fix: tolerate an empty calendar-user-address-set

CHANGELOG.md

@@ -17,6 +17,12 @@ This project should adhere to [Semantic Versioning](https://semver.org/spec/v2.0
 ### Added
 
 * `compatibility_hints`: `auth.www-authenticate` records whether the server sends the `WWW-Authenticate` header RFC7235 section 3.1 requires on a 401, and `auth.www-authenticate.usable-scheme` whether the schemes it offers include one this library implements.  A server failing either one never receives your password, and the 401 looks like a rejected one - so it needs `auth_type` pinned in the configuration, and a profile can now say which.  Probed by caldav-server-tester.  See https://github.com/python-caldav/caldav/issues/713.
+* `compatibility_hints`: new feature `scheduling.calendar-user-address-set.populated`, for a server that advertises `calendar-user-address-set` but returns it empty.  Graded `unsupported` for Xandikos.
+
+### Fixed
+
+* `Principal.get_vcal_address()` raised `IndexError: list index out of range` when the server returned an empty `calendar-user-address-set`.  It now falls back to the principal URL, as RFC 6638 section 2.4.1 provides for a user with no well-defined calendar user address.  `add_organizer()` and `add_attendee()` go through the same method, so they were affected too.  Seen on Xandikos 0.4.7, which advertises `calendar-auto-schedule` and serves schedule-inbox/outbox, but leaves the address set empty.  `change_attendee_status()` accepts that same URL back, so an event the library invited a principal to can still have its PARTSTAT changed.  A property that is *absent* still raises `NotFoundError`; per the same section that means the user is not enabled for scheduling.  En passant, the Xandikos profile is regraded for 0.4.7: scheduling is no longer declared unsupported, and `create-calendar.with-supported-component-types` no longer unsupported either, so `is_supported()` may answer differently with `features: xandikos` configured.
+
 
 ## [3.3.1] - 2026-09-16
 

caldav/calendarobjectresource.py

@@ -1270,6 +1270,11 @@ def change_attendee_status(self, attendee: Any | None = None, **kwargs) -> None:
         if isinstance(attendee, Principal):
             try:
                 attendee_emails = attendee.calendar_user_address_set()
+                ## Served but empty: the principal has no address of its own,
+                ## so it was invited under its URL - see get_vcal_address()
+                ## and RFC 6638 section 2.4.1.
+                if not attendee_emails:
+                    attendee_emails = [str(attendee.url)]
             except error.NotFoundError:
                 ## Server does not expose calendar-user-address-set (RFC6638 §2.4.1).
                 ## Fall back to client.username if it looks like an email address.

caldav/collection.py

@@ -647,7 +647,13 @@ def get_vcal_address(self) -> "vCalAddress | Coroutine[Any, Any, vCalAddress]":
         cn = self.get_display_name()
         ids = self.calendar_user_address_set()
         cutype = self.get_property(cdav.CalendarUserType())
-        ret = vCalAddress(ids[0])
+        ## A server may advertise calendar-user-address-set and still return it
+        ## empty (Xandikos does).  RFC 6638 section 2.4.1: "In the event that a
+        ## user has no well-defined identifier for his calendar user address,
+        ## the URI of his principal resource can be used."  An *absent*
+        ## property is a different thing - that means the user is not enabled
+        ## for scheduling at all, and calendar_user_address_set() raises.
+        ret = vCalAddress(next((i for i in ids if i), None) or str(self.url))
         ret.params["cn"] = vText(cn)
         ret.params["cutype"] = vText(cutype)
         return ret
@@ -666,7 +672,9 @@ async def _async_get_vcal_address(self) -> "vCalAddress":
         assert not [x for x in addresses_el if x.tag != dav.Href().tag]
         addresses = sorted(list(addresses_el), key=lambda x: -int(x.get("preferred", 0)))
         cutype = await self.get_property(cdav.CalendarUserType())
-        ret = vCalAddress(addresses[0].text)
+        ## empty-but-present property: the principal URL is the address,
+        ## see the comment in the sync get_vcal_address()
+        ret = vCalAddress(next((a.text for a in addresses if a.text), None) or str(self.url))
         ret.params["cn"] = vText(cn)
         ret.params["cutype"] = vText(cutype)
         return ret

caldav/compatibility_hints.py

@@ -689,6 +689,11 @@ class FeatureSet:
             "description": "Server provides the calendar-user-address-set property on the principal (RFC6638 section 2.4.1), used to identify a user's email/URI for scheduling purposes. When unsupported, calendar_user_address_set() raises NotFoundError.",
             "links": ["https://datatracker.ietf.org/doc/html/rfc6638#section-2.4.1"],
         },
+        "scheduling.calendar-user-address-set.populated": {
+            "description": "The calendar-user-address-set property actually carries at least one address.  A server can advertise the property (so scheduling.calendar-user-address-set is supported) and still return it empty - Xandikos does, while advertising calendar-auto-schedule and serving schedule-inbox/outbox.  When unsupported, the principal has no calendar user address of its own, and RFC 6638 section 2.4.1 has the URI of the principal resource used instead: that is what get_vcal_address() returns and what add_organizer() and add_attendee() put in ORGANIZER/ATTENDEE.",
+            "links": ["https://datatracker.ietf.org/doc/html/rfc6638#section-2.4.1"],
+            "default": {"support": "full"},
+        },
         "scheduling.mailbox.inbox-delivery": {
             "description": "Server delivers incoming scheduling REQUEST messages to the attendee's schedule-inbox (RFC6638 section 4.1). See also scheduling.auto-schedule for whether the server additionally auto-processes invitations into the attendee's calendar.",
             "links": [
@@ -1334,16 +1339,13 @@ def compare(self, observed):
     ## this only applies for very simple installations
     "auto-connect.url": {"domain": "localhost", "scheme": "http", "basepath": "/"},
 
-    "scheduling": {"support": "unsupported"},
-
-    ## Every collection reports and takes the same hardcoded component list
-    ## (xandikos/web.py), and the supported-calendar-component-set property has
-    ## no setter - yet MKCALENDAR still answers 201, though RFC 4791 section
-    ## 5.3.1 has it fail when a property cannot be set.  Measured on 0.4.5,
-    ## 2026-09-15.
-    "create-calendar.with-supported-component-types": {
+    ## Scheduling is implemented (calendar-auto-schedule in the DAV header,
+    ## schedule-inbox/outbox served), but the principal has no address of its
+    ## own: the property is returned as an empty <C:calendar-user-address-set/>.
+    ## Measured on 0.4.7, 2026-09-20.
+    "scheduling.calendar-user-address-set.populated": {
         "support": "unsupported",
-        "behaviour": "the component set is ignored: a VTODO-only calendar advertises VEVENT, VTODO, VJOURNAL, VFREEBUSY and VAVAILABILITY, and a VEVENT can be saved to it",
+        "behaviour": "the property is advertised but empty, so the principal URL is used as the calendar user address",
     },
 }
 

docs/source/async_tutorial.rst

@@ -332,6 +332,11 @@ Tasks work just like events, with ``await`` added:
             my_tasks = await cal.search(todo=True, include_completed=True)
             assert my_tasks
 
+            ## Some servers (i.e. Xandikos) will refuse to store events in a
+            ## VTODO-only calendar, so let's clean up and leave the server as
+            ## we found it.
+            await cal.delete()
+
     asyncio.run(main())
 
 The :meth:`~caldav.calendarobjectresource.Todo.complete` method is awaitable in

docs/source/tutorial.rst

@@ -320,6 +320,11 @@ There is some extra functionality around tasks, including the possibility to :me
             todo=True, include_completed=True)
         assert my_tasks
 
+        ## Some servers (i.e. Xandikos) will refuse to store events in a
+        ## VTODO-only calendar, so let's clean up and leave the server as we
+        ## found it.
+        cal.delete()
+
 Further Reading
 ---------------
 

pyproject.toml

@@ -130,10 +130,12 @@ test = [
 ignore = ["DEP002"]  # Test dependencies (pytest, coverage, etc.) are not imported in main code
 
 [tool.deptry.per_rule_ignores]
-DEP001 = ["conf", "h2"]  # conf: Local test config, h2: Optional HTTP/2 support.
+DEP001 = ["conf", "h2", "tests"]  # conf: Local test config, h2: Optional HTTP/2 support.
 ## httpxyz needed an entry here while it was imported by name; the httpx-family
 ## libraries are now imported dynamically (see _ASYNC_HTTPX_CANDIDATES), which
 ## deptry does not see at all.
+## tests: caldav/config.py optionally imports tests.test_servers.registry (source
+## tree only, guarded by ImportError) to auto-start a test server from a checkout.
 DEP003 = ["aiohttp", "h2"]  # aiohttp: optional dep used only in caldav/testing.py
 ## h2 needs an entry in both DEP001 and DEP003: which of the two the optional
 ## `import h2` in async_davclient.py trips depends on whether h2 happens to be

tests/test_caldav.py

@@ -1757,9 +1757,46 @@ def testAddOrganizer(self):
         org = event.icalendar_component.get("organizer")
         assert org is not None, "ORGANIZER should be set when add_organizer() uses principal"
         principal_addresses = self.principal.calendar_user_address_set()
-        assert any(addr in str(org) for addr in principal_addresses), (
-            f"ORGANIZER {org!r} should contain one of the principal's addresses {principal_addresses!r}"
-        )
+        populated = self.is_supported("scheduling.calendar-user-address-set.populated", str)
+        if populated == "unknown":
+            pytest.skip("nobody has probed whether this server populates the address set")
+        if populated != "unsupported":
+            assert any(addr in str(org) for addr in principal_addresses), (
+                f"ORGANIZER {org!r} should contain one of the principal's addresses {principal_addresses!r}"
+            )
+        else:
+            ## The server advertises the property but leaves it empty, so the
+            ## principal has no address of its own and RFC 6638 section 2.4.1
+            ## has its URL stand in.
+            assert str(self.principal.url) in str(org), (
+                f"ORGANIZER {org!r} should fall back to the principal URL {self.principal.url!r} "
+                f"when the address set is empty"
+            )
+
+    def testChangeAttendeeStatusWithEmptyAddressSet(self):
+        """add_attendee(principal) then change_attendee_status(principal) on a
+        server whose calendar-user-address-set is served but empty.
+
+        The library writes the principal URL as the ATTENDEE (RFC6638 section
+        2.4.1), so it has to accept that same URL back when asked to change
+        the PARTSTAT - otherwise it builds an event it cannot itself update.
+        """
+        self.skip_unless_support("scheduling.calendar-user-address-set")
+        if (
+            self.is_supported("scheduling.calendar-user-address-set.populated", str)
+            != "unsupported"
+        ):
+            pytest.skip("server populates calendar-user-address-set; nothing to fall back to")
+
+        cal = self._fixCalendar()
+        event = cal.save_event(ev1)
+        event.add_attendee(self.principal)
+        event.save()
+
+        event.change_attendee_status(self.principal, PARTSTAT="ACCEPTED")
+        attendee = event.icalendar_component.get("attendee")
+        assert attendee is not None
+        assert str(attendee.params.get("PARTSTAT")) == "ACCEPTED"
 
     def testIssue399ChangeAttendeeStatusUsernameEmailFallback(self):
         """change_attendee_status() works when the attendee is identified