slicing return iter?

[Gabriel Genellina]

En Sun, 18 Oct 2009 23:07:40 -0200, Raymond Hettinger <python at rcn.com>  
escribió:
> [Raymond]
>> > If it really is a sequence (with len and getitem), you can write your
>> > own indexing iterator:
>>
>> >   def myslice(seq, start, stop, step):
>> >        'Allow forward or backwards iteration over a subslice'
>> >        for i in range(start, stop, step):
>> >            yield seq[i]
>
> [StarWing]
>> Thank you. but it can't support negative index :-(
>
> The negative index is handled by the line, "yield seq[i]".
>
>>>> s[-2:-5:-1] == ''.join(myslice(s, -2, -5, -1))
> True
>>>> s[-5:-2:1] == ''.join(myslice(s, -5, -2, 1))
> True

But not always:

py> s[10:-1:1]==''.join(myslice(s, 10, -1, 1))
False
py> s[-10:25:1]==''.join(myslice(s, -10, 25, 1))
False

You have to take the sequence length into account before computing the  
range, not after.

-- 
Gabriel Genellina