TypeError：'NoneType'对象在执行“如果”条件时不可下标

Question

[当我在python 3中运行下面的代码并输入字母'a'时，出现错误：代码的最后一行显示” NoneType对象不可下标”。另一方面，当我在外壳中尝试类似的代码时，它将执行条件“ if”而不会出现问题。如果我能得到一些帮助，我会很高兴。 :)

letters_guessed = []
letters_guessed = letters_guessed.append(str.lower(str(input('Please guess a letter (you can only guess one letter at a time): '))))
            if letters_guessed[-1].isalpha() == True:
                ...

Answer 1

此行代码：

letters_guessed = letters_guessed.append(str.lower(str(input('Please guess a letter (you can only guess one letter at a time): '))))

错了。更改为：

letters_guessed.append(str.lower(str(input('Please guess a letter (you can only guess one letter at a time): '))))