python-peps/peps/pep-0281.rst

PEP: 281
Title: Loop Counter Iteration with range and xrange
Author: Magnus Lie Hetland <magnus@hetland.org>
Status: Rejected
Type: Standards Track
Content-Type: text/x-rst
Created: 11-Feb-2002
Python-Version: 2.3
Post-History:


Abstract
========

This PEP describes yet another way of exposing the loop counter in
for-loops. It basically proposes that the functionality of the
function ``indices()`` from :pep:`212` be included in the existing
functions ``range()`` and ``xrange()``.


Pronouncement
=============

In commenting on :pep:`279`'s ``enumerate()`` function, this PEP's author
offered, "I'm quite happy to have it make :pep:`281` obsolete."
Subsequently, :pep:`279` was accepted into Python 2.3.

On 17 June 2005, the BDFL concurred with it being obsolete and
hereby rejected the PEP.  For the record, he found some of the
examples to somewhat jarring in appearance::

   >>> range(range(5), range(10), range(2))
   [5, 7, 9]


Motivation
==========

It is often desirable to loop over the indices of a sequence.  PEP
212 describes several ways of doing this, including adding a
built-in function called indices, conceptually defined as::

   def indices(sequence):
       return range(len(sequence))

On the assumption that adding functionality to an existing built-in
function may be less intrusive than adding a new built-in function,
this PEP proposes adding this functionality to the existing
functions ``range()`` and ``xrange()``.


Specification
=============

It is proposed that all three arguments to the built-in functions
``range()`` and ``xrange()`` are allowed to be objects with a length
(i.e. objects implementing the ``__len__`` method).  If an argument
cannot be interpreted as an integer (i.e. it has no ``__int__``
method), its length will be used instead.

Examples::

   >>> range(range(10))
   [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
   >>> range(range(5), range(10))
   [5, 6, 7, 8, 9]
   >>> range(range(5), range(10), range(2))
   [5, 7, 9]
   >>> list(xrange(range(10)))
   [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
   >>> list(xrange(xrange(10)))
   [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

   # Number the lines of a file:
   lines = file.readlines()
   for num in range(lines):
       print num, lines[num]


Alternatives
============

A natural alternative to the above specification is allowing
``xrange()`` to access its arguments in a lazy manner.  Thus, instead
of using their length explicitly, ``xrange`` can return one index for
each element of the stop argument until the end is reached.  A
similar lazy treatment makes little sense for the start and step
arguments since their length must be calculated before iteration
can begin.  (Actually, the length of the step argument isn't needed
until the second element is returned.)

A pseudo-implementation (using only the stop argument, and assuming
that it is iterable) is::

   def xrange(stop):
       i = 0
       for x in stop:
           yield i
           i += 1

Testing whether to use ``int()`` or lazy iteration could be done by
checking for an ``__iter__`` attribute.  (This example assumes the
presence of generators, but could easily have been implemented as a
plain iterator object.)

It may be questionable whether this feature is truly useful, since
one would not be able to access the elements of the iterable object
inside the for loop through indexing.

Example::

   # Printing the numbers of the lines of a file:
   for num in range(file):
       print num # The line itself is not accessible

A more controversial alternative (to deal with this) would be to
let ``range()`` behave like the function ``irange()`` of :pep:`212` when
supplied with a sequence.

Example::

   >>> range(5)
   [0, 1, 2, 3, 4]
   >>> range('abcde')
   [(0, 'a'), (1, 'b'), (2, 'c'), (3, 'd'), (4, 'e')]


Backwards Compatibility
=======================

The proposal could cause backwards incompatibilities if arguments
are used which implement both ``__int__`` and ``__len__`` (or ``__iter__`` in
the case of lazy iteration with ``xrange``).  The author does not
believe that this is a significant problem.


Copyright
=========

This document has been placed in the public domain.