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Re: Negative array indicies and slice()

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On 10/29/2012 06:53 AM, Chris Angelico wrote:
> Can you provide links to these notes? I'm looking at
> cpython/Include/sliceobject.h that has this comment:
>
> /*
>
> A slice object containing start, stop, and step data members (the
> names are from range).  After much talk with Guido, it was decided to
> let these be any arbitrary python type.  Py_None stands for omitted values.
> */
>
> Also, the code for slice objects in CPython works with Py_ssize_t (a
> signed quantity of the same length as size_t), which will allow at
> least 2**31 for an index. I would guess that your crashes were nothing
> to do with 20 million elements and slices.
>
> ChrisA
Let's look at the source code rather than the web notes -- the source 
must be the true answer anyhow.

I downloaded the source code for python 3.3.0, as the tbz;
In the directory "Python-3.3.0/Python", look at Python-ast.c, line 2089 
& ff.

Clearly a slice is malloced for a slice_ty type.
It has four elements: kind, lower, upper, and step.

So, tracing it back to the struct definition...

"Include/Python-ast.h"  has "typedef struct _slice *slice_ty;"

And, here's the answer!:

enum _slice_kind {Slice_kind=1, ExtSlice_kind=2, Index_kind=3};
struct _slice {
         enum _slice_kind kind;
         union {
                 struct {
                         expr_ty lower;
                         expr_ty upper;
                         expr_ty step;
                 } Slice;

                 struct {
                         asdl_seq *dims;
                 } ExtSlice;

                 struct {
                         expr_ty value;
                 } Index;

         } v;
};


So, slice() does indeed have arbitrary python types included in it; 
contrary to what I read elsewhere.
expr_ty is a pointer to an arbitrary expression, so the actual structure 
is 4 pointers, at 32 bits each = 16 bytes.
The size of the structure itself, given in an earlier post, is 20 bytes 
-- which means one more pointer is involved, perhaps the one pointing to 
the slice structure itself.

Hmm...!

An empty tuple gives sys.getsizeof( () ) = 24.

But, I would expect a tuple to be merely a list of object pointers; 
hence I would expect 4 bytes for len(), and then a head pointer 4 bytes, 
and then a pointer for each object.
3 objects gives 12 bytes, + 8 = 16 bytes.

Then we need one more pointer so Python knows where the struct is...
So a Tuple of 3 objects ought to fit nicely into 20 bytes; the same size 
as slice() --

but it's 24, even when empty...
And 36 when initialized...
What are the extra 16 bytes for?

All I see is:
typedef struct { object** whatever } PyTupleObject;

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Negative array indicies and slice() andrewr3mail@gmail.com - 2012-10-28 20:12 -0700
  Re: Negative array indicies and slice() Ian Kelly <ian.g.kelly@gmail.com> - 2012-10-28 21:42 -0600
    Re: Negative array indicies and slice() andrewr3mail@gmail.com - 2012-10-28 21:00 -0700
      Re: Negative array indicies and slice() Ian Kelly <ian.g.kelly@gmail.com> - 2012-10-28 22:25 -0600
        Re: Negative array indicies and slice() Andrew <andrewr3mail@gmail.com> - 2012-10-29 00:54 -0700
          Re: Negative array indicies and slice() Chris Rebert <clp2@rebertia.com> - 2012-10-29 01:18 -0700
          Re: Negative array indicies and slice() Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-10-29 11:19 +0000
            Re: Negative array indicies and slice() Chris Angelico <rosuav@gmail.com> - 2012-10-29 22:32 +1100
            Re: Negative array indicies and slice() Andrew Robinson <andrew3@r3dsolutions.com> - 2012-10-28 21:52 -0700
            Re: Negative array indicies and slice() Chris Angelico <rosuav@gmail.com> - 2012-10-29 23:40 +1100
              Re: Negative array indicies and slice() Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-10-29 22:02 +0000
            Re: Negative array indicies and slice() Andrew Robinson <andrew3@r3dsolutions.com> - 2012-10-28 23:01 -0700
              Re: Negative array indicies and slice() Roy Smith <roy@panix.com> - 2012-10-29 09:52 -0400
                Re: Negative array indicies and slice() Andrew Robinson <andrew3@r3dsolutions.com> - 2012-10-29 08:20 -0700
                Re: Negative array indicies and slice() Ian Kelly <ian.g.kelly@gmail.com> - 2012-10-29 17:01 -0600
                Re: Negative array indicies and slice() Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2012-10-30 00:04 +0000
                Re: Negative array indicies and slice() Andrew Robinson <andrew3@r3dsolutions.com> - 2012-10-29 16:54 -0700
                Re: Negative array indicies and slice() Ian Kelly <ian.g.kelly@gmail.com> - 2012-10-30 02:15 -0600
            Re: Negative array indicies and slice() Chris Angelico <rosuav@gmail.com> - 2012-10-30 00:53 +1100
            Re: Negative array indicies and slice() Ian Kelly <ian.g.kelly@gmail.com> - 2012-10-29 11:09 -0600
            Re: Negative array indicies and slice() Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-10-29 22:14 +0000
            Re: Negative array indicies and slice() Andrew Robinson <andrew3@r3dsolutions.com> - 2012-10-29 08:42 -0700
              Re: Negative array indicies and slice() Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-10-30 00:02 +0000
                Re: Negative array indicies and slice() Andrew Robinson <andrew3@r3dsolutions.com> - 2012-10-29 12:34 -0700
                Re: Negative array indicies and slice() Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-10-30 08:17 +0000
                Re: Negative array indicies and slice() Andrew Robinson <andrew3@r3dsolutions.com> - 2012-10-30 08:47 -0700
                Re: Negative array indicies and slice() Mark Lawrence <breamoreboy@yahoo.co.uk> - 2012-10-30 23:48 +0000
                Re: Negative array indicies and slice() Michael Torrie <torriem@gmail.com> - 2012-10-30 23:29 -0600
                Re: Negative array indicies and slice() Michael Torrie <torriem@gmail.com> - 2012-10-29 23:53 -0600
                Re: Negative array indicies and slice() Andrew Robinson <andrew3@r3dsolutions.com> - 2012-10-29 17:04 -0700
            Re: Negative array indicies and slice() Chris Angelico <rosuav@gmail.com> - 2012-10-30 09:55 +1100
            Re: Negative array indicies and slice() Ian Kelly <ian.g.kelly@gmail.com> - 2012-10-29 17:07 -0600
              Re: Negative array indicies and slice() Roy Smith <roy@panix.com> - 2012-10-29 19:24 -0400
                Re: Negative array indicies and slice() Ian Kelly <ian.g.kelly@gmail.com> - 2012-10-29 17:43 -0600
                Re: Negative array indicies and slice() Roy Smith <roy@panix.com> - 2012-10-29 20:17 -0400
                Re: Negative array indicies and slice() Ian Kelly <ian.g.kelly@gmail.com> - 2012-10-29 18:05 -0600
            Re: Negative array indicies and slice() Andrew Robinson <andrew3@r3dsolutions.com> - 2012-10-29 11:00 -0700
            Re: Negative array indicies and slice() Chris Kaynor <ckaynor@zindagigames.com> - 2012-10-29 18:49 -0700
            Re: Negative array indicies and slice() Andrew Robinson <andrew3@r3dsolutions.com> - 2012-10-29 15:39 -0700
            Re: Negative array indicies and slice() Ian Kelly <ian.g.kelly@gmail.com> - 2012-10-29 23:55 -0600
            Re: Negative array indicies and slice() Ian Kelly <ian.g.kelly@gmail.com> - 2012-10-30 00:51 -0600
            Re: Negative array indicies and slice() Andrew Robinson <andrew3@r3dsolutions.com> - 2012-10-29 17:17 -0700
            Re: Negative array indicies and slice() Ian Kelly <ian.g.kelly@gmail.com> - 2012-10-30 01:21 -0600
            Re: Negative array indicies and slice() Ian Kelly <ian.g.kelly@gmail.com> - 2012-10-30 01:32 -0600
            Re: Negative array indicies and slice() Ian Kelly <ian.g.kelly@gmail.com> - 2012-10-30 02:46 -0600
            Re: Negative array indicies and slice() Ian Kelly <ian.g.kelly@gmail.com> - 2012-10-30 12:02 -0600
            Re: Negative array indicies and slice() Andrew Robinson <andrew3@r3dsolutions.com> - 2012-10-30 07:21 -0700
            Re: Negative array indicies and slice() Mark Lawrence <breamoreboy@yahoo.co.uk> - 2012-10-30 21:33 +0000
              Re: Negative array indicies and slice() Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-10-31 10:07 +0000
                Re: Negative array indicies and slice() Mark Lawrence <breamoreboy@yahoo.co.uk> - 2012-10-31 16:01 +0000
            Re: Negative array indicies and slice() Ian Kelly <ian.g.kelly@gmail.com> - 2012-10-30 15:47 -0600
            Re: Negative array indicies and slice() Ian Kelly <ian.g.kelly@gmail.com> - 2012-10-30 15:55 -0600
            Re: Negative array indicies and slice() Chris Angelico <rosuav@gmail.com> - 2012-10-31 09:00 +1100
            Re: Negative array indicies and slice() Ian Kelly <ian.g.kelly@gmail.com> - 2012-10-30 16:02 -0600
            Re: Negative array indicies and slice() Mark Lawrence <breamoreboy@yahoo.co.uk> - 2012-10-30 23:30 +0000
          Re: Negative array indicies and slice() Ian Kelly <ian.g.kelly@gmail.com> - 2012-10-29 11:27 -0600
          Re: Negative array indicies and slice() Andrew Robinson <andrew3@r3dsolutions.com> - 2012-10-29 16:33 -0700
        Re: Negative array indicies and slice() Andrew <andrewr3mail@gmail.com> - 2012-10-29 00:54 -0700
    Re: Negative array indicies and slice() andrewr3mail@gmail.com - 2012-10-28 21:00 -0700
    Re: Negative array indicies and slice() Andrew <andrewr3mail@gmail.com> - 2012-10-28 21:09 -0700
      Re: Negative array indicies and slice() alex23 <wuwei23@gmail.com> - 2012-10-28 21:44 -0700
        Re: Negative array indicies and slice() andrewr3mail@gmail.com - 2012-10-29 01:24 -0700
          Re: Negative array indicies and slice() Chris Rebert <clp2@rebertia.com> - 2012-10-29 01:37 -0700
            Re: Negative array indicies and slice() andrewr3mail@gmail.com - 2012-10-29 01:59 -0700
              Re: Negative array indicies and slice() Mark Lawrence <breamoreboy@yahoo.co.uk> - 2012-10-29 09:36 +0000
              Re: Negative array indicies and slice() Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-10-29 10:34 +0000
            Re: Negative array indicies and slice() andrewr3mail@gmail.com - 2012-10-29 01:59 -0700
      Re: Negative array indicies and slice() Paul Rubin <no.email@nospam.invalid> - 2012-10-28 22:14 -0700
        Re: Negative array indicies and slice() andrewr3mail@gmail.com - 2012-10-29 01:08 -0700
          Re: Negative array indicies and slice() Chris Rebert <clp2@rebertia.com> - 2012-10-29 01:26 -0700
    Re: Negative array indicies and slice() Andrew <andrewr3mail@gmail.com> - 2012-10-28 21:09 -0700
  Re: Negative array indicies and slice() MRAB <python@mrabarnett.plus.com> - 2012-10-29 03:45 +0000
  Re: Negative array indicies and slice() 88888 Dihedral <dihedral88888@googlemail.com> - 2012-11-01 18:08 -0700

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