[Python-Dev] sliceobject Py_None step inquiry

[Calvin Spealman]

I think that should not change. None is different than 0. It makes  
sense to use it as a "use the default value" kind of place holder.  
Silently using 1 when you pass 0 is a very different thing.

Maybe the slice was calculated and the developer should know about it  
being 0, because in this case they really don't want a step of 1, or  
the calculation was broken. There are lots of reasons.

On Dec 11, 2007, at 9:23 PM, Joseph Armbruster wrote:

>
> I was playing around with sliceobject.c this evening and noticed  
> the following
> behavior.  If you slice with a step 0, you receive a ValueError but  
> when you
> slice with a step of None, the step is set to 1.  As an example,  
> observe the
> following interactive session:
>
>>>> a = [1,2,3,4,5,6]
>>>> b = slice(0,5,None)
>>>> a[b]
> [1, 2, 3, 4, 5]
>>>> b = slice(0,5,0)
>>>> a[b]
> Traceback (most recent call last):
>    File "<stdin>", line 1, in <module>
> ValueError: slice step cannot be zero
>>>>
>
> Within the code, it looks like Py_None performs a step of 1.  Does  
> it make
> sense to create a patch so that None and 0 behave the same in this  
> respect?
>
>>>> a = [1,2,3,4,5,6]
>>>> b = slice(0,5,None)
>>>> a[b]
> Traceback (most recent call last):
>    File "<stdin>", line 1, in <module>
> ValueError: slice step cannot be None
>>>> b = slice(0,5,0)
>>>> a[b]
> Traceback (most recent call last):
>    File "<stdin>", line 1, in <module>
> ValueError: slice step cannot be zero
>>>> b = slice(0,5)
>>>> a[b]
> [1, 2, 3, 4, 5]
>>>>
>
>
> Joe
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