详解Python3中的Sequence type的使用

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

1.     list
2.     tuple
3.     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]
[3,1]
>>> a = (1,3)
>>> a[::-1]
(3,1)
>>> 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]
>>> lists[0] = 1
>>> lists
[1,0]

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:

（编辑：李大同）

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