[Python-Dev] PEP for adding an sq_index slot so that any object, a or b, can be used in X[a:b] notation

[Bengt Richter]

On Thu, 09 Feb 2006 01:00:22 -0700, Travis Oliphant <oliphant.travis at ieee.org> wrote:
>
>Abstract
>
>   This PEP proposes adding an sq_index slot in PySequenceMethods and
>   an __index__ special method so that arbitrary objects can be used
>   in slice syntax.
>
>Rationale
>
>   Currently integers and long integers play a special role in slice
>   notation in that they are the only objects allowed in slice
>   syntax. In other words, if X is an object implementing the sequence
>   protocol, then X[obj1:obj2] is only valid if obj1 and obj2 are both
>   integers or long integers.  There is no way for obj1 and obj2 to
>   tell Python that they could be reasonably used as indexes into a
>   sequence.  This is an unnecessary limitation.  
>
>   In NumPy, for example, there are 8 different integer scalars
>   corresponding to unsigned and signed integers of 8, 16, 32, and 64
>   bits.  These type-objects could reasonably be used as indexes into
>   a sequence if there were some way for their typeobjects to tell
>   Python what integer value to use.  
>
>Proposal
> 
>   Add a sq_index slot to PySequenceMethods, and a corresponding
>   __index__ special method.  Objects could define a function to
>   place in the sq_index slot that returns an C-integer for use in
>   PySequence_GetSlice, PySequence_SetSlice, and PySequence_DelSlice.
>
How about if SLICE byte code interpretation would try to call
obj.__int__() if passed a non-(int,long) obj ? Would that cover your use case?

BTW the slice type happily accepts anything for start:stop:step I believe,
and something[slice(whatever)] will call something.__getitem__ with the slice
instance, though this is neither a fast nor nicely spelled way to customize.

Regards,
Bengt Richter