[Python-Dev] Built in objects supporting slices (original) (raw)

Alex Martelli aleax@aleax.it
Sun, 28 Apr 2002 18:53:47 +0200

On Sunday 28 April 2002 15:20, Guido van Rossum wrote:

> At the moment, built in objects such as lists don't support the slice > notaton l[a:b:c]. Would there be support for including this in Python ... Yes. No PEP is needed, but I'm -0 on the idea -- I don't see what use there is for this besides theoretical neatness; I'm sure it complicates a lot of code.

IOW What's the use case?

If I understood the c.l.py discussion correctly, one key issue is with documentation -- currently, in:

http://www.python.org/dev/doc/devel/ref/sequence-types.html

the "right" way for a contained object to support slicing is said to be through getitem (with a slice argument), while getslice is said to be supported only for backwards compatibility. If you think getslice is best, then that page should be edited accordingly; otherwise, built-in sequences should probably respect the same "right way" as others.

The only use-case I would probably use substantially is a step of -1 for reverse looping. I do see in my existing code some number of: "for i in range(len(L)-1, 0, -1):" followed by use of L[i], and the like, that would be "smoother" as "for item in L[::-1]:". Not a huge win, for sure, but not terrible, it would seem to me. I see no such immediate application for other steps, although Python's standard libraries do have some cases with a step of 2 that might use a step-2 slice.

So, the benefits are admittedly modest, but, it seems to me, > 0.

Alex