何时以及为何在 Python 中使用 [:] [英] When and why to use [:] in python

问题描述

sentence = "你好"打印句子打印句子[:]

两者输出相同的东西，即Hello

那么，何时以及为什么使用/不使用 [:] ?

谢谢！:)

解决方案

正如 Nobi 在评论中指出的那样，有已经是一个 关于 Python 的切片符号的问题.正如对该问题的回答所述，没有开始和结束值 ([:]) 的切片基本上创建了原始序列的副本.

但是，您遇到了字符串的特殊情况.由于字符串是不可变的，因此创建字符串的副本是没有意义的.由于您将无法修改字符串的任何实例，因此无需在内存中拥有多个实例.所以，基本上，使用 s[:] (作为 s 一个字符串)你不是在创建字符串的副本；该语句返回与 s 引用的字符串完全相同的字符串.一个简单的方法是使用 id()(对象标识)函数:

<预><代码>>>>l1 = [1, 2, 3]>>>l2 = l1[:]>>>身份证(l1)3075103852L>>>身份证(l2)3072580172L

身份不同.但是，使用字符串:

<预><代码>>>>s1 = "你好">>>s2 = s1[:]>>>id(s1)3072585984L>>>id(s2)3072585984L

身份是相同的，这意味着两者是完全相同的对象.

sentence = "Hello"
print sentence
print sentence[:]

Both outputs the same thing, i.e. Hello

So, when and why to use/not use [:] ?

Thanks! :)

解决方案

As Nobi pointed out in the comments, there's already a question regarding Python's slicing notation. As stated in the answer to that question, the slicing without start and end values ([:]) basically creates a copy of the original sequence.

However, you have hit a special case with strings. Since strings are immutable, it makes no sense to create a copy of a string. Since you won't be able to modify any instance of the string, there's no need to have more than one in memory. So, basically, with s[:] (being s a string) you're not creating a copy of the string; that statement is returning the very same string referenced by s. An easy way to see this is by using the id() (object identity) function:

>>> l1 = [1, 2, 3]
>>> l2 = l1[:]
>>> id(l1)
3075103852L
>>> id(l2)
3072580172L

Identities are different. However, with strings:

>>> s1 = "Hello"
>>> s2 = s1[:]
>>> id(s1)
3072585984L
>>> id(s2)
3072585984L

Identity is the same, meaning both are the same exact object.

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