[Python-ideas] An ABC representing "Iterable, Sized, Container"

Chris Angelico rosuav at gmail.com
Fri Jul 22 15:05:50 EDT 2016

On Sat, Jul 23, 2016 at 3:58 AM, David Mertz <mertz at gnosis.cx> wrote:
>> What you might be looking at is a protocol for "bookmarking" or
>> "forking" an iterator. That might be more useful. For example:
> How is this different from itertools.tee()?

Functionally? Not at all. In fact, itertools.tee could make use of the
fork protocol. The idea would be efficiency; iterators that are
"restartable" by your definition would be "forkable", which would
effectively permit tee to just ask the iterator to tee itself. The
simple and naive implementation of tee() is "save all the elements
until they get asked for", which requires extra storage for those
elements; if your iterator can fork, that implies that you can get
those elements right back again from the original.

Imagine a list_iterator consists of two things, a base list and a
current index. Tee'ing that iterator means retrieving elements and
hanging onto them until needed; forking means creating another
iterator with the same index. Which means that a forked iterator would
reflect changes to the original list, for better or for worse.

Not saying it's necessarily a good thing, but it's a more general
version of the concept of "restarting" an iterator, being more
compatible with trimmed iterators and such. Ultimately, your "restart"
is no different from itertools.tee either.


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