Python slice how-to, I know the Python slice but how can I use built-in slice object for it?

While trying to answer Subset a string based on variable , I recalled that numpy has a syntactically nice way to define slice objects:

>>> import numpy as np
>>> s = "The long-string instrument is a musical instrument in which the string is of such a length that the fundamental transverse wave is below what a person can hear as a tone."
>>> z = np.s_[18:26]  # in this case same as slice(18, 26, None)
>>> s[z]
'strument'

The problem solved here is how to store the slice in a variable for later use, and np.s_ allows to do just that. Yes, it's not built-in, but as that original question was redirected here I feel like my answer belong here as well. Also, numpy was one of the reasons why so advanced slicing abilities were added to Python, IIRC.

An example of a more complex "slicing":

>>> data = np.array(range(6)).reshape((2, 3))
>>> z = np.s_[:1, 1:2]
>>> data[z]
array([[1]])
>>> data
array([[0, 1, 2],
       [3, 4, 5]])
>>> z
(slice(None, 1, None), slice(1, 2, None))

where z is now a tuple of slices.

Slice objects let you programmatically generate and manipulate slices. Especially for multidimensional numpy arrays, and especially if you don't know the dimensionality in advance, you might want to construct slices on-the-fly to specify the axes or dimensions that you want.

import numpy as np
dimension = np.random.randint(10) # Might have up to 10 dimensions
shape = []
for d in range(dimension):
    shape.append(np.random.randint(10))
zz = np.random.rand(tuple(shape))
print(zz)
>>> array([[0.68379351, 0.50854469, 0.64578775, 0.73441699, 0.28977396],
           [0.88797164, 0.81603025, 0.63978659, 0.22677299, 0.93455738],
           [0.0892855 , 0.28048706, 0.04262895, 0.9353467 , 0.13062249],
           [0.88561035, 0.93378367, 0.12124208, 0.25600301, 0.96035638]])

Here our data ended up being two dimensional (4-by-5), but there was no guarantee of that. How will you request slices from zz?

One problem is that I can't manipulate Python's slice notation. It's not valid syntax outside of a slicing operation.

my_slice = 2:3:1
>>> SyntaxError: Invalid Syntax

What if I could just build up the exact slice request I wanted in a loop, the way I can build up a string? Wouldn't that be great? I mean, sure you can use a string to do it, but it would be messy and requires eval.

your_slice_definitions = [(2,3,1), *[(None, None, None)]*(zz.ndim - 1)] 
my_slice_str = ""
for slice_start, slice_end, slice_step in your_slice_definitions:
    my_slice_str += "{}:{}:{},".format(slice_start, slice_end, slice_step)
eval("zz["+my_slice_str+"])

So here we are: slice objects let you do this. You can assemble lists and tuples of them on-the-fly, pass them as function parameters, sort them, shuffle them, and so on.

my_slices = []
for slice_start, slice_end, slice_step in your_slice_definitions:
    my_slices += slice(slice_start, slice_end, slice_step)
print(zz[my_slices])
>>> array([[0.0892855 , 0.28048706, 0.04262895, 0.9353467 , 0.13062249]])