<s>[INST] How do you use the ellipsis slicing syntax in Python? This came up in  Hidden features of Python, but I can't see good documentation or examples that explain how the feature works.
 [/INST] The ellipsis is used to slice higher-dimensional data structures. 
It's designed to mean at this point, insert as many full slices (:) to extend the multi-dimensional slice to all dimensions.
Example:
>>> from numpy import arange
>>> a = arange(16).reshape(2,2,2,2)

Now, you have a 4-dimensional matrix of order 2x2x2x2. To select all first elements in the 4th dimension, you can use the ellipsis notation
>>> a[..., 0].flatten()
array([ 0,  2,  4,  6,  8, 10, 12, 14])

which is equivalent to
>>> a[:,:,:,0].flatten()
array([ 0,  2,  4,  6,  8, 10, 12, 14])

In your own implementations, you're free to ignore the contract mentioned above and use it for whatever you see fit.
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