这段代码中list [:]的含义是什么? [英] What is the meaning of list[:] in this code?

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)

为什么这段代码会创建一个无限循环，而第一个却没有呢?

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，可以逃脱初学者.

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

words[:]是这里的魔术酱.

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']

这里要注意的主要事情是words[:]返回现有列表的copy，因此您要遍历未修改的副本.

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()检查是否引用了相同的列表:

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

在第一种情况下:

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

在第二种情况下:

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

值得注意的是，[i:j]被称为 slicing operator ，它的作用是从索引i直到(但不包括)返回列表的新副本.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']

省略起始索引意味着它默认为0，而省略最后一个索引意味着它默认为len(words)，最终结果是您收到了 entire 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.

如果您想使代码更具可读性，建议使用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)

这基本上与您的第一个代码段相同，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(如DSM在评论中所提到的)和在python> = 3上，您也可以使用words.copy()来执行相同的操作.

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

这篇关于这段代码中list [:]的含义是什么?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！