PEP: 322
Title: Reverse Iteration
Author: Raymond Hettinger <python@rcn.com>
Status: Final
Type: Standards Track
Created: 24-Sep-2003
Python-Version: 2.4
Post-History: 24-Sep-2003


Abstract
========

This proposal is to add a builtin function to support reverse
iteration over sequences.


Motivation
==========

For indexable objects, current approaches for reverse iteration are
error prone, unnatural, and not especially readable::

    for i in xrange(n-1, -1, -1):
        print seqn[i]

One other current approach involves reversing a list before iterating
over it.  That technique wastes computer cycles, memory, and lines of
code::

    rseqn = list(seqn)
    rseqn.reverse()
    for value in rseqn:
        print value

Extended slicing is a third approach that minimizes the code overhead
but does nothing for memory efficiency, beauty, or clarity.

Reverse iteration is much less common than forward iteration, but it
does arise regularly in practice.  See `Real World Use Cases`_ below.


Proposal
========

Add a builtin function called *reversed()* that makes a reverse
iterator over sequence objects that support __getitem__() and
__len__().

The above examples then simplify to::

    for i in reversed(xrange(n)):
        print seqn[i]

::

    for elem in reversed(seqn):
        print elem

The core idea is that the clearest, least error-prone way of specifying
reverse iteration is to specify it in a forward direction and then say
*reversed*.

The implementation could be as simple as::

    def reversed(x):
        if hasattr(x, 'keys'):
            raise ValueError("mappings do not support reverse iteration")
        i = len(x)
        while i > 0:
            i -= 1
            yield x[i]

No language syntax changes are needed.  The proposal is fully backwards
compatible.

A C implementation and unit tests are at:  https://bugs.python.org/issue834422

BDFL Pronouncement
==================

This PEP has been conditionally accepted for Py2.4.  The condition means
that if the function is found to be useless, it can be removed before
Py2.4b1.


Alternative Method Names
========================

* *reviter*  -- Jeremy Fincher's suggestion matches use of iter()
* *ireverse* -- uses the itertools naming convention
* *inreverse* -- no one seems to like this one except me

The name *reverse* is not a candidate because it duplicates the name
of the list.reverse() which mutates the underlying list.


Discussion
==========

The case against adoption of the PEP is a desire to keep the number of
builtin functions small.  This needs to weighed against the simplicity
and convenience of having it as builtin instead of being tucked away in
some other namespace.


Real World Use Cases
====================

Here are some instances of reverse iteration taken from the standard
library and comments on why reverse iteration was necessary:

* atexit.exit_handlers() uses::

    while _exithandlers:
        func, targs, kargs = _exithandlers.pop()
            . . .

  In this application popping is required, so the new function would
  not help.

* heapq.heapify() uses ``for i in xrange(n//2 - 1, -1, -1)`` because
  higher-level orderings are more easily formed from pairs of
  lower-level orderings.  A forward version of this algorithm is
  possible; however, that would complicate the rest of the heap code
  which iterates over the underlying list in the opposite direction.
  The replacement code ``for i in reversed(xrange(n//2))`` makes
  clear the range covered and how many iterations it takes.

* mhlib.test() uses::

    testfolders.reverse();
    for t in testfolders:
        do('mh.deletefolder(%s)' % `t`)

  The need for reverse iteration arises because the tail of the
  underlying list is altered during iteration.

* platform._dist_try_harder() uses
  ``for n in range(len(verfiles)-1,-1,-1)`` because the loop deletes
  selected elements from *verfiles* but needs to leave the rest of
  the list intact for further iteration.

* random.shuffle() uses ``for i in xrange(len(x)-1, 0, -1)`` because
  the algorithm is most easily understood as randomly selecting
  elements from an ever diminishing pool.  In fact, the algorithm can
  be run in a forward direction but is less intuitive and rarely
  presented that way in literature.  The replacement code
  ``for i in reversed(xrange(1, len(x)))`` is much easier
  to verify visually.

* rfc822.Message.__delitem__() uses::

    list.reverse()
    for i in list:
        del self.headers[i]

  The need for reverse iteration arises because the tail of the
  underlying list is altered during iteration.


Rejected Alternatives
=====================

Several variants were submitted that attempted to apply *reversed()*
to all iterables by running the iterable to completion, saving the
results, and then returning a reverse iterator over the results.
While satisfying some notions of full generality, running the input
to the end is contrary to the purpose of using iterators
in the first place.  Also, a small disaster ensues if the underlying
iterator is infinite.

Putting the function in another module or attaching it to a type object
is not being considered.  Like its cousins, *zip()* and *enumerate()*,
the function needs to be directly accessible in daily programming.  Each
solves a basic looping problem:  lock-step iteration, loop counting, and
reverse iteration.  Requiring some form of dotted access would interfere
with their simplicity, daily utility, and accessibility.  They are core
looping constructs, independent of any one application domain.


Copyright
=========

This document has been placed in the public domain.
