I believe the problem is not about negative strides but about negative
bounds. There should be a notion of "minus zero", something like
"abcde"[:-0:-1] =="edcba".
Here ":-" serves as a special syntax for negative stride; of course it is
not a real proposal.
The same awkwardness results when you take a negative upper bounds to the
limit of 0:
"abcde"[:-2] == "abc"
"abcde"[:-1] == "abcd"
"abcde"[:-0] == ""
(I once filed a bug for it, which was of course correctly rejected:
http://bugs.python.org/issue17287).
Elazar
2013/10/27 Guido van Rossum

In the comments of http://python-history.blogspot.com/2013/10/why-python-uses-0-based-indexing.... were some complaints about the interpretation of the bounds for negative strides, and I have to admin it feels wrong. Where did we go wrong? For example,

"abcde"[::-1] == "edcba"

as you'd expect, but there is no number you can put as the second bound to get the same result:

"abcde"[:1:-1] == "edc" "abcde"[:0:-1] == "edcb"

but

"abcde":-1:-1] == ""

I'm guessing it all comes from the semantics I assigned to negative stride for range() long ago, unthinkingly combined with the rules for negative indices.

Are we stuck with this forever? If we want to fix this in Python 4 we'd have to start deprecating negative stride with non-empty lower/upper bounds now. And we'd have to start deprecating negative step for range() altogether, recommending reversed(range(lower, upper)) instead.

Thoughts? Is NumPy also affected?

-- --Guido van Rossum (python.org/~guido)

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