[Python-Dev] PEP 322: Reverse Iteration

Samuele Pedroni pedronis at bluewin.ch
Tue Nov 4 19:47:33 EST 2003

At 16:07 04.11.2003 -0800, Guido van Rossum wrote:
> > > >     for i, value in reversed(enumerate(listofnum)):
> > >
> > > Sorry, this doesn't work.  enumerate() returns an iterator, reversed()
> > > requires a sequence.
> >
> > I believe the assumption is that enumerate (as well as the proposed 
> irange)
> > would grow an __reversed__ method to handle just that usage.
>Ah, so it is.  Then the PEP's abstract is wrong:
>This proposal is to add a builtin function to support reverse
>iteration over sequences.
>Also, the PEP should enumerate (:-) which built-in types should be
>modified in this way, to give an impression of the enormity (or not)
>of the task.

what is not clear to me is that the PEP is explicit about reversed() 
refusing general iterables and in particular infinite iterators, but then 
the combination reversed enumerate.__reversed__ would accept them or not?. 
Will enumerate implement __reversed__ in terms of keeping the enumerate 
argument around instead of just a iterator derived from it and reproducing 
then the reversed behavior: limits checks and implementation strategy on 
the original argument if/when __reversed__ is called?


for x in reversed(enumerate(itertools.count())):

would throw an exception instead of not terminating, OTHERWISE with the 
strategy of consuming
the iterator if x is a finite iterator but without __len__ then

reversed(x) would not work but

reversed(enumerate(x)) would.

Further enumerate.__iter__ does not enable re-iteration, simply it does not 
return a fresh iterator but what about enumerate.__reversed__ ?



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