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Task control for asyncio.

Project description

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quattro is an Apache 2 licensed library, written in Python, for task control in asyncio applications. quattro is influenced by structured concurrency concepts from the Trio framework.

quattro supports Python versions 3.8 - 3.10, and the 3.8 PyPy beta.

Installation

To install quattro, simply:

$ pip install quattro

Task Groups

quattro contains a TaskGroup implementation. TaskGroups are inspired by Trio nurseries.

from quattro import TaskGroup

async def my_handler():
    # We want to spawn some tasks, and ensure they are all handled before we return.
    async def task_1():
        ...

    async def task_2():
        ...

    async with TaskGroup() as tg:
        tg.start_soon(task_1)
        tg.start_soon(task_2)

    # The end of the `async with` block awaits the tasks, ensuring they are handled.

TaskGroups are essential building blocks for achieving the concept of structured concurrency. In simple terms, structured concurrency means your code does not leak tasks - when a coroutine finishes, all tasks spawned by that coroutine and all its children are also finished. (In fancy terms, the execution flow becomes a directed acyclic graph.)

Structured concurrency can be achieved by using TaskGroups instead of asyncio.create_task to start background tasks. TaskGroups essentially do two things:

  • when exiting from a TaskGroup async with block, the TaskGroup awaits all of its children, ensuring they are finished when it exits
  • when a TaskGroup child task raises an exception, all other children and the task inside the context manager are cancelled

The implementation has been borrowed from the EdgeDB project.

Cancel Scopes

Cancel scopes are not supported on Python 3.8, since the necessary underlying asyncio machinery is not present on that version.

quattro contains an asyncio implementation of Trio CancelScopes. Due to fundamental differences between asyncio and Trio the actual runtime behavior isn’t exactly the same, but close.

from quattro import move_on_after

async def my_handler():
    with move_on_after(1.0) as cancel_scope:
        await long_query()

    # 1 second later, the function continues running

quattro contains the following helpers:

  • move_on_after
  • move_on_at
  • fail_after
  • fail_at

All helpers produce instances of quattro.CancelScope, which is largely similar to the Trio variant.

CancelScopes have the following attributes:

  • cancel() - a method through which the scope can be cancelled manually
  • deadline - read/write, an optional deadline for the scope, at which the scope will be cancelled
  • cancelled_caught - a readonly bool property, whether the scope finished via cancellation

quattro also supports retrieving the current effective deadline in a task using quattro.current_effective_deadline. The current effective deadline is a float value, with float('inf') standing in for no deadline.

asyncio and Trio differences

fail_after and fail_at raise asyncio.Timeout instead of trio.Cancelled exceptions when they fail.

asyncio has edge-triggered cancellation semantics, while Trio has level-triggered cancellation semantics. The following example will behave differently in quattro and Trio:

with trio.move_on_after(TIMEOUT):
    conn = make_connection()
    try:
        await conn.send_hello_msg()
    finally:
        await conn.send_goodbye_msg()

In Trio, if the TIMEOUT expires while awaiting send_hello_msg(), send_goodbye_msg() will also be cancelled. In quattro, send_goodbye_msg() will run (and potentially block) anyway. This is a limitation of the underlying framework.

In quattro, cancellation scopes cannot be shielded.

Changelog

0.3.0 (2022-01-08)

  • Add py.typed to enable typing information.
  • Flesh out type annotations for TaskGroups.

0.2.0 (2021-12-27)

  • Add quattro.current_effective_deadline.

0.1.0 (2021-12-08)

  • Initial release, containing task groups and cancellation scopes.

Credits

The initial TaskGroup implementation has been taken from the EdgeDB project. The CancelScope implementation was heavily influenced by Trio, and inspired by the async_timeout package.

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