[Python-ideas] Where did we go wrong with negative stride?

[Terry Reedy]

On 10/28/2013 4:20 PM, Oscar Benjamin wrote:

> Also I'm not the only person to point out that a more common problem
> is with wraparound when doing something like a[:n].

I think it a mistake to think in terms of 'wraparound'. This implies to 
me that there is a mod len(s) operation applied, and there is not. A 
negative index or slice position, -n, is simply an abbreviation for 
len(s) - n.  Besides being faster to write, the abbreviation runs about 
3x as fast with 3.3.2 on my machine.

 >>> timeit.timeit('pass', "s='abcde'")
0.02394336495171956
 >>> timeit.timeit('pass', "s='abcde'")
0.02382040032352961
.024 timeit overhead

 >>> timeit.timeit('s[-3]', "s='abcde'")
0.06969358444349899
 >>> timeit.timeit('s[-3]', "s='abcde'")
0.06534832190172146
.068 - .024 = .044 net

 >>> timeit.timeit('s[len(s)-3]', "s='abcde'")
0.15656133106750403
 >>> timeit.timeit('s[len(s)-3]', "s='abcde'")
0.15518289758767878
.156 - .024 = .132 net

The trick works because Python, unlike some other languages, does not 
allow negative indexing from the start of the array.  If Python had 
required an explicit end marker from the beginning, conditional code 
would  be required if the sign were unknown. If Python gained one today, 
it would have to be optional for back compatibility.

-- 
Terry Jan Reedy