Proposed PEP: New style indexing, was Re: Bug in slice type

Steven Bethard steven.bethard at gmail.com
Sat Aug 20 23:33:22 CEST 2005


Bryan Olson wrote:
> Steven Bethard wrote:
>  > Well, I couldn't find where the general semantics of a negative stride
>  > index are defined, but for sequences at least[1]:
>  >
>  > "The slice of s from i to j with step k is defined as the sequence of
>  > items with index x = i + n*k such that 0 <= n < (j-i)/k."
>  >
>  > This seems to contradict list behavior though. [...]
> 
> The conclusion is inescapable: Python's handling of negative
> subscripts is a wart.

I'm not sure I'd go that far.  Note that my confusion above was the 
order of combination of points (3) and (5) on the page quoted above[1]. 
  I think the problem is not the subscript handling so much as the 
documentation thereof.  I posted a message about this [2], and a 
documentation patch based on that message [3].


[1] http://docs.python.org/lib/typesseq.html
[2] http://mail.python.org/pipermail/python-list/2005-August/295260.html
[3] http://www.python.org/sf/1265100


> Suppose instead of using semicolons as the PPEP proposes, we use
> commas, as in:
> 
>     sequence[start, stop, step]

This definitely won't work.  This is already valid syntax, and is used 
heavily by the numarray/numeric folks.

STeVe



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