the end.
+        """
+        canonical_map = dict((k, i) for i, k in enumerate(self.canonical_order or []))
+        return sorted(self.keys(), key=lambda k: canonical_map.get(k, 999))