[Python-Dev] Rationale behind lazy map/filter

[R. David Murray]

On Thu, 05 Nov 2015 03:59:05 +0000, Michael Selik <mike at selik.org> wrote:
> > I'm not suggesting restarting at the top (I've elsewhere suggested that
> > many such methods would be better as an *iterable* that can be restarted
> > at the top by calling iter() multiple times, but that's not the same
> > thing). I'm suggesting raising an exception other than StopIteration, so
> > that this situation can be detected. If you are writing code that tries
> > to resume iterating after the iterator has been exhausted, I have to
> > ask: why?
> 
> The most obvious case for me would be tailing a file. Loop over the lines
> in the file, sleep, then do it again. There are many tasks analogous to
> that scenario -- anything querying a shared resource.

The 'file' iterator actually breaks the rules of iterators: it does
*not* continue to raise StopIteration once it has been exhausted, if
more input becomes available.  Given that it is one of the most commonly
used iterators (and I would not be surprised if other special-purpose
iterators copied its design), this pattern does seem like a blocker for
the proposal.

--David