'Returns grades for the subsections in the course in
a dict keyed by subsection format types.'

@lazy
def graded_subsections_by_format(self):

    subsections_by_format = defaultdict(OrderedDict)
    for chapter in self.chapter_grades.itervalues():
        for subsection_grade in chapter['sections']:
            if subsection_grade.graded:
                graded_total = subsection_grade.graded_total
                if (graded_total.possible > 0):
                    subsections_by_format[subsection_grade.format][subsection_grade.location] = subsection_grade
    return subsections_by_format
