On Sun, 15 Apr 2018 00:05:58 -0500 Tim Peters wrote:

Just for fun - no complaint, no suggestion, just sharing a bit of code that tickled me.

The docs for `itertools.tee()` contain a Python work-alike, which is easy to follow. It gives each derived generator its own deque, and when a new value is obtained from the original iterator it pushes that value onto each of those deques.

Of course it's possible for them to share a single deque, but the code gets more complicated. Is it possible to make it simpler instead?

What it "really" needs is a shared singly-linked list of values, pointing from oldest value to newest. Then each derived generator can just follow the links, and yield its next result in time independent of the number of derived generators. But how do you know when a new value needs to be obtained from the original iterator, and how do you know when space for an older value can be recycled (because all of the derived generators have yielded it)?

I ended up with almost a full page of code to do that, storing with each value and link a count of the number of derived generators that had yet to yield the value, effectively coding my own reference-count scheme by hand, along with "head" and "tail" pointers to the ends of the linked list that proved remarkably tricky to keep correct in all cases.

Then I thought "this is stupid! Python already does reference counting." Voila! Vast swaths of tedious code vanished, giving this remarkably simple implementation:

This implementation doesn't work with Python 3.7 or 3.8. I've tried it here: https://gist.github.com/pitrou/b3991f638300edb6d06b5be23a4c66d6 and get: Traceback (most recent call last): File "mytee.py", line 14, in gen mylast = last[1] = last = [next(it), None] StopIteration The above exception was the direct cause of the following exception: Traceback (most recent call last): File "mytee.py", line 47, in <module> run(mytee1) File "mytee.py", line 36, in run lists[i].append(next(iters[i])) RuntimeError: generator raised StopIteration (Yuck!) In short, you want the following instead: try: mylast = last[1] = last = [next(it), None] except StopIteration: return

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]

That's smart and obscure :-o The way it works is that the `last` assignment changes the `last` value seen by all derived generators, while the `last[1]` assignment updates the bindings made in the other generators' `mylast` lists... It's difficult to find the words to explain it. The chained assignment makes it more difficult to parse as well (when I read this I don't know if `last[i]` or `last` gets assigned first; apparently the answer is `last[i]`, otherwise the recipe wouldn't work correctly). Perhaps like this: while True: mylast = mylast[1] if not mylast: try: # Create new list link mylast = [next(it), None] except StopIteration: return else: # Append to other generators `mylast` linked lists last[1] = mylast # Update shared list link last = last[1] yield mylast[0] Regards Antoine.

On Sun, Apr 15, 2018 at 8:05 AM, Tim Peters 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 ;-) _______________________________________________ Python-ideas mailing list Python-ideas@python.org https://mail.python.org/mailman/listinfo/python-ideas Code of Conduct: http://python.org/psf/codeofconduct/

-- + Koos Zevenhoven + http://twitter.com/k7hoven +

The topic reminded me of Stephan Houben's streamtee. https://github.com/stephanh42/streamtee/blob/master/streamtee.py It was an attempt at a memory-efficient tee, but it turned out tee was efficient enough. It uses a thunk-like method and recursion to remove the need for an explicit linked list. In the spirit of the topic, I tried to strip it down to the bare essentials: def tee(iterable, n): stream = _streamiter(iter(iterable)) return tuple(_wrapper(stream) for _ in range(n)) def _streamiter(itr): result = (next(itr), _streamiter(itr)) del itr # Release unneeded reference. while True: yield result def _wrapper(stream): while True: result, stream = next(stream) yield result Unfortunately, this code, too, breaks with the new StopIteration propagation rules. On Sun, Apr 15, 2018 at 1:05 AM, Tim Peters wrote:

Just for fun - no complaint, no suggestion, just sharing a bit of code that tickled me.

The docs for `itertools.tee()` contain a Python work-alike, which is easy to follow. It gives each derived generator its own deque, and when a new value is obtained from the original iterator it pushes that value onto each of those deques.

Of course it's possible for them to share a single deque, but the code gets more complicated. Is it possible to make it simpler instead?

What it "really" needs is a shared singly-linked list of values, pointing from oldest value to newest. Then each derived generator can just follow the links, and yield its next result in time independent of the number of derived generators. But how do you know when a new value needs to be obtained from the original iterator, and how do you know when space for an older value can be recycled (because all of the derived generators have yielded it)?

I ended up with almost a full page of code to do that, storing with each value and link a count of the number of derived generators that had yet to yield the value, effectively coding my own reference-count scheme by hand, along with "head" and "tail" pointers to the ends of the linked list that proved remarkably tricky to keep correct in all cases.

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").

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.

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 ;-) _______________________________________________ Python-ideas mailing list Python-ideas@python.org https://mail.python.org/mailman/listinfo/python-ideas Code of Conduct: http://python.org/psf/codeofconduct/