Creating and managing tasks

A task is a unit of execution that lets you do many things concurrently that need waiting on. This works so that while you can have any number of tasks, the asynchronous event loop can only run one of them at a time. When the task encounters an await statement that requires the task to sleep until something happens, the event loop is then free to work on another task. When the thing the first task was waiting is complete, the event loop will resume the execution of that task on the first opportunity it gets.

Task handling in AnyIO loosely follows the Trio model. Tasks can be created (spawned) using task groups. A task group is an asynchronous context manager that makes sure that all its child tasks are finished one way or another after the context block is exited. If a child task, or the code in the enclosed context block raises an exception, all child tasks are cancelled. Otherwise the context manager just waits until all child tasks have exited before proceeding.

Here’s a demonstration:

from anyio import sleep, create_task_group, run


async def sometask(num: int) -> None:
    print('Task', num, 'running')
    await sleep(1)
    print('Task', num, 'finished')


async def main() -> None:
    async with create_task_group() as tg:
        for num in range(5):
            tg.start_soon(sometask, num)

    print('All tasks finished!')

run(main)

Working with task handles

As an alternative way to spawn tasks, you can use the create_task() method which will return a TaskHandle. This handle can be used to:

  1. Wait for the task to finish before exiting the task group

  2. Retrieve the task’s return value or exception

  3. Cancel the task

  4. Check the task’s status

from anyio import TaskHandle, create_task_group, run, sleep


async def sometask(num: int) -> str:
    await sleep(1)
    return str(num)


async def main() -> None:
    async with create_task_group() as tg:
        handles = [
            tg.create_task(sometask(num)) for num in range(5)
        ]
        handles[1].cancel()
        print(await handles[2])

    print(
        'Tasks finished:',
        ', '.join(
            handle.return_value for handle in handles
            if handle.status is TaskHandle.Status.FINISHED
        )
    )
    # Should print:
    #   2
    #   Tasks finished: 0, 2, 3, 4

run(main)

Starting and initializing tasks

Sometimes it is very useful to be able to wait until a task has successfully initialized itself. For example, when starting network services, you can have your task start the listener and then signal the caller that initialization is done. That way, the caller can now start another task that depends on that service being up and running. Also, if the socket bind fails or something else goes wrong during initialization, the exception will be propagated to the caller which can then catch and handle it.

This can be done with TaskGroup.start():

from anyio import (
    TASK_STATUS_IGNORED,
    create_task_group,
    connect_tcp,
    create_tcp_listener,
    run,
)
from anyio.abc import TaskStatus


async def handler(stream):
    ...


async def start_some_service(
    port: int, *, task_status: TaskStatus[None] = TASK_STATUS_IGNORED
):
    async with await create_tcp_listener(
        local_host="127.0.0.1", local_port=port
    ) as listener:
        task_status.started()
        await listener.serve(handler)


async def main():
    async with create_task_group() as tg:
        await tg.start(start_some_service, 5000)
        async with await connect_tcp("127.0.0.1", 5000) as stream:
            ...


run(main)

The target coroutine function must call task_status.started() because the task that is calling with TaskGroup.start() will be blocked until then. If the spawned task never calls it, then the TaskGroup.start() call will raise a RuntimeError.

Note

Unlike start_soon(), start() needs an await.

Handling multiple errors in a task group

It is possible for more than one task to raise an exception in a task group. This can happen when a task reacts to cancellation by entering either an exception handler block or a finally: block and raises an exception there. This raises the question: which exception is propagated from the task group context manager? The answer is “both”. In practice this means that a special exception, ExceptionGroup (or BaseExceptionGroup) is raised which contains both exception objects.

To catch such exceptions potentially nested in groups, special measures are required. On Python 3.11 and later, you can use the except* syntax to catch multiple exceptions:

from anyio import create_task_group

try:
    async with create_task_group() as tg:
        tg.start_soon(some_task)
        tg.start_soon(another_task)
except* ValueError as excgroup:
    for exc in excgroup.exceptions:
        ...  # handle each ValueError
except* KeyError as excgroup:
    for exc in excgroup.exceptions:
        ...  # handle each KeyError

If compatibility with older Python versions is required, you can use the catch() function from the exceptiongroup package:

from anyio import create_task_group
from exceptiongroup import catch

def handle_valueerror(excgroup: ExceptionGroup) -> None:
    for exc in excgroup.exceptions:
        ...  # handle each ValueError

def handle_keyerror(excgroup: ExceptionGroup) -> None:
    for exc in excgroup.exceptions:
        ...  # handle each KeyError

with catch({
    ValueError: handle_valueerror,
    KeyError: handle_keyerror
}):
    async with create_task_group() as tg:
        tg.start_soon(some_task)
        tg.start_soon(another_task)

If you need to set local variables in the handlers, declare them as nonlocal:

async def yourfunc():
    somevariable: str | None = None

    def handle_valueerror(exc):
        nonlocal somevariable
        somevariable = 'whatever'

    with catch({
        ValueError: handle_valueerror,
        KeyError: handle_keyerror
    }):
        async with create_task_group() as tg:
            tg.start_soon(some_task)
            tg.start_soon(another_task)

    print(f"{somevariable=}")

Context propagation

Whenever a new task is spawned, context will be copied to the new task. It is important to note which context will be copied to the newly spawned task. It is not the context of the task group’s host task that will be copied, but the context of the task that calls TaskGroup.start() or TaskGroup.start_soon().

Differences with asyncio.TaskGroup

The asyncio.TaskGroup class, added in Python 3.11, is very similar in design to the AnyIO TaskGroup class. The asyncio counterpart has some important differences in its semantics, however:

  • The task group itself is instantiated directly, rather than using a factory function

  • Tasks are spawned solely through create_task(); there is no start() or start_soon() method

  • The create_task() method returns a task object which can be awaited on (or cancelled)

  • When a task spawned via create_task() is cancelled before its coroutine has started running, it will not get a chance to handle the cancellation exception

  • asyncio.TaskGroup does not allow starting new tasks after an exception in one of the tasks has triggered a shutdown of the task group

  • Tasks spawned from asyncio.TaskGroup use different cancellation semantics (see the notes on asyncio cancellation semantics)

Asyncio call graph introspection support

Added in version 4.12.0.

Python 3.14 added support for call graph introspection on asyncio which lets various tools display the tree of Tasks or Futures that are waiting for a specific Future. AnyIO supports this mechanism in its own task groups by adding and removing the task’s “waiter” as appropriate, just like asyncio’s own task group class does. Here’s a small demonstration:

import asyncio

from anyio import create_task_group


async def foo(level=0):
    if level == 3:
        asyncio.print_call_graph(asyncio.current_task())
        return

    async with create_task_group() as tg:
        tg.start_soon(foo, level + 1, name=f"Nesting level {level + 1}")

asyncio.run(foo())

This results in an output like:

* Task(name='Nesting level 3', id=0x7f94f01c43f0)
  + Call stack:
  |   File '/usr/lib64/python3.14/asyncio/graph.py', line 276, in print_call_graph()
  |   File '/tmp/foo.py', line 8, in async foo()
  + Awaited by:
    * Task(name='Nesting level 2', id=0x7f94f03334e0)
      + Call stack:
      |   File '/tmp/venv/lib64/python3.14/site-packages/anyio/_backends/_asyncio.py', line 755, in async TaskGroup.__aexit__()
      |   File '/tmp/foo.py', line 11, in async foo()
      + Awaited by:
        * Task(name='Nesting level 1', id=0x7f94f03172f0)
          + Call stack:
          |   File '/tmp/venv/lib64/python3.14/site-packages/anyio/_backends/_asyncio.py', line 755, in async TaskGroup.__aexit__()
          |   File '/tmp/foo.py', line 11, in async foo()
          + Awaited by:
            * Task(name='Task-1', id=0x7f94f0316f30)
              + Call stack:
              |   File '/tmp/venv/lib64/python3.14/site-packages/anyio/_backends/_asyncio.py', line 755, in async TaskGroup.__aexit__()
              |   File '/tmp/foo.py', line 11, in async foo()