snarfed/activitystreams-unofficial — pull request #14 by kylewm (2014-07-27 16:51:10+00:00)
Title: added Source.can_create() to check for known unsupported types

- allows us to give intelligent errors when the user tries to publish
something to a silo that just doesn't support it (e.g. Twitter and RSVPs)
- Source.preview_create() should raise the new type
CannotPublishTypeError to indicate a known unpublishable type