chore: Work on the compatibility_hints

New "features" added:

* 'save-load.mutable' (replaces an old "compatibility flag", this has been observed unsupported for the old google API, long time ago)

Compatibilities changed:

* search.recurrences.expanded.exception was wrongly taggd as
  unsupported on many servers due to a bug in the checker script.

Other:

The old unique_calendar_ids "compatibility flag" seems not to be needed anymore by any servers I test, so it was removed (but possibly we will need it at some point in the future ...).

AI-disclaimer:

The unique_calendar_ids yanking job was hand-made, the other was
AI-generated.  The compatibility_hints.py is one of the places where
AI-maintenance is considered OK.  It's not considered to be a part of
the sharp edge of the library, and some of the updates may be tedious.

prompt: Create tests for a new feature checking if it's possible to
overwrite a calendar event with new data. Should replace the old
"overwrite" compatibility flag. (Perhaps this "incompatibility" was due
to the caldav library not supporting etags and not updating from no
sequence to SEQUENCE:1 ... i somehow doubt we'll find any servers not
supporting this feature, but we should have the check anyway).
load-save.mutable ? Update the compatibility_hints.py in
/tmp/caldav-async-tests/ and update the tests there as well.
prompt: On the main branch in the current directory, we get
AssertionError: expectation is unsupported, observation is full for
search.recurrences.expanded.exception from many servers now when running
pytest -k compat in ~/caldav. Reproducable by doing cd ~/caldav ; status=good ;
pytest tests/test_caldav.py::TestForServerRadicale::testCheckCompatibility ||
status=bad; cd ~/caldav-server-tester ; echo $status. Checking out tag v3.2.0,
the test passes. There aren't that many differences between main and v3.2.0.
Please investigate.
followup-prompt: Throw it into the current async-test-infrastructure branch.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: tobixen

caldav/compatibility_hints.py

@@ -77,6 +77,9 @@ class FeatureSet:
                 ## TODO: in the future, templates for the principal URL, calendar URLs etc may also be added.
             }
         },
+        "url": {
+            "type": "client-hints",
+        },
         "get-current-user-principal": {
             "description": "Support for RFC5397, current principal extension.  Most CalDAV servers have this, but it is an extension to the DAV standard.  Possibly observed missing on mail.ru, DavMail gateway and it is possible to configure the support in some sabre-based servers",
             "links": ["https://datatracker.ietf.org/doc/html/rfc5397"],
@@ -177,6 +180,10 @@ class FeatureSet:
             "default": {"support": "full"},
             "links": ["https://datatracker.ietf.org/doc/html/rfc5545#section-3.8.4.5"],
         },
+        "save-load.mutable": {
+            "description": "A saved calendar object resource can be modified and PUT back to the server; the server accepts the update and returns the modified data on the next GET/REPORT. When 'unsupported', the server treats calendar objects as immutable after initial creation (e.g. Google Calendar's legacy CalDAV API). Replaces the old 'no_overwrite' compatibility flag.",
+            "default": {"support": "full"},
+        },
         "search": {
             "description": "calendar MUST support searching for objects using the REPORT method, as specified in RFC4791, section 7",
             "links": ["https://datatracker.ietf.org/doc/html/rfc4791#section-7"],
@@ -855,9 +862,6 @@ def dotted_feature_set_list(self, compact=False):
     'vtodo-cannot-be-uncompleted':
         """If a VTODO object has been set with STATUS:COMPLETE, it's not possible to delete the COMPLTEDED attribute and change back to STATUS:IN-ACTION""",
 
-    'unique_calendar_ids':
-        """For every test, generate a new and unique calendar id""",
-
     'sticky_events':
         """Events should be deleted before the calendar is deleted, """
         """and/or deleting a calendar may not have immediate effect""",
@@ -927,7 +931,7 @@ def dotted_feature_set_list(self, compact=False):
     "search.text.case-sensitive": {"support": "unsupported"},
     "search.recurrences.includes-implicit.todo.pending": {"support": "fragile", "behaviour": "inconsistent results between runs"},
     "search.recurrences.expanded.todo": {"support": "unsupported"},
-    "search.recurrences.expanded.exception": {"support": "unsupported"},
+    "search.recurrences.expanded.exception": {"support": "full"},
     "principal-search": {"support": "unsupported"},
     ## this only applies for very simple installations
     "auto-connect.url": {"domain": "localhost", "scheme": "http", "basepath": "/"},
@@ -947,7 +951,6 @@ def dotted_feature_set_list(self, compact=False):
     ## I'm surprised, I'm quite sure this was reported ungraceful earlier.  Passed with caldav commit a98d50490b872e9b9d8e93e2e401c936ad193003, caldav server checker commit 3cae24cf99da1702b851b5a74a9b88c8e5317dad 2026-02-15.  The commit 3cae24cf99da1702b851b5a74a9b88c8e5317dad was however development done on the wrong branch and has been force-pushed awway.  It was again observed ungraceful at commits be26d42b1ca3ff3b4fd183761b4a9b024ce12b84 / 537a23b145487006bb987dee5ab9e00cdebb0492
     'search.comp-type.optional': {'support': 'ungraceful'},
     'search.recurrences.expanded.todo': {'support': 'unsupported'},
-    'search.recurrences.expanded.exception': {'support': 'unsupported'}, ## TODO: verify
     "search.recurrences.includes-implicit.infinite-scope": False,
     'delete-calendar': {
         'support': 'fragile',
@@ -961,7 +964,6 @@ def dotted_feature_set_list(self, compact=False):
     'principal-search.by-name.self': {'support': 'unsupported'},
     'principal-search': {'support': 'ungraceful'},
     'search.time-range.open.start.duration': 'broken',
-    #'old_flags': ['unique_calendar_ids'],
     ## I'm surprised, I'm quite sure this was passing earlier.  Caldav commit a98d50490b872e9b9d8e93e2e401c936ad193003, caldav server checker commit 3cae24cf99da1702b851b5a74a9b88c8e5317dad
     'search.combined-is-logical-and': False,
     ## Observed with Nextcloud 33: server delivers iTIP notification to the inbox AND
@@ -993,10 +995,11 @@ def dotted_feature_set_list(self, compact=False):
 zimbra = {
     'auto-connect.url': {'basepath': '/dav/'},
     'delete-calendar': {'support': 'fragile', 'behaviour': 'may move to trashbin instead of deleting immediately'},
-    'save-load.get-by-url': {'support': 'fragile', 'behaviour': '404 most of the time - but sometimes 200.  Weird, should be investigated more'},
+    ## This is a zimbra bug when creating calendars with a display
+    ## name.  Now mitigated in the calendar creation code.
+    #'save-load.get-by-url': {'support': 'fragile', 'behaviour': '404 most of the time - but sometimes 200.  Weird, should be investigated more'},
     ## Zimbra treats same-UID events across calendars as aliases of the same event
     'save.duplicate-uid.cross-calendar': {'support': 'unsupported'},
-    'search.recurrences.expanded.exception': {'support': 'unsupported'}, ## TODO: verify
     'create-calendar.set-displayname': {'support': 'unsupported'},
     'save-load.todo.mixed-calendar': {'support': 'unsupported'},
     'save-load.todo.recurrences.count': {'support': 'unsupported'}, ## This is a new problem?
@@ -1060,7 +1063,6 @@ def dotted_feature_set_list(self, compact=False):
     "search.text": False, ## sometimes ungraceful
     "search.recurrences.includes-implicit": False,
     "sync-token": { "support": "fragile" },
-    "search.recurrences.expanded.exception": False,
     "search.recurrences.expanded.event": False,
     "search.recurrences.expanded.todo": False,
     'search.comp-type': {'support': 'broken', 'behaviour': 'Server returns everything when searching for events and nothing when searching for todos'},
@@ -1095,7 +1097,6 @@ def dotted_feature_set_list(self, compact=False):
     'search.is-not-defined': {'support': 'fragile', 'behaviour': 'works for CLASS but not for CATEGORIES'},
     'search.text.case-sensitive': {'support': 'unsupported'},
     'search.time-range.alarm': {'support': 'unsupported'},
-    "search.recurrences.expanded.exception": False,
     'old_flags': ['vtodo_datesearch_nodtstart_task_is_skipped'],
     'test-calendar': {'cleanup-regime': 'wipe-calendar'},
     'scheduling.schedule-tag': False,
@@ -1109,7 +1110,6 @@ def dotted_feature_set_list(self, compact=False):
     "http.multiplexing": "fragile", ## ref https://github.com/python-caldav/caldav/issues/564
     'search.comp-type.optional': {'support': 'ungraceful'},
     'search.recurrences.expanded.todo': {'support': 'unsupported'},
-    'search.recurrences.expanded.exception': {'support': 'unsupported'},
     'search.recurrences.includes-implicit.todo': {'support': 'unsupported'},
     "search.recurrences.includes-implicit.infinite-scope": False,
     'save-load.journal.mixed-calendar': {'support': 'unsupported'},
@@ -1134,7 +1134,6 @@ def dotted_feature_set_list(self, compact=False):
 
 cyrus = {
     "search.comp-type.optional": {"support": "ungraceful"},
-    "search.recurrences.expanded.exception": {"support": "unsupported"},
     "search.recurrences.includes-implicit.infinite-scope": False,
     "search.time-range.alarm": {"support": "ungraceful"},
     'principal-search': {'support': 'ungraceful'},
@@ -1156,7 +1155,6 @@ def dotted_feature_set_list(self, compact=False):
 
 ## See comments on https://github.com/python-caldav/caldav/issues/3
 #icloud = [
-#    'unique_calendar_ids',
 #    'duplicate_in_other_calendar_with_same_uid_breaks',
 #    'sticky_events',
 #    'no_journal', ## it threw a 500 internal server error!
@@ -1175,7 +1173,6 @@ def dotted_feature_set_list(self, compact=False):
     # into their calendar.
     "scheduling.schedule-tag": False,
     "search.comp-type.optional": { "support": "fragile" },
-    "search.recurrences.expanded.exception": { "support": "unsupported" },
     "search.time-range.alarm": { "support": "unsupported" },
     'sync-token': {'support': 'fragile'},
     'principal-search': {'support': 'unsupported'},
@@ -1285,7 +1282,6 @@ def dotted_feature_set_list(self, compact=False):
     "search.comp-type.optional": { "support": "ungraceful" },
     "search.recurrences.expanded.todo": { "support": "unsupported" },
     "search.recurrences.expanded.event": { "support": "fragile" },
-    "search.recurrences.expanded.exception": { "support": "unsupported" },
     'search.recurrences.includes-implicit.todo': {'support': 'unsupported'},
     'principal-search': {'support': 'ungraceful'},
     'freebusy-query': {'support': 'ungraceful'},
@@ -1327,7 +1323,6 @@ def dotted_feature_set_list(self, compact=False):
     ## foo ... "full" observed, 70938dc1cbb6a839978eee4315699746d38ee5f0/3cae24cf99da1702b851b5a74a9b88c8e5317dad, 2026-02-17
     #'search.time-range.todo.old-dates': {'support': 'unsupported'},
     'search.recurrences.expanded.todo': {'support': 'unsupported'},
-    'search.recurrences.expanded.exception': {'support': 'unsupported'},
     'search.recurrences.includes-implicit.todo': {'support': 'unsupported'},
     'search.combined-is-logical-and': {'support': 'unsupported'},
     'sync-token': {'support': 'ungraceful'},
@@ -1356,7 +1351,6 @@ def dotted_feature_set_list(self, compact=False):
     # attendee inbox AND auto-schedules into their calendar.
     "scheduling.schedule-tag": False,
     "search.recurrences.expanded.todo": {"support": "unsupported"},
-    "search.recurrences.expanded.exception": {"support": "unsupported"},
     "search.recurrences.includes-implicit.todo": {"support": "unsupported"},
     "search.recurrences.includes-implicit.infinite-scope": False,
     "principal-search.by-name.self": {"support": "unsupported"},
@@ -1467,7 +1461,6 @@ def dotted_feature_set_list(self, compact=False):
     'search.time-range.event': {'support': 'fragile'},
     ## was: ungraceful - observed unsupported 2026-02 (for .old-dates)
     'search.time-range.todo': {'support': 'fragile'},
-    'search.recurrences.expanded.exception': {'support': 'unsupported'},
     'principal-search': {'support': 'ungraceful'},
     'principal-search.by-name.self': {'support': 'ungraceful'},
     'principal-search.list-all': {'support': 'ungraceful'},

tests/test_caldav.py

@@ -1322,12 +1322,8 @@ def setup_method(self):
         for flag in self.old_features:
             assert flag in incompatibility_description
 
-        if self.check_compatibility_flag("unique_calendar_ids"):
-            self.testcal_id = "testcalendar-" + str(uuid.uuid4())
-            self.testcal_id2 = "testcalendar-" + str(uuid.uuid4())
-        else:
-            self.testcal_id = "pythoncaldav-test"
-            self.testcal_id2 = "pythoncaldav-test2"
+        self.testcal_id = "pythoncaldav-test"
+        self.testcal_id2 = "pythoncaldav-test2"
 
         foo = self.is_supported("rate-limit", dict)
         if foo.get("enable"):
@@ -1408,8 +1404,6 @@ def _cleanup(self, mode=None):
                     pass
             else:
                 cal.delete()
-        if self.check_compatibility_flag("unique_calendar_ids") and mode == "pre":
-            a = self._teardownCalendar(name="Yep")
         for calid in (self.testcal_id, self.testcal_id2, self.testcal_id + "-tasks"):
             self._teardownCalendar(cal_id=calid)
         if self.cleanup_regime == "thorough":
@@ -1471,10 +1465,7 @@ def _fixCalendar_(self, **kwargs):
 
         # Pre-processing: set up defaults for name and cal_id
         if "name" not in kwargs:
-            if not self.check_compatibility_flag("unique_calendar_ids") and self.cleanup_regime in (
-                "light",
-                "pre",
-            ):
+            if self.cleanup_regime in ("light", "pre"):
                 self._teardownCalendar(cal_id=self.testcal_id)
             if not self.is_supported("create-calendar.set-displayname"):
                 kwargs["name"] = None
@@ -2103,7 +2094,7 @@ def testObjectBySyncToken(self):
             assert len(list(my_changed_objects)) == 0
 
         ## I was unable to run the rest of the tests towards Google using their legacy caldav API
-        self.skip_on_compatibility_flag("no_overwrite")
+        self.skip_unless_support("save-load.mutable")
 
         ## MODIFYING an object
         if is_time_based:
@@ -2234,7 +2225,7 @@ def testSync(self):
             time.sleep(1)
 
         ## I was unable to run the rest of the tests towards Google using their legacy caldav API
-        self.skip_on_compatibility_flag("no_overwrite")
+        self.skip_unless_support("save-load.mutable")
 
         ## MODIFYING an object
         obj.icalendar_instance.subcomponents[0]["SUMMARY"] = "foobar"
@@ -2291,10 +2282,7 @@ def testSync(self):
     def testLoadEvent(self):
         self.skip_unless_support("save-load.event")
         self.skip_unless_support("create-calendar")
-        if not self.check_compatibility_flag("unique_calendar_ids") and self.cleanup_regime in (
-            "light",
-            "pre",
-        ):
+        if self.cleanup_regime in ("light", "pre"):
             self._teardownCalendar(cal_id=self.testcal_id)
             self._teardownCalendar(cal_id=self.testcal_id2)
         c1 = self._fixCalendar(name="Yep", cal_id=self.testcal_id)
@@ -2307,20 +2295,14 @@ def testLoadEvent(self):
         if not self.check_compatibility_flag("event_by_url_is_broken"):
             assert e1.url == e1_.url
             e1.load()
-        if (
-            not self.check_compatibility_flag("unique_calendar_ids")
-            and self.cleanup_regime == "post"
-        ):
+        if self.cleanup_regime == "post":
             self._teardownCalendar(cal_id=self.testcal_id)
             self._teardownCalendar(cal_id=self.testcal_id2)
 
     def testCopyEvent(self):
         self.skip_unless_support("save-load.event")
         self.skip_unless_support("create-calendar")
-        if not self.check_compatibility_flag("unique_calendar_ids") and self.cleanup_regime in (
-            "light",
-            "pre",
-        ):
+        if self.cleanup_regime in ("light", "pre"):
             self._teardownCalendar(cal_id=self.testcal_id)
             self._teardownCalendar(cal_id=self.testcal_id2)
 
@@ -2366,10 +2348,7 @@ def testCopyEvent(self):
         else:
             assert len(c1.get_events()) == 2
 
-        if (
-            not self.check_compatibility_flag("unique_calendar_ids")
-            and self.cleanup_regime == "post"
-        ):
+        if self.cleanup_regime == "post":
             self._teardownCalendar(cal_id=self.testcal_id)
             self._teardownCalendar(cal_id=self.testcal_id2)
 
@@ -3496,10 +3475,7 @@ def testUtf8Event(self):
         # TODO: split up in creating a calendar with non-ascii name
         # and an event with non-ascii description
         self.skip_unless_support("create-calendar")
-        if not self.check_compatibility_flag("unique_calendar_ids") and self.cleanup_regime in (
-            "light",
-            "pre",
-        ):
+        if self.cleanup_regime in ("light", "pre"):
             self._teardownCalendar(cal_id=self.testcal_id)
 
         c = self._fixCalendar(name="Yølp", cal_id=self.testcal_id)
@@ -3519,19 +3495,13 @@ def testUtf8Event(self):
         if "zimbra" not in str(c.url):
             assert len(events) == 1
 
-        if (
-            not self.check_compatibility_flag("unique_calendar_ids")
-            and self.cleanup_regime == "post"
-        ):
+        if self.cleanup_regime == "post":
             self._teardownCalendar(cal_id=self.testcal_id)
 
     def testUnicodeEvent(self):
         self.skip_unless_support("save-load.event")
         self.skip_unless_support("create-calendar")
-        if not self.check_compatibility_flag("unique_calendar_ids") and self.cleanup_regime in (
-            "light",
-            "pre",
-        ):
+        if self.cleanup_regime in ("light", "pre"):
             self._teardownCalendar(cal_id=self.testcal_id)
         c = self._fixCalendar(name="Yølp", cal_id=self.testcal_id)
 
@@ -3565,18 +3535,15 @@ def testSetCalendarProperties(self):
 
         # Creating a new calendar with different ID but with existing name
         # TODO: why do we do this?
-        if not self.check_compatibility_flag("unique_calendar_ids") and self.cleanup_regime in (
-            "light",
-            "pre",
-        ):
+        # TODO: we're doing this all over the placee, it should be consolidated
+        # TODO: it should be in the test setup/teardown
+        if self.cleanup_regime in ("light", "pre"):
             self._teardownCalendar(cal_id=self.testcal_id2)
         cc = self._fixCalendar(name="Yep", cal_id=self.testcal_id2)
         try:
             cc.delete()
         except error.DeleteError:
-            if not self.is_supported("delete-calendar") or self.check_compatibility_flag(
-                "unique_calendar_ids"
-            ):
+            if not self.is_supported("delete-calendar"):
                 raise
 
         c.set_properties(
@@ -3696,7 +3663,7 @@ def testCreateOverwriteDeleteEvent(self):
 
         ## add same event again.  As it has same uid, it should be overwritten
         ## (but some calendars may throw a "409 Conflict")
-        if not self.check_compatibility_flag("no_overwrite"):
+        if self.is_supported("save-load.mutable"):
             e2 = c.add_event(ev1)
             if todo_ok:
                 t2 = c.add_todo(todo)
@@ -3742,7 +3709,7 @@ def testCreateOverwriteDeleteEvent(self):
         # Verify that we can't look it up, both by URL and by ID
         with pytest.raises(self._notFound()):
             c.event_by_url(e1.url)
-        if not self.check_compatibility_flag("no_overwrite"):
+        if self.is_supported("save-load.mutable"):
             with pytest.raises(self._notFound()):
                 c.event_by_url(e2.url)
         if not self.check_compatibility_flag("event_by_url_is_broken"):
@@ -3799,7 +3766,7 @@ def testDateSearchAndFreeBusy(self):
         ## (But events should not be immutable!  One should be able to change an event, push the changes
         ## out to all participants and all copies of the calendar, and let everyone know that it's a
         ## changed event and not a cancellation and a new event).
-        self.skip_on_compatibility_flag("no_overwrite")
+        self.skip_unless_support("save-load.mutable")
 
         # ev2 is same UID, but one year ahead.
         # The timestamp should change.