为什么当函数“ None”不返回时，python在NoneType上失败？

Question

I am trying to create a list of keys from the filteredKeys() function, but when I call the function I get a TypeError: 'NoneType' object is not iterable.

'None' doesnt exist when I print the result of filteredKeys() but when I convert it to a list. What am i missing? It's driving me crazy!

        key_list = []
        for line in stdout.read().splitlines():
            input_line = line
            input_line = input_line.replace(b'[sudo] password for ufitadmin: ', b'')
            key_individual = str(input_line, 'utf-8')
            key_individual = key_individual.replace(
                'cat: /home/%s/.ssh/authorized_keys: No such file or directory' % user,
                'No key exists for this user.')
            key_list.append(key_individual)


        global keyDict
        keyDict = {}
        keyDict[host] = key_list
        filterString = "No key exists for this user."
        filteredKeylist = []

        def filteredKeys():
            for key,value in keyDict.items():
               for values in value:
                   if values == filterString:
                       return key



        filteredKeylist = list(filteredKeys())

        print(filteredKeylist)

Answer 1

Your filteredKeys() function is actually very likely to return None - when none of the values matches filterString .

Also, you state that you want to "create a list of keys from the filteredKeys() function", but since it returns a single value (either None or the first key having a value matching filterString ), it just WontWork as you expect.

A better implementation would be to make it a generator:

def filteredKeys():
    for key,values in keyDict.items():
       if filterString in values:
           yield key

filteredKeylist = list(filteredKeys())

but that's still a uselessly complicated way to write:

 filteredKeylist = [key for key, values in keyDict.items() if filterString in values]

Also globally your code seems overly complicated and I can already spot an obvious bug with keyDict being overwritten on each iteration.