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

Greg Ewing greg@cosc.canterbury.ac.nz
Tue, 01 Aug 2000 11:45:02 +1200 (NZST)

I think there are some big conceptual problems with allowing negative steps in a slice.

With ordinary slices, everything is very clear if you think of the indices as labelling the points between the list elements.

With a step, this doesn't work any more, and you have to think in terms of including the lower index but excluding the upper index.

But what do "upper" and "lower" mean when the step is negative? There are several things that a[i:j:-1] could plausibly mean:

[a[i], a[i-1], ..., a[j+1]]

[a[i-1], a[i-2], ..., a[j]]

[a[j], a[j-1], ..., a[i+1]]

[a[j-1], a[j-2], ..., a[i]]

And when you consider negative starting and stopping values, it just gets worse. These have no special meaning to range(), but in list indexing they do. So what do they mean in a slice with a step? Whatever is chosen, it can't be consistent with both.

In the face of such confusion, the only Pythonic thing would seem to be to disallow these things.

Greg Ewing, Computer Science Dept, +--------------------------------------+ University of Canterbury, | A citizen of NewZealandCorp, a | Christchurch, New Zealand | wholly-owned subsidiary of USA Inc. | greg@cosc.canterbury.ac.nz +--------------------------------------+