25课一些函数的作用

def break_words(stuff):
    """This function will break uo words for us."""
    words = stuff.split(' ')
    return words#返回分割后的字符串列表

def sort_words(words):
    """Sorts the words."""
    return sorted(words)#返回重新排序的列表

def print_first_word(words):
    """Prints the first word after popping it off."""
    word = words.pop(0)  #pop() 函数用于移除列表中的一个元素（默认最后一个元素），并且返回该元素的值

def print_last_word(words):
    """Prints the first word after popping it off."""
    word = words.pop(-1)#语法：list.pop(obj=list[-1])     #默认为 index=-1，删除最后一个列表值。
                                       
    print(word)

def sort_sentence(sentence):
    """Prints the last word after popping it off"""
    words = break_words(sentence)
    return sort_words(words)

def print_first_and_last(sentence):
    """Prints the first and last words of the sentence."""
    words = break_words(sentence)
    print_first_word(words)
    print_last_word(words)

def print_first_and_last_sorted(sentence):
    """Sorts the words then prints the first and last one."""
    words = sort_sentence(sentence)
    print_first_word(words)
    print_last_word(words)