distinguish "list" & "array" in python

list

&

array

It seems

array

is a subconcept of

numpy

package.

list

&

array

are with some different properties.

The most apparent difference is lists do concatenate and arrays do math which is shown as follow.

import numpy as np

list1 = [1, 2]
list2 = [3,4]
type(list1)                 # key point
Out[26]: list
list1 + list2               # key point
Out[27]: [1, 2, 3, 4]

array1 = np.array([1,2])
array2 = np.array([3,4])
type(array1)                # key point
Out[31]: numpy.ndarray
array1 + array2             # key point
Out[32]: array([4, 6])

what is

aarray[None,0]

for

It makes me comfused that what the grammer

aarray[None,0]

stands for. After some trying, I thought I found the meaning. For a 2-dimensional array,

aarray[None,0]

retrun

aarray[0]

in 2 dimensional space, the length of one dimension of which is 1.

Differences show as below:

import numpy as np

a = np.array([[1,2,3],[4,5,6]])
print(type(a))
print('a[0] ============',a[0])
print('a[:,0] ============',a[:,0])
print('a[None,0] ============',a[None,0])
print('a[0][None,:] ============',a[0][None,:])

output:

<class 'numpy.ndarray'>
a[0] ============ [1 2 3]
a[:,0] ============ [1 4]
a[None,0] ============ [[1 2 3]]
a[0][None,:] ============ [[1 2 3]]