[Python-Dev] Rationale behind lazy map/filter (original) (raw)
R. David Murray rdmurray at bitdance.com
Tue Oct 13 11:41:37 EDT 2015
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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
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