Probable bug in DURATION getter

[jrd]

icalendar/src/icalendar/attr.py

Lines 956 to 960 in 2f6ce64

	if duration is not default and not isinstance(duration, timedelta):
	raise InvalidCalendar(
	f"DURATION must be a timedelta, not {type(duration).__name__}."
	)
	return None

While reading the source code I think I found a little error.

If the duration property exist (so it’s not default object) and it’s already a timedelta instance, then currently None is returned.

Maybe it should be something like that:

    if isinstance(duration, timedelta):  # it cannot be default as default is not a timedelta instance
        return duration
    elif duration is not default:
        raise InvalidCalendar(
            f"DURATION must be a timedelta, not {type(duration).__name__}."
        )
    return None

But maybe I didn’t understand the code correctly…

[niccokunzmann]

Hi, thanks forn reporting this!
That seems weird indeed. Would you like to create a pull request with a test?