[Python-Dev] Rationale behind lazy map/filter

[R. David Murray]

On Tue, 13 Oct 2015 11:26:09 -0400, Random832 <random832 at fastmail.com> wrote:
> "R. David Murray" <rdmurray at bitdance.com> writes:
> 
> > On Tue, 13 Oct 2015 14:59:56 +0300, Stefan Mihaila
> > <stefanmihaila91 at gmail.com> wrote:
> >> Maybe it's just python2 habits, but I assume I'm not the only one
> >> carelessly thinking that "iterating over an input a second time will 
> >> result in the same thing as the first time (or raise an error)".
> >
> > This is the way iterators have always worked.
> 
> It does raise the question though of what working code it would actually
> break to have "exhausted" iterators raise an error if you try to iterate
> them again rather than silently yield no items.

They do raise an error: StopIteration.  It's just that the iteration
machinery uses that to stop iteration :).

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.

--David