为什么附加到列表会在 Python 中引发 NoneType 错误? [英] Why does appending to a list raise a NoneType error in Python?

问题描述

这是我的代码:

text="""col1 col2 col3
a 1 $
b 2 @
c 3 &
"""
mList = []
for line in text.splitlines():
    for item in line.split(" "):
        mList = mList.append(item)

这会引发错误，'NoneType' 对象没有属性 'append'.我尝试了其他方法来创建列表并执行此操作，但我得到的最好的方法是将列表变为 None.这是怎么回事?

This raises an error, 'NoneType' object has no attribute 'append'. I've tried other ways of creating the list and doing this, but the best I get is the list turned to None. What's going on here?

推荐答案

list.append() 是一个就地方法，它不返回任何内容(因此默认情况下它返回 None ，因为所有函数 calls 必须返回一些值，如果某个函数没有显式返回任何值，则调用返回`None).

list.append() is an in-place method, it does not return anything ( and hence by default it returns None , as all function calls must return some value, and if some function does not explicitly return anything, the call returns `None).

因此，当您将其分配回 mList 时，它将变为 None 并且在您执行的下一次迭代中 - mList.append()> ，它会出错，因为 mList 是 None.

Hence, when you assign it back to mList, it becomes None and in the next iteration when you do - mList.append() , it errors out as mList is None.

你应该试试——

mList.append(item)

<小时>

或者你可以简单地在列表理解中做你想做的事情 -

---

Or you can simply do what you are trying to do in a list comprehension -

mList = [item for line in text.splitlines() for item in line.split(" ")]

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