'Computes and returns the course letter grade given the
inputs, as defined in the grading_policy (e.g. \'A\' \'B\' \'C\')
or None if not passed.'

@staticmethod
def _compute_letter_grade(grade_cutoffs, percent):

    letter_grade = None
    descending_grades = sorted(grade_cutoffs, key=(lambda x: grade_cutoffs[x]), reverse=True)
    for possible_grade in descending_grades:
        if (percent >= grade_cutoffs[possible_grade]):
            letter_grade = possible_grade
            break
    return letter_grade
