笨方法学python3 习题25，给sorted_words重新赋值，为什么不行，告诉我list不能分割

这是代码：
def break_words(stuff):
"""This function will break up words for us."""
words = stuff.split(' ')
return words

def sort_words(words):
"""Sorts the words"""
return sorted(words)

def print_first_word(words):
"""Print the first word after popping it off"""
word = words.pop(0)
print(word)

def print_lat_word(words):
"""Prints the last word after poping it off"""
word = words.pop(-1)
print(word)

def sort_sentence(sentence):
"""Takes in a full sentence and returns the sorted words"""
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_lat_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_lat_word(words)
这是返回结果：
sorted_words = test25.sort_sentence(sentence)
Traceback (most recent call last):
File "", line 1, in
File "F:\Python\test25.py", line 23, in sort_sentence
return sort_words(words)
File "F:\Python\test25.py", line 22, in sort_sentence
words = break_words(sentence)
File "F:\Python\test25.py", line 3, in break_words
words = stuff.split(' ')
AttributeError: 'list' object has no attribute 'split'
为什么？求大神指教，用的是pycharm