python-peps/pep-0357.txt

PEP: 357
Title: Allowing Any Object to be Used for Slicing
Version: $Revision$
Last-Modified: $Date$
Author: Travis Oliphant <oliphant@ee.byu.edu>
Status: Final
Type: Standards Track
Created: 09-Feb-2006
Python-Version: 2.5

Abstract

    This PEP proposes adding an nb_index slot in PyNumberMethods and an
    __index__ special method so that arbitrary objects can be used
    whenever integers are explicitly needed in Python, such as in slice
    syntax (from which the slot gets its name).

Rationale

    Currently integers and long integers play a special role in
    slicing 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 integers in
    many places where Python expects true integers but cannot inherit from 
    the Python integer type because of incompatible memory layouts.  
    There should be some way to be able to tell Python that an object can 
    behave like an integer.

    It is not possible to use the nb_int (and __int__ special method)
    for this purpose because that method is used to *coerce* objects
    to integers.  It would be inappropriate to allow every object that
    can be coerced to an integer to be used as an integer everywhere
    Python expects a true integer.  For example, if __int__ were used
    to convert an object to an integer in slicing, then float objects
    would be allowed in slicing and x[3.2:5.8] would not raise an error
    as it should.

Proposal
 
    Add an nb_index slot to PyNumberMethods, and a corresponding
    __index__ special method.  Objects could define a function to place
    in the nb_index slot that returns an appropriate C-integer (Py_ssize_t
    after PEP 353).  This C-integer will be used whenever Python needs
    one such as in PySequence_GetSlice, PySequence_SetSlice, and
    PySequence_DelSlice.  

Specification:
    
    1) The nb_index slot will have the signature

       Py_ssize_t index_func (PyObject *self)

    2) The __index__ special method will have the signature

       def __index__(self):
           return obj
       
       where obj must be either an int or a long. 

    3) A new C-API function PyNumber_Index will be added with signature

       Py_ssize_t PyNumber_Index (PyObject *obj)

       which will return obj->ob_type->tp_as_number->nb_index(obj) if it is available. 
       A -1 will be returned and an exception set on an error. 

    4) A new operator.index(obj) function will be added that calls
       equivalent of obj.__index__() and raises an error if obj does not implement
       the special method.
       
Implementation Plan

    1) Add the nb_index slot in object.h and modify typeobject.c to 
       create the __index__ method

    2) Change the ISINT macro in ceval.c to ISINDEX and alter it to 
       accomodate objects with the index slot defined.

    3) Change the _PyEval_SliceIndex function to accommodate objects
       with the index slot defined.

    4) Change all builtin objects (e.g. lists) that use the as_mapping 
       slots for subscript access and use a special-check for integers to 
       check for the slot as well.

    5) Add the nb_index slot to integers and long_integers.

    6) Add PyNumber_Index C-API to return an integer from any 
       Python Object that has the nb_index slot.  

    7) Add the operator.index(x) function.


Discussion Questions

    Speed: 

    Implementation should not slow down Python because integers and long
    integers used as indexes will complete in the same number of
    instructions.  The only change will be that what used to generate
    an error will now be acceptable.

    Why not use nb_int which is already there?

    The nb_int method is used for coercion and so means something
    fundamentally different than what is requested here.  This PEP
    proposes a method for something that *can* already be thought of as
    an integer communicate that information to Python when it needs an
    integer.  The biggest example of why using nb_int would be a bad
    thing is that float objects already define the nb_int method, but
    float objects *should not* be used as indexes in a sequence.

    Why the name __index__?

    Some questions were raised regarding the name __index__ when other
    interpretations of the slot are possible.  For example, the slot
    can be used any time Python requires an integer internally (such
    as in "mystring" * 3).  The name was suggested by Guido because
    slicing syntax is the biggest reason for having such a slot and 
    in the end no better name emerged. See the discussion thread:
    http://mail.python.org/pipermail/python-dev/2006-February/thread.html#60594 
    for examples of names that were suggested such as "__discrete__" and 
    "__ordinal__".

    Why return Py_ssize_t from nb_index?  

    The nb_index slot is primarily intended to return an integer
    needed by the sequence interface.  In Python 2.5 this is
    Py_ssize_t.  As this is the primary purpose of the slot, it
    makes sense to return the C-integer directly and not wrapped
    in a Python int object. 

    Why can't __index__ return any object with the nb_index method?

    This would allow infinite recursion in many different ways that are not
    easy to check for.  This restriction is similar to the requirement that 
    __nonzero__ return an int or a bool. 

Reference Implementation
 
    Submitted as patch 1436368 to SourceForge.  

Copyright

    This document is placed in the public domain.