python - 类型错误 : object of type 'NoneType' has no len() while iterating through a list

Question

这个问题已经有答案了:

Why do these list methods (append, sort, extend, remove, clear, reverse) return None rather than the resulting list? (5 个回答)

How can I collect the results of a repeated calculation in a list, dictionary etc. (or make a copy of a list with each element modified)? (2 个回答)

已关闭 7 年前。

对于下面的程序，我收到以下错误“TypeError:类型为‘NoneType’的对象没有 len()”。为什么我不能迭代一个列表并将其与另一个列表进行比较？

word_list = list() 

while True: 
    file_name = raw_input('Enter file name: ')
    if len(file_name) < 1: exit() 
    try: 
        file = open(file_name)
        break

    except: 
        print 'Please enter a valid file name.'
        continue

for line in file: 
    line = line.rstrip() 
    words = line.split()
    for word in words: 
        if len(word_list) <1:
            word_list = word_list.append(word)

        else: 
            if not word in word_list:
                word_list = word_list.append(word) 

word_list = word_list.sort() 
print word_list

Answer 1

list.append返回无。

以下行:

word_list = word_list.append(word)

应替换为:

word_list.append(word)

否则，word_list 会变为 None，导致稍后出现 TypeError。