[Python-Dev] Rationale behind lazy map/filter (original) (raw)

Steven D'Aprano steve at pearwood.info
Tue Oct 13 19:29:35 EDT 2015

On Tue, Oct 13, 2015 at 11:26:09AM -0400, Random832 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.

Anything which looks like this:

for item in iterator: if condition: break do_this() ... for item in iterator: do_that()

If the condition is never true, the iterator is completely processed by the first loop, and the second loop is a no-op by design.

I don't know how common it is, but I've written code like that.

Had we been designing the iterator protocol from scratch, perhaps we might have had two exceptions:

class EmptyIterator(Exception): ... class StopIteration(EmptyIterator): ...

and have StopIteration only raised the first time you call next() on an empty iterator. But would it have been better? I don't know. I suspect not. I think that although it might avoid a certain class of errors, it would add complexity to other situations which are currently simple.

-- Steve

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