From 996d6204200cd0a3ca40ff8bc10a2d7674f72fe7 Mon Sep 17 00:00:00 2001
From: Raymond Hettinger <python@rcn.com>
Date: Thu, 25 Sep 2003 16:18:28 +0000
Subject: [PATCH] Updated to reflect comments on comp.lang.python.

---
 pep-0322.txt | 60 +++++++++++++++++++++++++++++-----------------------
 1 file changed, 34 insertions(+), 26 deletions(-)

diff --git a/pep-0322.txt b/pep-0322.txt
index 71fe5f493..b6bfbc2c2 100644
--- a/pep-0322.txt
+++ b/pep-0322.txt
@@ -29,16 +29,15 @@ error prone, unnatural, and not especially readable::
 
 One other current approach involves reversing a list before iterating
 over it.  That technique wastes computer cycles, memory, and lines of
-code.  Also, it only works with lists (strings, for example, do not
-define a reverse method)::
+code::
 
     rseqn = list(seqn)
     rseqn.reverse()
     for value in rseqn:
         print value
 
-Extending slicing minimizes the code overhead but does nothing for
-memory efficiency, beauty, or clarity.
+Extending 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.
@@ -109,6 +108,9 @@ library and comments on why reverse iteration was necessary:
         . . .
     del _exithandlers
 
+  Note, if the order of deletion is important, then the first form
+  is still needed.
+
 * difflib.get_close_matches() uses::
 
     result.sort()           # Retain only the best n.
@@ -149,18 +151,15 @@ library and comments on why reverse iteration was necessary:
 * 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.  This use case could be
-  addressed with *itertools.ifilter()* but would require the
-  selection predicate to be in a *lambda* expression.  The net
-  result is less clear and readable than the original.  A better
-  reformulation is to replace the first line with the proposed
-  method.
+  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.
+  presented that way in literature.  The replacement code
+  ``for i in xrange(1, len(x)).iter_backwards()`` is much easier
+  to mentally verify.
 
 * rfc822.Message.__delitem__() uses::
 
@@ -172,23 +171,32 @@ library and comments on why reverse iteration was necessary:
   underlying list is altered during iteration.
 
 
-Rejected Alternative Ideas
-==========================
+Alternative Ideas
+=================
 
-* Add a builtin function, *reverse()* which calls a magic method,
-  __riter__.  I see this as more overhead for no additional benefit.
+* Add a builtin function, *riter()* which calls a magic method,
+  *__riter__*.  I see this as more overhead for no additional benefit.
 
-* Add a builtin function, *reverse()* which does the above, and
-  if *__riter__* is not found, constructs its own using
-  *__getitem__*, and if *__getitem__* is not found, builds a list
-  from *__iter__* and returns a reverse iterator over the new list.
-  The advantage is that one function takes care of almost everything
-  that is potentially reversible.  A disadvantage is that it can
-  invisibility slip in to a low performance mode (in terms of time
-  and memory) which would be more visible with an explicit
-  ``list(obj).reverse()``.  Another problem is that *__getitem__*
-  is also used in mappings as well as sequences and that could lead
-  to bizarre results.
+* Several variants were submitted that provided fallback behavior
+  when *__riter__* is not defined:
+
+  - fallback to:  ``for i in xrange(len(obj)-1,-1,-1): yield obj[i]``
+  - fallback to:  ``for i in itertools.count(): yield[obj[-i]]``
+  - fallback to:  ``tmp=list(obj); tmp.reverse();  return iter(tmp)``
+
+  All of these attempt to save implementing some object methods at the
+  expense of adding a new builtin function and of creating a new magic
+  method name.
+
+  The approaches using *__getitem__()* are slower than using a custom
+  method for each object.  Also, the *__getitem__()* variants produce
+  bizarre results when applied to mappings.
+
+  All of the variants crash when applied to an infinite iterator.
+
+  The last variant can invisibly slip into a low performance mode
+  (in terms of time and memory) which could be made more visible with
+  an explicit ``list(obj).reverse()``.
 
 
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