详解Python3中的Sequence type的使用

发表于 5年以前 | 总阅读数：1090 次

其实本来是要reverse一下list的，就去查了一下list[::-1]是什么意思，发现还有很多要注意的地方，所以就记一下。
主要是参照https://docs.python.org/3/library/stdtypes.html?highlight=list#list

首先Sequence type有三种

```
list
```
tuple
range

slice

[i:j:k]表示的是slice of s from i to j with step k, 对三种类型都有用


    >>> a = [1, 2, 3]
    >>> a[::-1]
    [3, 2, 1]
    >>> a = (1, 2, 3)
    >>> a[::-1]
    (3, 2, 1)
    >>> a = range(3)
    >>> a[::-1]
    range(2, -1, -1)

range中参数是range(start, stop[, step])
initialize a list

s * n表示的是n shallow copies of s concatenated
注意是浅拷贝哦，所以会有如下情况


    >>> lists = [[]] * 3
    >>> lists
    [[], [], []]
    >>> lists[0].append(3)
    >>> lists
    [[3], [3], [3]]

如果元素不是对象的话就没关系


    >>> lists = [0] * 3
    >>> lists
    [0, 0, 0]
    >>> lists[0] = 1
    >>> lists
    [1, 0, 0]

正确的初始化嵌套list的方法应该是


    >>> lists = [[] for i in range(3)]
    >>> lists[0].append(3)
    >>> lists[1].append(5)
    >>> lists[2].append(7)
    >>> lists
    [[3], [5], [7]]

concatenation pitfall

(感觉还是英文说的清楚些，这一点跟Java是一样的)
Concatenating immutable sequences always results in a new object. This means that building up a sequence by repeated concatenation will have a quadratic runtime cost in the total sequence length. To get a linear runtime cost, you must switch to one of the alternatives below: