用列表理解替换字符串时出现类型错误

['found' if 'some_string' in word else word for word in wordlist]

但它又回来了

    TypeError                                 
Traceback (most recent call last)
    <ipython-input-108-990cad51d935> in <module>()
    ----> 1 [word  for word in wordlist if 'some_string' in word ]

    <ipython-input-108-990cad51d935> in <listcomp>(.0)
    ----> 1 [word  for word in wordlist if 'some_string' in word ]

    TypeError: argument of type 'NoneType' is not iterable

相比之下,此代码的运行完全符合预期:

for word in wordlist:
    if 'some_string' in word:
        print ('found')
    else:
        print(word)

进一步的上下文:wordlist是Jupyter笔记本中Pandas数据框的一列中的值。

wordlist = list(df.sender.values)
type(wordlist)
 list