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Suggested feature: slice syntax within tuples (or even more generally)?

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  Suggested feature: slice syntax within tuples (or even more generally)? stephenwlin@gmail.com - 2013-02-13 11:00 -0800
    Re: Suggested feature: slice syntax within tuples (or even more generally)? Terry Reedy <tjreedy@udel.edu> - 2013-02-14 00:23 -0500
      Re: Suggested feature: slice syntax within tuples (or even more generally)? stephenwlin@gmail.com - 2013-02-13 21:54 -0800
      Re: Suggested feature: slice syntax within tuples (or even more generally)? stephenwlin@gmail.com - 2013-02-13 21:54 -0800
        Re: Suggested feature: slice syntax within tuples (or even more generally)? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-02-14 07:32 +0000
          Re: Suggested feature: slice syntax within tuples (or even more generally)? stephenwlin@gmail.com - 2013-02-14 00:36 -0800
        Re: Suggested feature: slice syntax within tuples (or even more generally)? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-02-14 08:03 +0000
          Re: Suggested feature: slice syntax within tuples (or even more generally)? stephenwlin@gmail.com - 2013-02-14 01:08 -0800
            Re: Suggested feature: slice syntax within tuples (or even more generally)? stephenwlin@gmail.com - 2013-02-14 01:26 -0800
          Re: Suggested feature: slice syntax within tuples (or even more generally)? Ian Kelly <ian.g.kelly@gmail.com> - 2013-02-14 11:58 -0700
            Re: Suggested feature: slice syntax within tuples (or even more generally)? stephenwlin@gmail.com - 2013-02-14 14:01 -0800
            Re: Suggested feature: slice syntax within tuples (or even more generally)? stephenwlin@gmail.com - 2013-02-14 14:01 -0800
              Re: Suggested feature: slice syntax within tuples (or even more generally)? Rick Johnson <rantingrickjohnson@gmail.com> - 2013-02-14 17:46 -0800
              Re: Suggested feature: slice syntax within tuples (or even more generally)? Rick Johnson <rantingrickjohnson@gmail.com> - 2013-02-14 17:46 -0800
    Re: Suggested feature: slice syntax within tuples (or even more generally)? stephenwlin@gmail.com - 2013-02-13 22:06 -0800
    Re: Suggested feature: slice syntax within tuples (or even more generally)? Duncan Booth <duncan.booth@invalid.invalid> - 2013-02-14 12:25 +0000
      Re: Suggested feature: slice syntax within tuples (or even more generally)? stephenwlin@gmail.com - 2013-02-14 07:56 -0800
    Re: Suggested feature: slice syntax within tuples (or even more generally)? Andrew Robinson <andrew3@r3dsolutions.com> - 2013-02-25 01:10 +0000
    Re: Suggested feature: slice syntax within tuples (or even more generally)? Ian Kelly <ian.g.kelly@gmail.com> - 2013-02-25 03:28 -0700
    Re: Suggested feature: slice syntax within tuples (or even more generally)? Terry Reedy <tjreedy@udel.edu> - 2013-02-25 06:23 -0500
    Re: Suggested feature: slice syntax within tuples (or even more generally)? Ian Kelly <ian.g.kelly@gmail.com> - 2013-02-25 09:54 -0700
    Re: Suggested feature: slice syntax within tuples (or even more generally)? Andrew Robinson <andrew3@r3dsolutions.com> - 2013-02-25 09:47 +0000
    Re: Suggested feature: slice syntax within tuples (or even more generally)? Nobody <nobody@nowhere.com> - 2013-02-26 07:38 +0000

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#39895

On Mon, Feb 25, 2013 at 12:41 AM, Andrew Robinson
<andrew3@r3dsolutions.com> wrote:
>> Intuitively, it should result in an infinite loop starting at 0.  But
>> ranges require a stop value for a very good reason -- it should not be
>> this easy to accidentally create an infinite for loop.
>
> ...
> and, besides, the same is true with other constructions of loops....
>
> while a:  # Damn easy, if a is accidentally true!

Notice I specifically said an "infinite *for* loop".  While loops are
meant to be indeterminate in the number of iterations they will take
going into the loop; for loops are not.

>> The next problem you run into is that the semantics of negative
>> numbers are completely different between slices and ranges.  Consider
>> this code:
>>
>> s = (-5:6)
>> for i in s:
>>      print(i)
>> for i in range(6)[s]:
>>      print(i)
>
> I don't find this difference to be necessary, nor objectionable.
>
> It is less inconsistent, in my view, to allow that
> ([ 1,2,3,4,5 ])[-1:2]  produce [5,1,2] than an empty list;
> and ([ 1,2,3,4,5])[2:-1] does produce an empty list.
>
> I have been looking for actual programs that this would break for over two
> months now, and I haven't been finding any.  I am willing to run any
> mainstream application you can find on test-patched python!

Yes, I recollect now that we have already had this discussion.

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#39901

On 02/25/2013 04:54 PM, Ian Kelly wrote:
> On Mon, Feb 25, 2013 at 12:41 AM, Andrew Robinson
> <andrew3@r3dsolutions.com> wrote:
>>> Intuitively, it should result in an infinite loop starting at 0.  But
>>> ranges require a stop value for a very good reason -- it should not be
>>> this easy to accidentally create an infinite for loop.
>> ...
>> and, besides, the same is true with other constructions of loops....
>>
>> while a:  # Damn easy, if a is accidentally true!
> Notice I specifically said an "infinite *for* loop".
OK, so tit for tat.

Notice I already showed an effective *accidental* "infinite" for loop 
because I did notice you spoke about a *for* loop.

And, obviously, in the case of the while loop I showed -- it was not 
meant to be True forever.
It's a variable, which is subject to change.

I really do respect your opinion; but it's one of about 5 people that 
dominate this list, albeit the same spend a lot of time helping others;  
Stephen is someone new to me, and I want to encourage his probing of the 
issue more than I want to advance my view.

P.S.
I apologize about the e-mail clock, it seems I am sending my local time 
again -- and it's different from your timezone; I *wish* the python list 
computer would politely adjust it when *accidents* happen, or my OS's 
distribution would fix their bug -- but cest la vie.  I limp along with 
the status quo for now.

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#39936

On Wed, 13 Feb 2013 11:00:15 -0800, stephenwlin wrote:

> Would it be feasible to modify the Python grammar to allow ':' to generate
> slice objects everywhere rather than just indexers and top-level tuples of
> indexers?

If you need to be able to easily construct indexing objects, create a
helper like:

	> class Slicer(object):
	=     def __getitem__(self, s):
	=         return s
	= 
	> s_ = Slicer()
	> s_[1,2,3]
	(1, 2, 3)
	> s_[:]
	slice(None, None, None)
	> s_[1:2:3,4:5:6]
	(slice(1, 2, 3), slice(4, 5, 6))
	> s_[...]
	Ellipsis

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