On 26 Feb 2021, at 08:31, Jonathan Slenders <jonathan@slenders.be> wrote:

Barry,

What you describe sounds like `asyncio.gather(...)` if I understand correctly.

The thing with a Barier is that it's usable in situations where we don't know the other tasks. Maybe there is no reference to them from the current scope. Maybe they are even not yet created.

It certainly can be done with a list of `asyncio.Future` and `asyncio.gather(...)`, but that's a lot of boilerplate.

IMHO, Yves is right. For both asyncio and threading, we have Lock, Event, Condition, Semaphore and BoundedSemaphore. Only Barier is missing among the asyncio primitives. (RLock doesn't make sense.)

(I guess we can probably go to bugs.python.org with this proposal.)

Jonathan

Le jeu. 25 févr. 2021 à 23:38, Barry Scott <barry@barrys-emacs.org> a écrit :

On 25 Feb 2021, at 17:15, Jonathan Slenders <jonathan@slenders.be> wrote:

It does make sense to have a barrier synchronization primitive for asyncio.

The idea is to make a coroutine block until at least X coroutines are waiting to enter the barrier.

This is very useful, if certain actions need to be synchronized.

I do most of my async coding with twisted where what you calling a barrier is a DeferredList.

The way its used is that you add in all the deferreds that you want to complete before you continue

into the list. Once all the deferered have competed the DefferedList completes and its callback is run.

Barry

Recently, I had to implement a barier myself for our use case. See code below:

It is simple to implement, but I too would like to have one for asyncio, in order to be consistent with the concurrency primitives we have for threading.

Jonathan

class Barier:
    """
    Make a coroutine block until there are at least X waiters.

    Similar to the threading Barier objects but for asyncio:
    https://docs.python.org/3/library/threading.html#barrier-objects
    """

    def __init__(self, parties: int) -> None:
        self.parties = parties
        self._waiting: int
        self._event = asyncio.Event()

    def add_one(self) -> None:
        self._waiting += 1
        if self._waiting == self.parties:
            self._event.set()

    async def wait(self, worker: "Worker") -> None:
        """
        Wait until all we have at least `parties` waiters.
        """
        self.add_one()
        await self._event.wait()

Le jeu. 25 févr. 2021 à 16:42, Barry Scott <barry@barrys-emacs.org> a écrit :

> On 25 Feb 2021, at 13:14, Yves Duprat <yduprat@gmail.com> wrote:
>
> Hi,the list,
>
> I'm wondering why Barrier object does not exist in the synchronization primitives of the asyncio lib while it is present in threading and multiprocessing libs ?
> This may not be the right place to ask this question, but I never found an answer on the web.
> Thanks for your help.


I'm assuming that the barrier you are speaking of is the mechanism that is used to
synchronise threads/processes running in parallel to prevent data races.

With async code that is never an issue. Each function runs to completion uninterrupted.
There are no data races. Each time a async function runs it can know that the state of
the objects it uses will not be changed while it is running.

Barry



>
> Yves
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