问题描述

此代码来自 Python 的文档.我有点困惑.

This code is from Python's Documentation. I'm a little confused.

words = ['cat', 'window', 'defenestrate']
for w in words[:]:
    if len(w) > 6:
        words.insert(0, w)
print(words)

以下是我最初的想法:

words = ['cat', 'window', 'defenestrate']
for w in words:
    if len(w) > 6:
        words.insert(0, w)
print(words)

为什么这段代码创建了一个无限循环而第一个没有?

Why does this code create a infinite loop and the first one doesn't?

推荐答案

这是陷阱之一！python的，可以逃脱初学者.

This is one of the gotchas! of python, that can escape beginners.

words[:] 是这里的魔法酱.

观察:

>>> words =  ['cat', 'window', 'defenestrate']
>>> words2 = words[:]
>>> words2.insert(0, 'hello')
>>> words2
['hello', 'cat', 'window', 'defenestrate']
>>> words
['cat', 'window', 'defenestrate']

现在没有 [:]:

>>> words =  ['cat', 'window', 'defenestrate']
>>> words2 = words
>>> words2.insert(0, 'hello')
>>> words2
['hello', 'cat', 'window', 'defenestrate']
>>> words
['hello', 'cat', 'window', 'defenestrate']

这里要注意的主要是 words[:] 返回现有列表的 copy，因此您正在迭代未修改的副本.

The main thing to note here is that words[:] returns a copy of the existing list, so you are iterating over a copy, which is not modified.

您可以使用 id() 检查您是否引用了相同的列表:

You can check whether you are referring to the same lists using id():

第一种情况:

>>> words2 = words[:]
>>> id(words2)
4360026736
>>> id(words)
4360188992
>>> words2 is words
False

第二种情况:

>>> id(words2)
4360188992
>>> id(words)
4360188992
>>> words2 is words
True

值得注意的是，[i:j]被称为切片操作符，它的作用是返回一个从索引<开始的列表的新副本code>i，直到(但不包括)索引 j.

It is worth noting that [i:j] is called the slicing operator, and what it does is it returns a fresh copy of the list starting from index i, upto (but not including) index j.

所以，words[0:2] 给你

>>> words[0:2]
['hello', 'cat']

省略起始索引表示默认为0，省略最后一个索引表示默认为len(words)，最终结果是收到一个整个列表的副本.

Omitting the starting index means it defaults to 0, while omitting the last index means it defaults to len(words), and the end result is that you receive a copy of the entire list.

如果你想让你的代码更具可读性，我推荐 copy 模块.

If you want to make your code a little more readable, I recommend the copy module.

from copy import copy 

words = ['cat', 'window', 'defenestrate']
for w in copy(words):
    if len(w) > 6:
        words.insert(0, w)
print(words)

这基本上和你的第一个代码片段做同样的事情，并且更具可读性.

This basically does the same thing as your first code snippet, and is much more readable.

或者(正如 DSM 在评论中提到的)和在 python >=3 上，您也可以使用 words.copy() 来做同样的事情.

Alternatively (as mentioned by DSM in the comments) and on python >=3, you may also use words.copy() which does the same thing.

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