[Python-Dev] Negative slice steps considered unhealthy (extended slicing for lists) (original) (raw)

Michael Hudson mwh21@cam.ac.uk
01 Aug 2000 07:59:11 +0100

"Fredrik Lundh" <effbot@telia.com> writes:

greg wrote:

> I think there are some big conceptual problems with allowing > negative steps in a slice. wasn't "slices" supposed to work the same way as "ranges"?

The problem is that for slices (& indexes in general) that negative indices have a special interpretation:

range(10,-1,-1) range(10)[:-1]

Personally I don't think it's that bad (you just have to remember to write :: instead of :-1: when you want to step all the way back to the beginning). More serious is what you do with out of range indices - and NumPy is a bit odd with this one, it seems:

l = Numeric.arrayrange(10) l[30::-2] array([0, 8, 6, 4, 2, 0])

What's that initial "0" doing there? Can someone who actually understands NumPy explain this?

Cheers, M.

(PS: PySlice_GetIndices is in practice a bit useless because when it fails it offers no explanation of why! Does any one use this function, or should I submit a patch to make it a bit more helpful (& support longs)?)

-- -Dr. Olin Shivers, Ph.D., Cranberry-Melon School of Cucumber Science -- seen in comp.lang.scheme