当len <ndim时，使用元组（或列表）为numpy数组建立索引？

Question

I have a 3d numpy array, eg:

>>> A = np.arange(24).reshape(2,3,4)

I want to take a 1d slice along axis 0 based on a pair of coordinates for axes 1 and 2:

>>> h = 1
>>> l = 2
>>> A[:,h,l]
array([ 6, 18])

So far so good. But what if my coordinate pair is stored as a tuple or a list, rather than two integers? I've experimented with a few obvious options, to no avail:

>>> coords = (1,2)
>>> A[coords]
array([20, 21, 22, 23])
>>> A[:,coords]
array([[[ 4,  5,  6,  7],
        [ 8,  9, 10, 11]],

       [[16, 17, 18, 19],
        [20, 21, 22, 23]]])
>>> A[...,coords]
array([[[ 1,  2],
        [ 5,  6],
        [ 9, 10]],

       [[13, 14],
        [17, 18],
        [21, 22]]])

I've googled around on this and not found anything, but it's entirely possible that I'm not searching with the appropriate jargon. So, apologies if this is an overly simplistic question!

Answer 1

You can construct the slice tuple directly, with something like:

In [11]: A[(slice(None),) + coords]
Out[11]: array([ 6, 18])

---

This is because calling A[:, 1, 2] is equivalent / calls:

In [12]: A.__getitem__((slice(None, None, None), 1, 2))
Out[12]: array([ 6, 18])