<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def to_vobjects(self, filename, uids=None):
 """Return iCal objects and etags of all Taskwarrior entries in uids filename -- the Taskwarrior project uids -- the UIDs of the Taskwarrior tasks (all if None) """

        self._update()

        if not uids:
            uids = self.get_uids(filename)

        project = basename(filename)
        items = []

        for uid in uids:
            vtodos = iCalendar()
            uuid = uid.split('@')[0]
            self._gen_vtodo(self._tasks[project][uuid], vtodos.add('vtodo'))
            items.append((uid, vtodos, '"%s"' % self._tasks[project][uuid]['modified']))
        return items