[Python-ideas] Previous item from iterator.

[Steven D'Aprano]

On Wed, Apr 15, 2015 at 05:26:50PM +0100, Krystian Kichewko wrote:
> Hi all!
> 
> I hope I'm posting in correct place.
> 
> I would like to propose an idea for iterating over an iterator in reverse.
> 
> prev(iterator[, default])
> 
> Built-in function: it should retrieve previous element from an
> iterator by calling its __prev__() method.

For experimentation purposes, you can easily write your own:


_sentinel = object()

def prev(it, default=_sentinel):
    try:
        return type(it).__prev__(it)
    except StopIteration:
        if default is _sentinel:
            raise
        else:
            return _sentinel


ought to do it. Then you can write your own iterators, give them a 
__prev__ method as well as a __next__ method, and experiment to see what 
advantages it gives you (if any).

Can you demonstrate some useful tasks that are easier with this 
bidirectional iteration than they currently are?


-- 
Steve