Using synchronization primitives

Synchronization primitives are objects that are used by tasks to communicate and coordinate with each other. They are useful for things like distributing workload, notifying other tasks and guarding access to shared resources.

Semaphores

Semaphores are used for limiting access to a shared resource. A semaphore starts with a maximum value, which is decremented each time the semaphore is acquired by a task and incremented when it is released. If the value drops to zero, any attempt to acquire the semaphore will block until another task frees it.

Example:

from anyio import create_task_group, create_semaphore, sleep, run


async def use_resource(tasknum, semaphore):
    async with semaphore:
        print('Task number', tasknum, 'is now working with the shared resource')
        await sleep(1)


async def main():
    semaphore = create_semaphore(2)
    async with create_task_group() as tg:
        for num in range(10):
            await tg.spawn(use_resource, num, semaphore)

run(main)

Locks

Locks are used to guard shared resources to ensure sole access to a single task at once. They function much like semaphores with a maximum value of 1.

Example:

from anyio import create_task_group, create_lock, sleep, run


async def use_resource(tasknum, lock):
    async with lock:
        print('Task number', tasknum, 'is now working with the shared resource')
        await sleep(1)


async def main():
    lock = create_lock()
    async with create_task_group() as tg:
        for num in range(4):
            await tg.spawn(use_resource, num, lock)

run(main)

Events

Events are used to notify tasks that something they’ve been waiting to happen has happened. An event object can have multiple listeners and they are all notified when the event is triggered.

Example:

from anyio import create_task_group, create_event, run


async def notify(event):
    await event.set()


async def main():
    event = create_event()
    async with create_task_group() as tg:
        await tg.spawn(notify, event)
        await event.wait()
        print('Received notification!')

run(main)

Note

Unlike standard library Events, AnyIO events cannot be reused, and must be replaced instead. This practice prevents a class of race conditions, and matches the semantics of the trio library.

Conditions

A condition is basically a combination of an event and a lock. It first acquires a lock and then waits for a notification from the event. Once the condition receives a notification, it releases the lock. The notifying task can also choose to wake up more than one listener at once, or even all of them.

Example:

from anyio import create_task_group, create_condition, sleep, run


async def listen(tasknum, condition):
    async with condition:
        await condition.wait()
        print('Woke up task number', tasknum)


async def main():
    condition = create_condition()
    async with create_task_group() as tg:
        for tasknum in range(6):
            await tg.spawn(listen, tasknum, condition)

        await sleep(1)
        async with condition:
            await condition.notify(1)

        await sleep(1)
        async with condition:
            await condition.notify(2)

        await sleep(1)
        async with condition:
            await condition.notify_all()

run(main)

Capacity limiters

Capacity limiters are like semaphores except that a single borrower (the current task by default) can only hold a single token at a time. It is also possible to borrow a token on behalf of any arbitrary object, so long as that object is hashable.

Example:

from anyio import create_task_group, create_capacity_limiter, sleep, run


async def use_resource(tasknum, limiter):
    async with limiter:
        print('Task number', tasknum, 'is now working with the shared resource')
        await sleep(1)


async def main():
    limiter = create_capacity_limiter(2)
    async with create_task_group() as tg:
        for num in range(10):
            await tg.spawn(use_resource, num, limiter)

run(main)

To adjust the number of total tokens, you can use the set_total_tokens() method.