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

Guido van Rossum guido@python.org
Sun, 28 Apr 2002 18:30:22 -0400

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.

+1 on making the built-in types accept slice objects in their getitem, setitem, delitem methods.

+0 on subsequently getting rid of their getslice, setslice, delslice methods. PyObject_GetSlice c.s. may have to be adapted to fall back to getitem with a slice object. The slice opcodes may possibly be simplified.

Possible issue: I believe getslice and getitem deal differently with negative indices (one passes them on, the other doesn't, or something like that). There is a bit of a rat's nest trying to keep this compatible in all ways.

--Guido van Rossum (home page: http://www.python.org/~guido/)