[Python-3000] Making more effective use of slice objects in Py3k (original) (raw)

Ron Adam rrr at ronadam.com
Tue Aug 29 07:47:24 CEST 2006

Greg Ewing wrote:

Ron Adam wrote:

1. Remove None stored as indices in slice objects. Depending on the step value, Any Nones can be converted to 0 or -1 immediately, But None isn't the same as -1 in a slice. None means the end of the sequence, whereas -1 means one less than the end.

Yes, you are correct, thats one of those things I get caught on when I haven't had enough sleep. ;-)

'abcdefg'[-1] 'g'

'abcdefg'[0:-1] 'abcdef'

And in addition to that... 0 is not the beginning if the step is -1.

'abcdefg'[-1:0:-1] 'gfedcb'

So None for the start index can be 0 or -1. But for the end index it can't be determined.

In the first case above, the stop index would need to be one greater than -1 which is 0, and that causes a problem.

In the second case above, the stop index would need to be one less than 0, then that would again cause a problem.

I'm also not all that happy about forcing slice indices to be ints. Traditionally they are, but someone might want to define a class that uses them in a more general way.

Hmmmm, thanks for pointing this out. It sounds interesting and is something I hadn't thought about. In most cases I've seen only integers and None are ever used. And I'm used to seeing an exception if anything else is used.

'abc'[1.0] Traceback (most recent call last): File "", line 1, in ? TypeError: string indices must be integers

That is a string method that is generating the exception then and not the slice object?

But then what about the slice.indices() method? It does generate exceptions.

slc = slice(1.0) slc.indices(10) Traceback (most recent call last): File "", line 1, in ? TypeError: slice indices must be integers

Once the slice is created the Nones are not needed, valid index values can be determined. I don't understand what you mean here. Slice objects themselves know nothing about what object they're going to be used to slice, so there's no way they can determine "valid index values" (or even types -- see above).

Ok, I hadn't considered the possibility of methods being defined to read the slice object. Do you know where I could find an example of that?

Cheers, Ron

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