python 切片 逗号_带有逗号/列表的Python切片符号

I have come across some python code with slice notation that I am having trouble figuring out.

It looks like slice notation but uses a comma and a list:

list[:, [1, 2, 3]]

Is this syntax valid? If so what does it do?

edit looks like it is a 2D numpy array

解决方案

Assuming that the object is really a numpy array, this is known as advanced indexing, and picks out the specified columns:

>>> import numpy as np

>>> a = np.arange(12).reshape(3,4)

>>> a

array([[ 0, 1, 2, 3],

[ 4, 5, 6, 7],

[ 8, 9, 10, 11]])

>>> a[:, [1,2,3]]

array([[ 1, 2, 3],

[ 5, 6, 7],

[ 9, 10, 11]])

>>> a[:, [1,3]]

array([[ 1, 3],

[ 5, 7],

[ 9, 11]])

Note that this won't work with the standard Python list:

>>> a.tolist()

[[0, 1, 2, 3], [4, 5, 6, 7], [8, 9, 10, 11]]

>>> a.tolist()[:,[1,2,3]]

Traceback (most recent call last):

File "", line 1, in

a.tolist()[:,[1,2,3]]

TypeError: list indices must be integers, not tuple