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[Python-Dev] Rationale behind lazy map/filter

R. David Murray rdmurray at bitdance.com
Tue Oct 13 14:32:10 EDT 2015 
On Tue, 13 Oct 2015 12:08:12 -0400, Random832 <random832 at fastmail.com> wrote:
> "R. David Murray" <rdmurray at bitdance.com> writes:
> > On Tue, 13 Oct 2015 11:26:09 -0400, Random832 <random832 at fastmail.com> wrote:
> >
> > And the answer to the question is: lots of code.  I've written some:
> > code that iterates an iterator, breaks that loop on a condition, then
> > resumes iterating, breaking that loop on a different condition, and so
> > on, until the iterator is exhausted.  If the iterator restarted at the
> > top once it was exhausted, that code would break
> 
> 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?

Because the those second &c loops don't run if the iterator is already
exhausted, the else clause is executed instead (or nothing happens,
depending on the code).

Now, likely such code isn't common (so I shouldn't have said "lots"),
but the fact that I've done it at least once, maybe twice (but I can't
remember what context, it was a while ago), argues it isn't vanishingly
uncommon.

--David
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