aiotools

Idiomatic asyncio utilties

NOTE: This project is under early stage of development. The public APIs may break version by version.

Modules

I also recommend to try the following asyncio libraries for your happier life.

aiojobs: Provides a concurrency-limited scheduler for asyncio tasks with graceful shutdown.
trio: An alternative implementation of asynchronous IO stack for Python, with focus on cancellation scopes and task groups called "nursery".

Examples

Async Context Manager

This is an asynchronous version of contextlib.contextmanager to make it easier to write asynchronous context managers without creating boilerplate classes.

import asyncio
import aiotools

@aiotools.actxmgr
async def mygen(a):
   await asyncio.sleep(1)
   yield a + 1
   await asyncio.sleep(1)

async def somewhere():
   async with mygen(1) as b:
       assert b == 2

Note that you need to wrap yield with a try-finally block to ensure resource releases (e.g., locks), even in the case when an exception is ocurred inside the async-with block.

import asyncio
import aiotools

lock = asyncio.Lock()

@aiotools.actxmgr
async def mygen(a):
   await lock.acquire()
   try:
       yield a + 1
   finally:
       lock.release()

async def somewhere():
   try:
       async with mygen(1) as b:
           raise RuntimeError('oops')
   except RuntimeError:
       print('caught!')  # you can catch exceptions here.

You can also create a group of async context managers, which are entered/exited all at once using asyncio.gather().

import asyncio
import aiotools

@aiotools.actxmgr
async def mygen(a):
   yield a + 10

async def somewhere():
   ctxgrp = aiotools.actxgroup(mygen(i) for i in range(10))
   async with ctxgrp as values:
       assert len(values) == 10
       for i in range(10):
           assert values[i] == i + 10

Async Server

This implements a common pattern to launch asyncio-based server daemons.

import asyncio
import aiotools

async def echo(reader, writer):
   data = await reader.read(100)
   writer.write(data)
   await writer.drain()
   writer.close()

@aiotools.server
async def myworker(loop, pidx, args):
   server = await asyncio.start_server(echo, '0.0.0.0', 8888,
       reuse_port=True, loop=loop)
   print(f'[{pidx}] started')
   yield  # wait until terminated
   server.close()
   await server.wait_closed()
   print(f'[{pidx}] terminated')

if __name__ == '__main__':
   # Run the above server using 4 worker processes.
   aiotools.start_server(myworker, num_workers=4)

It handles SIGINT/SIGTERM signals automatically to stop the server, as well as lifecycle management of event loops running on multiple processes. Internally it uses aiotools.fork module to get kernel support to resolve potential signal/PID related races via PID file descriptors on supported versions (Python 3.9+ and Linux kernel 5.4+).

Async TaskGroup

A TaskGroup object manages the lifecycle of sub-tasks spawned via its create_task() method by guarding them with an async context manager which exits only when all sub-tasks are either completed or cancelled.

This is motivated from trio's nursery API and a draft implementation is adopted from EdgeDB's Python client library.

import aiotools

async def do():
    async with aiotools.TaskGroup() as tg:
        tg.create_task(...)
        tg.create_task(...)
        ...
    # at this point, all subtasks are either cancelled or done.

Async Timer

import aiotools

i = 0

async def mytick(interval):
   print(i)
   i += 1

async def somewhere():
   t = aiotools.create_timer(mytick, 1.0)
   ...
   t.cancel()
   await t

t is an asyncio.Task object. To stop the timer, call t.cancel(); await t. Please don't forget await-ing t because it requires extra steps to cancel and await all pending tasks. To make your timer function to be cancellable, add a try-except clause catching asyncio.CancelledError since we use it as a termination signal.

You may add TimerDelayPolicy argument to control the behavior when the timer-fired task takes longer than the timer interval. DEFAULT is to accumulate them and cancel all the remainings at once when the timer is cancelled. CANCEL is to cancel any pending previously fired tasks on every interval.

import asyncio
import aiotools

async def mytick(interval):
   await asyncio.sleep(100)  # cancelled on every next interval.

async def somewhere():
   t = aiotools.create_timer(mytick, 1.0, aiotools.TimerDelayPolicy.CANCEL)
   ...
   t.cancel()
   await t

Virtual Clock

It provides a virtual clock that advances the event loop time instantly upon any combination of asyncio.sleep() calls in multiple coroutine tasks, by temporarily patching the event loop selector.

This is also used in our timer test suite.

import aiotools
import pytest

@pytest.mark.asyncio
async def test_sleeps():
    loop = aiotools.compat.get_running_loop()
    vclock = aiotools.VirtualClock()
    with vclock.patch_loop():
        print(loop.time())  # -> prints 0
        await asyncio.sleep(3600)
        print(loop.time())  # -> prints 3600

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