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

Guido van Rossum guido@beopen.com
Mon, 31 Jul 2000 23:01:34 -0500

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.

You have a point! I just realized today that my example L[9:-1:-1] does not access L[0:10] backwards, because of the way the first -1 is interpreted as one before the end of the list L. :(

But I'm not sure we can forbid this behavior (in general) because the NumPy folks are already using this. Since these semantics are up to the object, and no built-in objects support extended slices (yet), I'm not sure that this behavior has been documented anywhere except in NumPy.

However, for built-in lists I think it's okay to forbid a negative step until we've resolved this...

This is something to consider for patch 100998 which currently implements (experimental) extended slices for lists...

--Guido van Rossum (home page: http://www.pythonlabs.com/~guido/)