xrange issue 7721

[Mark Lawrence]

Hi Martin, thanks for the response, please see below.

On 29/05/2010 20:12, Martin Manns wrote:
> On Sat, 29 May 2010 19:46:28 +0100
> Mark Lawrence<breamoreboy at yahoo.co.uk>  wrote:
>
>> I've had an OverflowError using xrange with Python 2.6.5 on Windows.
>> Googling got me to the subject line.
>>
>> msg97928 gives a code snippet to overcome the limitations of xrange,
>> allowing for negative steps, however it doesn't raise a ValueError
>> for a zero step. msg99624 gives a docs change that has been
>> implemented for V2.6, but this doesn't refer to the msg97928 code
>> snippet, rather it refers to a one liner that only works for positive
>> steps.  The docs for V2.7 haven't been changed at all.
>
> Mark:
>
> Thank you for posting.
>
> 2.7 is not affected by issue 7721 because itertools.islice behavior is
> changed. Therefore, the original snippet should work in 2.7 (I have not
> tested this).

 From http://docs.python.org/dev/library/itertools.html
"Unlike regular slicing, islice()  does not support negative values for 
start, stop, or step."

Rule 1 of programming never assume anything, particularly wrt testing. I 
assume that you are ok with this. :) Dreadful I know :)

>
> I found the msg97928 code pretty obvious when seeing the snippet.
> If you disagree you may consider re-opening the issue.

Try running this on Python 2.6.5 in file irange.py

from itertools import takewhile, count

def irange(start, stop, step):
     if step < 0:
         cond = lambda x: x > stop
     else:
         cond = lambda x: x < stop
     return takewhile(cond, (start + i * step for i in count()))

if __name__=='__main__':
     for i in irange(0, 10, 0): print i

My output from the command line

c:\Users\Mark\python>irange
0
0
etc etc etc
I trust that you get my point regarding the failure to raise a 
ValueError :)  Or am I wearing my extremely stupid hat today?

>
> Martin
>

Kindest regards.

Mark Lawrence.