blocks|key|1174279|text|它的意思是“第一个参数没有任何东西，第二个参数没有任何东西，跳三个”。它每隔一段时间对序列中的每三个项目进行切片。Extended+slices就是你想要的。Python+2.3中的新特性|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1174280|entityMap|0|LINK|mutability|MUTABLE|url|http://docs.python.org/release/2.3.5/whatsnew/section-slices.html^0|1L|F|0|0^^$0|@$1|2|3|4|5|6|7|L|8|@]|9|@$A|M|B|N|1|O]]|C|$]]|$1|D|3|-4|5|6|7|P|8|@]|9|@]|C|$]]]|E|$F|$5|G|H|I|C|$J|K]]]]

it means 'nothing for the first argument, nothing for the second, and jump by three'. It gets every third item of the sequence sliced.
<a href="http://docs.python.org/release/2.3.5/whatsnew/section-slices.html">Extended slices</a> is what you want. New in Python 2.3

blocks|key|1174221|text|Python序列切片地址可以写为a[start:end:step]，并且可以删除开始、停止或结束中的任何一个。a[::3]是序列的第三个元素。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1174222|entityMap^0|G|H|1J|6|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]|$9|K|A|L|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|M|8|@]|D|@]|E|$]]]|G|$]]

Python sequence slice addresses can be written as <code>a[start:end:step]</code> and any of start, stop or end can be dropped. <code>a[::3]</code> is every third element of the sequence.

blocks|key|960019|text|seq[::n]是每个n-th项在整个序列中的序列。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|960020|示例：|960021|>>>+range(10)[::2]
[0,+2,+4,+6,+8]|code-block|syntax|javascript|960022|语法为：|960023|seq[start:end:step]|960024|所以你可以这样做(在Python+2中)：|960025|>>>+range(100)[5:18:2]
[5,+7,+9,+11,+13,+15,+17]|960026|entityMap^0|0|8|B|1|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|W|8|@$9|X|A|Y|B|C]|$9|Z|A|10|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|11|8|@]|D|@]|E|$]]|$1|H|3|I|5|J|7|12|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|13|8|@]|D|@]|E|$]]|$1|O|3|P|5|J|7|14|8|@]|D|@]|E|$K|L]]|$1|Q|3|R|5|6|7|15|8|@]|D|@]|E|$]]|$1|S|3|T|5|J|7|16|8|@]|D|@]|E|$K|L]]|$1|U|3|-4|5|6|7|17|8|@]|D|@]|E|$]]]|V|$]]

<code>seq[::n]</code> is a sequence of each <code>n</code>-th item in the entire sequence.
Example:
<pre><code>&gt;&gt;&gt; range(10)[::2]
[0, 2, 4, 6, 8]
</code></pre>
The syntax is:
<pre><code>seq[start:end:step]
</code></pre>
So you can do (in Python 2):
<pre><code>&gt;&gt;&gt; range(100)[5:18:2]
[5, 7, 9, 11, 13, 15, 17]
</code></pre>

blocks|key|1174168|text|在Python中进行切片时，第三个参数是step。正如其他人所提到的，请参阅Extended+Slices以获得一个很好的概述。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1174169|有了这些知识，[::3]就意味着您没有为您的切片指定任何开始或结束索引。由于您已经指定了一个步骤3，因此它将从第一个索引开始，每隔三个条目获取一次something。例如：|style|CODE|1174170|>>>+'123123123'[::3]
'111'|code-block|syntax|javascript|1174171|entityMap|0|LINK|mutability|MUTABLE|url|http://docs.python.org/release/2.3.5/whatsnew/section-slices.html^0|12|F|0|0|7|5|1C|1|21|9|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@]|9|@$A|V|B|W|1|X]]|C|$]]|$1|D|3|E|5|6|7|Y|8|@$A|Z|B|10|F|G]|$A|11|B|12|F|G]|$A|13|B|14|F|G]]|9|@]|C|$]]|$1|H|3|I|5|J|7|15|8|@]|9|@]|C|$K|L]]|$1|M|3|-4|5|6|7|16|8|@]|9|@]|C|$]]]|N|$O|$5|P|Q|R|C|$S|T]]]]

When slicing in Python the third parameter is the step. As others mentioned, see <a href="http://docs.python.org/release/2.3.5/whatsnew/section-slices.html" rel="noreferrer">Extended Slices</a> for a nice overview.

With this knowledge, <code>[::3]</code> just means that you have not specified any start or end indices for your slice. Since you have specified a step, <code>3</code>, this will take every third entry of <code>something</code> starting at the first index. For example:

<pre><code>&gt;&gt;&gt; '123123123'[::3]
'111'
</code></pre>

blocks|key|1174352|text|第三个参数是step。所以::3将返回列表/字符串的第三个元素。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1174353|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

The third parameter is the step. So [::3] would return every 3rd element of the list/string.

blocks|key|1174380|text|Python使用:：分隔结束值、开始值和步长值。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1174381|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

Python uses the :: to separate the End, the Start, and the Step value.

blocks|key|1174432|text|我是不是错过了，或者这里没有人提到用[::-1]进行反向操作？|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1174433|#+Operating+System+List
systems+=+['Windows',+'macOS',+'Linux']
print('Original+List:',+systems)

#+Reversing+a+list++
#Syntax:+reversed_list+=+systems[start:stop:step]+
reversed_list+=+systems[::-1]

#+updated+list
print('Updated+List:',+reversed_list)|code-block|syntax|javascript|1174434|来源：https://www.programiz.com/python-programming/methods/list/reverse|1174435|entityMap|0|LINK|mutability|MUTABLE|url|https://www.programiz.com/python-programming/methods/list/reverse^0|I|6|0|0|3|1T|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@$9|V|A|W|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|X|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|Y|8|@]|D|@$9|Z|A|10|1|11]]|E|$]]|$1|M|3|-4|5|6|7|12|8|@]|D|@]|E|$]]]|N|$O|$5|P|Q|R|E|$S|T]]]]

Did I miss or nobody mentioned reversing with <code>[::-1]</code> here?
<pre><code># Operating System List
systems = ['Windows', 'macOS', 'Linux']
print('Original List:', systems)

# Reversing a list 
#Syntax: reversed_list = systems[start:stop:step] 
reversed_list = systems[::-1]

# updated list
print('Updated List:', reversed_list)
</code></pre>
source:
<a href="https://www.programiz.com/python-programming/methods/list/reverse" rel="nofollow noreferrer">https://www.programiz.com/python-programming/methods/list/reverse</a>

blocks|key|1174484|text|记住，基础就是a[start:end:step]的意思。从那里你可以得到a[1::2]得到每个奇数索引，a[::2]得到每个偶数，a[2::2]得到每个偶数从2开始，a[2:4:2]得到每个偶数从2开始到4结束。灵感来自https://stackoverflow.com/a/3453102/1601580|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1174485|entityMap|0|LINK|mutability|MUTABLE|url|https://stackoverflow.com/a/3453102/1601580^0|7|H|10|7|1G|6|1T|7|2B|8|32|17|0|0^^$0|@$1|2|3|4|5|6|7|N|8|@$9|O|A|P|B|C]|$9|Q|A|R|B|C]|$9|S|A|T|B|C]|$9|U|A|V|B|C]|$9|W|A|X|B|C]]|D|@$9|Y|A|Z|1|10]]|E|$]]|$1|F|3|-4|5|6|7|11|8|@]|D|@]|E|$]]]|G|$H|$5|I|J|K|E|$L|M]]]]

remember that the foundations is what <code>a[start:end:step]</code> means. From there you can get <code>a[1::2]</code> get every odd index, <code>a[::2]</code> get every even, <code>a[2::2]</code> get every even starting at 2, <code>a[2:4:2]</code> get every even starting at 2 and ending at 4. Inspired by <a href="https://stackoverflow.com/a/3453102/1601580">https://stackoverflow.com/a/3453102/1601580</a>

I know that I can use something like <code>string[3:4]</code> to get a substring in Python, but what does the 3 mean in <code>somesequence[::3]</code>?

What is :: (double colon) in Python when subscripting sequences?

Windows

Python

Linux

我知道在Python语言中可以使用像string[3:4]这样的东西来获取子字符串，但是在somesequence[::3]中3是什么意思呢

问在Python中下标序列时:：(双冒号)是什么？
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