Python 追加将我的列表变为 NoneType
声明：本页面是StackOverFlow热门问题的中英对照翻译，遵循CC BY-SA 4.0协议，如果您需要使用它，必须同样遵循CC BY-SA许可，注明原文地址和作者信息，同时你必须将它归于原作者（不是我）：StackOverFlow
原文地址: http://stackoverflow.com/questions/3840784/
Warning: these are provided under cc-by-sa 4.0 license. You are free to use/share it, But you must attribute it to the original authors (not me):
StackOverFlow
Appending turns my list to NoneType
提问by user460847
In Python Shell, I entered:
在 Python Shell 中，我输入：
aList = ['a', 'b', 'c', 'd']
for i in aList:
print(i)
and got
并得到
a
b
c
d
but when I tried:
但是当我尝试：
aList = ['a', 'b', 'c', 'd']
aList = aList.append('e')
for i in aList:
print(i)
and got
并得到
Traceback (most recent call last):
File "<pyshell#22>", line 1, in <module>
for i in aList:
TypeError: 'NoneType' object is not iterable
Does anyone know what's going on? How can I fix/get around it?
有谁知道发生了什么？我该如何解决/解决它？
采纳答案by Thomas Wouters
list.appendis a method that modifies the existing list. It doesn't return a new list -- it returns None, like most methods that modify the list. Simply do aList.append('e')and your list will get the element appended.
list.append是一种修改现有列表的方法。它不返回新列表——它返回None，就像大多数修改列表的方法一样。只需这样做aList.append('e')，您的列表就会附加元素。
回答by aletelecomm
Delete your second line aList = aList.append('e')and use only aList.append("e"), this should get rid of that problem.
删除你的第二行aList = aList.append('e')并使用 only aList.append("e")，这应该可以解决这个问题。
回答by SCB
Generally, what you want is the accepted answer. But if you want the behavior of overriding the value and creating a new list (which is reasonable in some cases^), what you could do instead is use the "splat operator", also known as list unpacking:
通常，您想要的是公认的答案。但是，如果您想要覆盖值并创建新列表的行为（在某些情况下这是合理的^），您可以做的是使用“splat 运算符”，也称为列表解包：
aList = [*aList, 'e']
#: ['a', 'b', 'c', 'd', 'e']
Or, if you need to support python 2, use the +operator:
或者，如果您需要支持 python 2，请使用+运算符：
aList = aList + ['e']
#: ['a', 'b', 'c', 'd', 'e']
^ There are many cases where you want to avoid the side effects of mutating with .append(). For one, imagine you want to append something to a list you've taken as a function argument. Whoever is using the function probably doesn't expect that the list they provided is going to be changed. Using something like this keeps your function "pure"without "side effects".
^ 在很多情况下，您希望避免使用.append(). 一方面，假设您想将某些内容附加到您作为函数参数的列表中。使用该功能的人可能不希望他们提供的列表会发生变化。使用这样的东西可以让你的功能“纯粹”而没有“副作用”。
