'No students have grade entries.'

def test_all_empty_grades(self):

    with patch.object(BlockStructureFactory, 'create_from_store', wraps=BlockStructureFactory.create_from_store) as mock_create_from_store:
        (all_course_grades, all_errors) = self._course_grades_and_errors_for(self.course, self.students)
        self.assertEquals(mock_create_from_store.call_count, 1)
    self.assertEqual(len(all_errors), 0)
    for course_grade in all_course_grades.values():
        self.assertIsNone(course_grade.letter_grade)
        self.assertEqual(course_grade.percent, 0.0)
