syntax - "Slicing" in Python Expressions documentation

Question

I don't understand the following part of the Python docs:

http://docs.python.org/reference/expressions.html#slicings

Is this referring to list slicing ( x=[1,2,3,4]; x[0:2] )..? Particularly the parts referring to ellipsis..

slice_item       ::=  expression | proper_slice | ellipsis

The conversion of a slice item that is an expression is that expression. The conversion of an ellipsis slice item is the built-in Ellipsis object.

Answer 1

Ellipsis is used mainly by the numeric python extension, which adds a multidimensional array type. Since there are more than one dimensions, slicing becomes more complex than just a start and stop index; it is useful to be able to slice in multiple dimensions as well. eg, given a 4x4 array, the top left area would be defined by the slice "[:2,:2]"

>>> a
array([[ 1,  2,  3,  4],
       [ 5,  6,  7,  8],
       [ 9, 10, 11, 12],
       [13, 14, 15, 16]])

>>> a[:2,:2]  # top left
array([[1, 2],
       [5, 6]])

Ellipsis is used here to indicate a placeholder for the rest of the array dimensions not specified. Think of it as indicating the full slice [:] for dimensions not specified, so for a 3d array, a[...,0] is the same as a[:,:,0] and for 4d, a[:,:,:,0].

Note that the actual Ellipsis literal (...) is not usable outside the slice syntax in python2, though there is a builtin Ellipsis object. This is what is meant by "The conversion of an ellipsis slice item is the built-in Ellipsis object." ie. "a[...]" is effectively sugar for "a[Ellipsis]". In python3, ... denotes Ellipsis anywhere, so you can write:

>>> ...
Ellipsis

If you're not using numpy, you can pretty much ignore all mention of Ellipsis. None of the builtin types use it, so really all you have to care about is that lists get passed a single slice object, that contains "start","stop" and "step" members. ie:

l[start:stop:step]   # proper_slice syntax from the docs you quote.

is equivalent to calling:

l.__getitem__(slice(start, stop, step))