`get_supported_components` throws error if tag is missing

[ColonelThirtyTwo]

I'm getting this error from HomeAssistant's CalDav integration, which uses this library:

Error while setting up caldav platform for calendar: '{urn:ietf:params:xml:ns:caldav}supported-calendar-component-set'
Traceback (most recent call last):
  File "/usr/src/homeassistant/homeassistant/helpers/entity_platform.py", line 455, in _async_setup_platform
    await asyncio.shield(awaitable)
  File "/usr/src/homeassistant/homeassistant/components/caldav/calendar.py", line 158, in async_setup_entry
    calendars = await async_get_calendars(hass, entry.runtime_data, SUPPORTED_COMPONENT)
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/src/homeassistant/homeassistant/components/caldav/api.py", line 20, in async_get_calendars
    return await hass.async_add_executor_job(_get_calendars)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.14/concurrent/futures/thread.py", line 86, in run
    result = ctx.run(self.task)
  File "/usr/local/lib/python3.14/concurrent/futures/thread.py", line 73, in run
    return fn(*args, **kwargs)
  File "/usr/src/homeassistant/homeassistant/components/caldav/api.py", line 17, in _get_calendars
    if component in calendar.get_supported_components()
                    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^
  File "/usr/local/lib/python3.14/site-packages/caldav/collection.py", line 500, in get_supported_components
    prop = response_list[unquote(self.url.path)][
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^
        cdav.SupportedCalendarComponentSet().tag
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    ]
    ^
KeyError: '{urn:ietf:params:xml:ns:caldav}supported-calendar-component-set'

RFC 4791 specifies that "This property MAY be defined on any calendar collection", so it is an optional element. However, Calendar.get_supported_components uses a standard attribute access to get the element in the fallback path, which fails with a KeyError if it does not exist.

get_supported_components should gracefully handle this case and return an empty list if the tag is missing.

[tobixen]

Wow - async support in the caldav library was released in v3.0.0 just some weeks ago, and HomeAssistant is already using it? Despite the warning in my CHANGELOG that "if you're using the Python CalDAV client library in some sharp production environment, I would recommend to wait for two months before upgrading"? :-)

I will look into this and come up with a fix - hope to be able to do it this week, but no guarantees given.

[tobixen]

Oh, I read the traceback wrongly. The async components is actually from the homeassistant API. That explains, I could not remember to have made an api.py. :-) Anyway, I'm onto it.

[tobixen]

The RFC reads:

In the absence of this property, the server MUST accept all component types, and the client can assume that all component types are accepted.

Hence, returning an empty list is the wrong solution. It basically means "this calendar does not support any components - don't try saving anything at all here". The default fallback should be to return the full list (VJOURNAL, VTODO, VEVENT and VTIMEZONE). Now, that's a trivial change - but I'd like to go a bit further, I'd like to implement is so that if it's well-known that the server only supports VEVENT and VTODO - or perhaps only VEVENT - then it should only return that. And I need to also write a check-script to see what servers supports it. And then I'd like to test it on some 16 different server implementations. So it's a bit more than a five-minute job indeed.