Python TypeError: sequence item 0: expected str instance, NoneType found

问题

I copied this code from a book:

lyst = {'Hi':'Hello'}
def changeWord(sentence):
    def getWord(word):
        lyst.get(word, word)
    return ''.join(map(getWord, sentence.split()))

I get this error:

Traceback (most recent call last):
  File "<pyshell#4>", line 1, in <module>
    cambiaPronome('ciao')
  File "C:\Python33\2.py", line 6, in cambiaPronome
    return ''.join(map(getWord, list(frase)))
TypeError: sequence item 0: expected str instance, NoneType found

What is wrong?

回答1:

The getWord function doesn't explicitly return anything, so it implicitly returns None. Try returning something, e.g.:

def getWord(word):
    return lyst.get(word, word)

回答2:

The problem is you're trying to write to a string a type which is NoneType. That's not allowed.

If you're interested in getting the None values as well one of the things you can do is convert them to strings.

And you can do it with a list comprehension, like:

return ''.join([str(x) for x in map(getWord, sentence.split())])

But to do it properly in this case, you have to return something on the inner function, else you have the equivalent to return None

来源：https://stackoverflow.com/questions/25222697/python-typeerror-sequence-item-0-expected-str-instance-nonetype-found