Python > a[0] > for multidimensional a, a[0] is interpreted by taking all elements in the unspecified dimensions.

@ Scipy lecture notes, Edition 2017.1
https://www.scipy-lectures.org/_downloads/ScipyLectures-simple.pdf
p54
https://www.scipy-lectures.org/intro/numpy/array_object.html#indexing-and-slicing

for multidimensional a, a[0] is interpreted by taking all elements in the unspecified dimensions.

意訳「多次元aの場合、a[0]は不定次元の全要素と解釈される」

import numpy as np

ary = np.eye(3)
print(ary)
print(ary[0])

run

[[ 1.  0.  0.]
 [ 0.  1.  0.]
 [ 0.  0.  1.]]
[ 1.  0.  0.]

a[0]を使うと間違いにつながる、という認識を持った。