On Sun, Apr 15, 2018 at 8:05 AM, Tim Peters <tim.pet...@gmail.com> wrote:
[...]
> Then I thought "this is stupid! Python already does reference
> counting." Voila! Vast swaths of tedious code vanished, giving this
> remarkably simple implementation:
>
> def mytee(xs, n):
> last = [None, None]
>
> def gen(it, mylast):
> nonlocal last
> while True:
> mylast = mylast[1]
> if not mylast:
> mylast = last[1] = last = [next(it), None]
> yield mylast[0]
>
> it = iter(xs)
> return tuple(gen(it, last) for _ in range(n))
>
> There's no need to keep a pointer to the start of the shared list at
> all - we only need a pointer to the end of the list ("last"), and each
> derived generator only needs a pointer to its own current position in
> the list ("mylast").
>
>
Things here remind me of my implementation design for PEP 555: the
"contexts" present in the process are represented by a singly-linked tree
of assignment objects. It's definitely possible to write the above in a
more readable way, and FWIW I don't think it involves "assignments as
expressions".
> What I find kind of hilarious is that it's no help at all as a
> prototype for a C implementation: Python recycles stale `[next(it),
> None]` pairs all by itself, when their internal refcounts fall to 0.
> That's the hardest part.
>
>
Why can't the C implementation use Python refcounts? Are you talking about
standalone C code? Or perhaps you are thinking about overhead? (In PEP 555
that was not a concern, though). Surely it would make sense to reuse the
refcounting code that's already there. There are no cycles here, so it's
not particulaly complicated -- just duplication.
Anyway, the whole linked list is unnecessary if the iterable can be
iterated over multiple times. But "tee" won't know when to do that. *That*
is what I call overhead (unless of course all the tee branches are consumed
in an interleaved manner).
> BTW, I certainly don't suggest adding this to the itertools docs
> either. While it's short and elegant, it's too subtle to grasp easily
> - if you think "it's obvious", you haven't yet thought hard enough
> about the problem ;-)
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--
+ Koos Zevenhoven + http://twitter.com/k7hoven +
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