[Tutor] 'slice', etc

[eryksun]

On Sat, Dec 7, 2013 at 9:01 PM, Steven D'Aprano <steve at pearwood.info> wrote:
> For the record, slice objects existed in Python 1.5, so they have been
> around and used for extended (three argument) slicing for a long time.
> It's only the two argument slicing that called __getslice__.

According to the NEWS for 1.4b2 (1996), slice objects and Ellipsis
were added to support Jim Hugunin's Numeric, the ancestor of NumPy.

http://hg.python.org/cpython/file/129f1299d4e9/Misc/NEWS

Here's the 1.4 slice/ellipsis implementation by Jim Hugunin and Chris Chase:

http://hg.python.org/cpython/file/129f1299d4e9/Objects/sliceobject.c

In NumPy, Ellipsis is used to insert as many ':' as needed based on
the array shape. For example:

    >>> import numpy as np
    >>> a = np.reshape(np.arange(8), (1,2,2,2))
    >>> a
    array([[[[0, 1],
             [2, 3]],

            [[4, 5],
             [6, 7]]]])

    >>> a[:,:,:,1]
    array([[[1, 3],
            [5, 7]]])

    >>> a[...,1]
    array([[[1, 3],
            [5, 7]]])

Naturally the comma in the above examples creates a tuple:

    class Test(object):
        def __getitem__(self, key):
            return key, type(key)

    >>> Test()[:,1]
    ((slice(None, None, None), 1), <type 'tuple'>)

    >>> Test()[...,1]
    ((Ellipsis, 1), <type 'tuple'>)