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A simple asyncio wrapper attempting to look like Trio

Project description

A simple asyncio wrapper attempting to look like Trio

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When going deeper and deeper with asyncio, and managing a lot of tasks in parallel, you notice that on top of having a lot to deal with to keep an eye on all your task, you also end up always doing the same kind of boiler plate, and the code can become easily twisted and unreadable.

Trio is there to make asynchronous programming easy and more "for humans".

Traio (as kind of Trio on top of Asyncio) let you use asyncio with a little of the philosophy of Trio, mostly the Nursery concept (called Scope here). It also synthesize the most common pattern we are using for handling async tasks in a sane way.

Disclaimer

This is not a replacement for Trio: if you do a full Trio-like project, just use Trio! It's just truly awesome, but in some cases you get stuck with asyncio, but still want to have a code you can read and manage...

Because we run on top of asyncio, we are quite limited in how we can handle cancellation, scopes, and coroutines. This is just on top of asyncio, this is not an alternative! But on the good side, you can mix this with regular asyncio code, which can look appealing.

Examples

Simple Scope

The main way to use the Scope is (like in Trio) as a context manager

import asyncio
from traio import Scope

async def fetch_url(x):
    # Do something long
    await asyncio.sleep(3)

async def main():
    async with Scope(timeout=10) as scope:
        for i in range(10):
            scope.spawn(fetch_url(i))

The Scope.spawn method, called on an awaitable thing, will spawn a task and register it on the scope. An equivalent exist using the << operator (see next example).

When reaching the end of the context block, the code will block until:

  • all tasks are done
  • or the timeout is over
  • or the scope get's cancelled.

You can also use the Scope without context manager:

import asyncio
from traio import Scope

async def fetch_url(x):
    # Do something long
    await asyncio.sleep(3)

async def main():
    # Equivalent to previous example
    scope = Scope(timeout=10)

    for i in range(10):
        scope << fetch_url(i)

    await scope.join()

Names and logger

If you went deep enough in asyncio misteries, you know that tracing code is (for now) kind of a nightmare... For that reason, Scope as well as tasks can be instantiated with a name. The Scope can also take a logger which will be used for tracing most of the calls and task life cycle, mostly with debug level.

Special tasks

Scope.spawn can be called with different parameters with interesting effects:

  • The bubble boolean parameter controls task error bubbling. A task will bubble by default. This means that an error in the taskwill cause the task to stop (of course), but the scope will be cancelled as well and raise the given error. This is the desired default behavior. But it can be useful in some cases not to do that, and just ignore a task. Not that if you await manually a task, this cancels bubbling automatically: if you take the pain of waiting for a task, it's not to get all the rest cancelled!
import asyncio
from traio import Scope

async def fetch_url():
    # Do something long
    await asyncio.sleep(10)

async def trivial():
    await asyncio.sleep(0.01)
    raise ValueError('not interesting')

async def main():

    async with Scope(timeout=0.5) as n:
        # This will try to run for 10 seconds
        n << fetch_url()

        # This will run a bit then raise (but not 'bubble')
        n.spawn(trivial(), bubble=False)

    # Eventually after 0.5 seconds the Scope times out and
    # gives a TImeoutError
  • A task can be marked as master, in that case the scope will die with the task when done. This is typically useful when you have one main task to be performed and other background ones, which have no meaning if the main one stops.
import asyncio
from traio import Scope

async def fetch_url():
    # Do something long
    await asyncio.sleep(10)

async def trivial():
    await asyncio.sleep(0.01)

async def main():

    async with Scope(timeout=0.5) as n:
        # This will try to run for 10 seconds
        n << fetch_url()

        # This will run a bit then scope gets cancelled when it ends
        n.spawn(trivial(), master=True)

Note you can combine both flags; If bubble is False and master is True, the scope will exit silently when the task is done, even with an exception.

Nested Scope

That's one of the most exciting ones: you can spawn a sub-scope from the original one, which will follow the same rules as any Scope but will as well die if the parent is cancelled!

import asyncio
from traio import Scope

async def fetch_url():
    # Do something long
    await asyncio.sleep(10)

async def main():

    async with Scope(timeout=0.2):
        async with Scope(timeout=0.5) as inner:
            # This will try to run for 10 seconds
            inner << fetch_url()

    # Here the outer Scope will timeout first and will cancel the inner one!

Getting current Scope

Using the awesome contextvars and the current backport of it aiocontextvars, we can keep track of the current active scope without necessarily passing the scope variable all along the call chain.

async def handler(client):
    # This runs in the scope `main_scope`
    # Create a subscope of the current one
    async with Scope.get_current().fork() as s:
        # Here we are in the scope of `s`, as well as spawned tasks
        s << do_something(client)
        s << do_something_else(client)

async def server(main_scope):
    # This runs in the scope `main_scope`
    client = await new_connection()
    main_scope.spawn(handler(client))

async def main():
    async with Scope() as main_scope:
        main_scope << server(main_scope)
        main_scope << do_something_else()

Status

This is beta. We are not going to change the API (much) anymore.

TODOS

  • write more examples
  • extend the API:
    • new kinds of tasks to investigate, for example executors (although we have no way to stop a thread executor...)
  • play more with the real Trio and get a better feeling of it
  • get some user feedback if possible!

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