aiotools 2.2.3

Idiomatic asyncio utilities

aiotools

Idiomatic asyncio utilities

Modules

• Safe Cancellation
• Async Context Manager
• Async Defer
• Async Fork
• Async Functools
• Async Itertools
• Async Server
• Async Timer
• TaskContext
• TaskScope
• (alias of TaskScope) Supervisor
• (deprecated since 2.0) (Persistent)TaskGroup
• High-level Coroutine Utilities

Full API documentation: https://aiotools.readthedocs.io

See also

• anyio: High level asynchronous concurrency and networking framework that works on top of either Trio or asyncio
• trio: An alternative implementation of asyncio focusing on structured concurrency
• aiometer: A Python concurrency scheduling library, compatible with asyncio and trio.
• aiojobs: A concurrency-limiting, task-shielding scheduler for asyncio tasks for graceful shutdown

Currently aiotools targets the vanilly asyncio ecosystem only (with some tight coupling with asyncio internals), but you may find similar problem definitions and alternative solutions in the above libraries.

Examples

Below are some highlighted usecases of aiotools. Please refer more detailed logic and backgrounds in the documentation.

Safe Cancellation

Consider the following commonly used pattern:

task = asyncio.create_task(...)
task.cancel()
await task  # PROBLEM: would it raise CancelledError or not? should we propagate it or not?

It has been the reponsibility of the author of tasks and the caller of them to coordinate whether to re-raise injected cancellation error.

Now we can use the structured cancellation introduced in Python 3.11:

task = asyncio.create_task(...)
await aiotools.cancel_and_wait(task)

which will re-raise the cancellation when there is an external cancellation request and absorb it otherwise.

Relying on this API whenever you need to cancel asyncio tasks will make your codebase more consistent because you no longer need to decide whether to (re-)raise or suppress CancelledError in your task codes.

You may also combine cancel_and_wait() with ShieldScope to guard a block of codes from cancellation in the middle but defer the cancellation to the end of the block.

async def work():
    try:
        ...
    except asyncio.CancelledError:
        with aiotools.ShieldScope():
            await cleanup()  # any async code here is not affected by multiple cancellation
            raise

async def parent():
    work_task = asyncio.create_task(work())
    ...
    await cancel_and_wait(work_task)

parent_task = asyncio.create_task(parent())
...
await cancel_and_wait(parent_task)  # it may trigger double cancellation, but it will return after the shielded block completes.

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)
    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 TaskScope

TaskScope is a variant of TaskGroup which ensures all child tasks run to completion (either having results or exceptions) unless the context manager is cancelled.

This could be considered as a safer version (in terms of lifecycle tracking) of asyncio.gather(*, return_exceptions=True) and a more convenient version of it because you can decouple the timing of task creation and their scheduling within the TaskScope context.

import aiotools

async def do():
    async with aiotools.TaskScope() as ts:
        ts.create_task(...)
        ts.create_task(...)
        # each task will run to completion regardless sibling failures.
        ...
    # at this point, all subtasks are either cancelled or done.

You may customize exception handler for each scope to receive and process unhandled exceptions in child tasks. For use in long-running server contexts, TaskScope does not store any exceptions or results by itself.

See also high-level coroutine utilities such as as_completed_safe(), gather_safe(), and race() functions in the utils module.

TaskScope itself and these utilities integrate with the call-graph inspection introduced in Python 3.14.

Async Timer

import aiotools

i = 0

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

async def somewhere():
    task = aiotools.create_timer(mytick, 1.0)
    ...
    await aiotools.cancel_and_wait(task)

The returned task is an asyncio.Task object. To stop the timer, it should be cancelled explicitly. Use cancel_and_wait() to ensure complete shutdown of any ongoing tick 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)
    ...
    await aiotools.cancel_and_wait(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