用字典调用函数的意义

Question

我对函数和词典的使用非常困惑。我正在使用的书中有一个例子来学习python，但它没有解释为什么作者这样写的原因。这就是为什么我希望有人可以向我解释为什么脚本是用这种形式写的。这是脚本：

def get_specials():
    monday = {'B': 'Horseradish omelet. Note: better than it sounds', \
    'L': 'Momma\'s curry. Note: can be made spicy.',\
    'D': 'Beef brisket. Note: Comes with au jus. That\'s pronounced "Oh jhoo", not "Ow Juice"'}
    tuesday = {'B': 'Sausage gravy over biscuits. Note: Toast can be subbed.',\
    'L': 'Grilled cheese and tomato soup. Note: We have vegan cheese.',
    'D': 'Meatloaf. Note: Comes with catsup on the top. Not optional.'}
    wednesday = {'B': 'Horseradish omelet. Note: better than it sounds', \
    'L': 'Momma\'s curry. Note: can be made spicy.',\
    'D': 'Beef brisket. Note: Comes with au jus. That\'s pronounced "Oh jhoo", not "Ow Juice"'}
    thursday = {'B': 'Horseradish omelet. Note: better than it sounds', \
    'L': 'Momma\'s curry. Note: can be made spicy.',\
    'D': 'Beef brisket. Note: Comes with au jus. That\'s pronounced "Oh jhoo", not "Ow Juice"'}
    friday = {'B': 'Horseradish omelet. Note: better than it sounds', \
    'L': 'Momma\'s curry. Note: can be made spicy.',\
    'D': 'Beef brisket. Note: Comes with au jus. That\'s pronounced "Oh jhoo", not "Ow Juice"'}
    saturday = {'B': 'Horseradish omelet. Note: better than it sounds', \
    'L': 'Momma\'s curry. Note: can be made spicy.',\
    'D': 'Beef brisket. Note: Comes with au jus. That\'s pronounced "Oh jhoo", not "Ow Juice"'}
    sunday = {'B': 'Horseradish omelet. Note: better than it sounds', \
    'L': 'Momma\'s curry. Note: can be made spicy.',\
    'D': 'Beef brisket. Note: Comes with au jus. That\'s pronounced "Oh jhoo", not "Ow Juice"'}

    specials = {'M': monday, 'T': tuesday, 'W': wednesday, 'R': thursday, 'F': friday, 'St': saturday, 'Sn': sunday}
    return specials

def print_special(special):
    print "The special is: "
    print special
    print "*"*15

def get_day():
    while True:
        day = raw_input("Day (M/T/W/R/F/St/Sb): ")
        if day.upper() in ['M', 'T', 'W', 'R', 'F', 'ST', 'SN']:
            return day.upper()
        else:
            print "I'm sorry, but {} isn't valid.".format(day)

def get_time():
    while True:
        time =raw_input("Time (B/L/D): ")

        if time.upper() in ['B', 'L', 'D']:
            return time.upper()
        else:
            print "I'm sorry, but {} isn't a valid time.".format(time)

def main():
    specials = get_specials()
    print "This script will tell you the specials for any day of the week, and any time."

    while True:
        day = get_day()
        special = specials[day]

        time = get_time()
        print_special(special[time])
        another = raw_input("Do you want to check another day and time? (Y/N)")

        if another.lower() == 'n':
            break

if __name__ == '__main__':
    main()

我的怀疑主要在def main（）部分。有人可以向我解释这件事是如何被召集的吗？在此先感谢!!!