Pre-PEP: reverse iteration methods

Andrew Dalke adalke at
Wed Sep 24 03:47:57 CEST 2003

Raymond Hettinger:
> Proposal
> ========
> Add a method called iter_backwards() to sequence objects that can benefit
> from it.  The above examples then simplify to::
>     for i in xrange(n).iter_backwards():
>         print seqn[i]
>     for elem in seqn.iter_backwards():
>         print elem

What about letting 'iter_backwards' be a builtin which
looks for __riter__ and if it doesn't exist, get the length
and do old-fashioned offset indexing?

Something like this untested code

def iter_backwards(obj):
     riter = getattr(obj, "__riter__")
  except AttributeError:
      n = len(obj)
      while n != 0:
          n = n -1
          yield obj[n]
    for term in riter():
        yield term

which would be used like

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

     for elem in iter_backwards(seqn):
         print elem

                    dalke at

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