python Sequence type

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2023-07-31 01:50:14

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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:

str.join()或者写入io.StringIO
bytes.join()或者写入io.BytesIO或者使用可变的bytearray
使用list代替tuple

python

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python Sequence type

slice

initialize a list

concatenation pitfall

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