python_doc_md/Intermediate/library/asyncio-sync.md

::: currentmodule asyncio :::

Synchronization Primitives {#asyncio-sync}

Source code: Lib/asyncio/locks.py{.interpreted-text role="source"}

---

asyncio synchronization primitives are designed to be similar to those of the threading{.interpreted-text role="mod"} module with two important caveats:

• asyncio primitives are not thread-safe, therefore they should not be used for OS thread synchronization (use threading{.interpreted-text role="mod"} for that);
• methods of these synchronization primitives do not accept the timeout argument; use the asyncio.wait_for{.interpreted-text role="func"} function to perform operations with timeouts.

asyncio has the following basic synchronization primitives:

• Lock{.interpreted-text role="class"}
• Event{.interpreted-text role="class"}
• Condition{.interpreted-text role="class"}
• Semaphore{.interpreted-text role="class"}
• BoundedSemaphore{.interpreted-text role="class"}
• Barrier{.interpreted-text role="class"}

---

Lock

::::::: Lock() Implements a mutex lock for asyncio tasks. Not thread-safe.

An asyncio lock can be used to guarantee exclusive access to a shared resource.

The preferred way to use a Lock is an async with{.interpreted-text role="keyword"} statement:

lock = asyncio.Lock()

# ... later
async with lock:
    # access shared state

which is equivalent to:

lock = asyncio.Lock()

# ... later
await lock.acquire()
try:
    # access shared state
finally:
    lock.release()

::: versionchanged 3.10 Removed the loop parameter. :::

::: {.method async=""} acquire()

Acquire the lock.

This method waits until the lock is unlocked, sets it to locked and returns True.

When more than one coroutine is blocked in acquire{.interpreted-text role="meth"} waiting for the lock to be unlocked, only one coroutine eventually proceeds.

Acquiring a lock is fair: the coroutine that proceeds will be the first coroutine that started waiting on the lock. :::

::: method release()

Release the lock.

When the lock is locked, reset it to unlocked and return.

If the lock is unlocked, a RuntimeError{.interpreted-text role="exc"} is raised. :::

::: method locked()

Return True if the lock is locked. ::: :::::::

Event

::::::::: Event() An event object. Not thread-safe.

An asyncio event can be used to notify multiple asyncio tasks that some event has happened.

An Event object manages an internal flag that can be set to true with the ~Event.set{.interpreted-text role="meth"} method and reset to false with the clear{.interpreted-text role="meth"} method. The ~Event.wait{.interpreted-text role="meth"} method blocks until the flag is set to true. The flag is set to false initially.

::: versionchanged 3.10 Removed the loop parameter. :::

::: {#asyncio_example_sync_event} Example:

async def waiter(event):
    print('waiting for it ...')
    await event.wait()
    print('... got it!')

async def main():
    # Create an Event object.
    event = asyncio.Event()

    # Spawn a Task to wait until 'event' is set.
    waiter_task = asyncio.create_task(waiter(event))

    # Sleep for 1 second and set the event.
    await asyncio.sleep(1)
    event.set()

    # Wait until the waiter task is finished.
    await waiter_task

asyncio.run(main())

:::

::: {.method async=""} wait()

Wait until the event is set.

If the event is set, return True immediately. Otherwise block until another task calls ~Event.set{.interpreted-text role="meth"}. :::

::: method set()

Set the event.

All tasks waiting for event to be set will be immediately awakened. :::

::: method clear()

Clear (unset) the event.

Subsequent tasks awaiting on ~Event.wait{.interpreted-text role="meth"} will now block until the ~Event.set{.interpreted-text role="meth"} method is called again. :::

::: method is_set()

Return True if the event is set. ::: :::::::::

Condition

::::::::::: Condition(lock=None) A Condition object. Not thread-safe.

An asyncio condition primitive can be used by a task to wait for some event to happen and then get exclusive access to a shared resource.

In essence, a Condition object combines the functionality of an Event{.interpreted-text role="class"} and a Lock{.interpreted-text role="class"}. It is possible to have multiple Condition objects share one Lock, which allows coordinating exclusive access to a shared resource between different tasks interested in particular states of that shared resource.

The optional lock argument must be a Lock{.interpreted-text role="class"} object or None. In the latter case a new Lock object is created automatically.

::: versionchanged 3.10 Removed the loop parameter. :::

The preferred way to use a Condition is an async with{.interpreted-text role="keyword"} statement:

cond = asyncio.Condition()

# ... later
async with cond:
    await cond.wait()

which is equivalent to:

cond = asyncio.Condition()

# ... later
await cond.acquire()
try:
    await cond.wait()
finally:
    cond.release()

::: {.method async=""} acquire()

Acquire the underlying lock.

This method waits until the underlying lock is unlocked, sets it to locked and returns True. :::

::: method notify(n=1)

Wake up n tasks (1 by default) waiting on this condition. If fewer than n tasks are waiting they are all awakened.

The lock must be acquired before this method is called and released shortly after. If called with an unlocked lock a RuntimeError{.interpreted-text role="exc"} error is raised. :::

::: method locked()

Return True if the underlying lock is acquired. :::

::: method notify_all()

Wake up all tasks waiting on this condition.

This method acts like notify{.interpreted-text role="meth"}, but wakes up all waiting tasks.

The lock must be acquired before this method is called and released shortly after. If called with an unlocked lock a RuntimeError{.interpreted-text role="exc"} error is raised. :::

::: method release()

Release the underlying lock.

When invoked on an unlocked lock, a RuntimeError{.interpreted-text role="exc"} is raised. :::

::: {.method async=""} wait()

Wait until notified.

If the calling task has not acquired the lock when this method is called, a RuntimeError{.interpreted-text role="exc"} is raised.

This method releases the underlying lock, and then blocks until it is awakened by a notify{.interpreted-text role="meth"} or notify_all{.interpreted-text role="meth"} call. Once awakened, the Condition re-acquires its lock and this method returns True.

Note that a task may return from this call spuriously, which is why the caller should always re-check the state and be prepared to ~Condition.wait{.interpreted-text role="meth"} again. For this reason, you may prefer to use ~Condition.wait_for{.interpreted-text role="meth"} instead. :::

::: {.method async=""} wait_for(predicate)

Wait until a predicate becomes true.

The predicate must be a callable which result will be interpreted as a boolean value. The method will repeatedly ~Condition.wait{.interpreted-text role="meth"} until the predicate evaluates to true. The final value is the return value. ::: :::::::::::

Semaphore

::::::: Semaphore(value=1) A Semaphore object. Not thread-safe.

A semaphore manages an internal counter which is decremented by each acquire{.interpreted-text role="meth"} call and incremented by each release{.interpreted-text role="meth"} call. The counter can never go below zero; when acquire{.interpreted-text role="meth"} finds that it is zero, it blocks, waiting until some task calls release{.interpreted-text role="meth"}.

The optional value argument gives the initial value for the internal counter (1 by default). If the given value is less than 0 a ValueError{.interpreted-text role="exc"} is raised.

::: versionchanged 3.10 Removed the loop parameter. :::

The preferred way to use a Semaphore is an async with{.interpreted-text role="keyword"} statement:

sem = asyncio.Semaphore(10)

# ... later
async with sem:
    # work with shared resource

which is equivalent to:

sem = asyncio.Semaphore(10)

# ... later
await sem.acquire()
try:
    # work with shared resource
finally:
    sem.release()

::: {.method async=""} acquire()

Acquire a semaphore.

If the internal counter is greater than zero, decrement it by one and return True immediately. If it is zero, wait until a release{.interpreted-text role="meth"} is called and return True. :::

::: method locked()

Returns True if semaphore can not be acquired immediately. :::

::: method release()

Release a semaphore, incrementing the internal counter by one. Can wake up a task waiting to acquire the semaphore.

Unlike BoundedSemaphore{.interpreted-text role="class"}, Semaphore{.interpreted-text role="class"} allows making more release() calls than acquire() calls. ::: :::::::

BoundedSemaphore

:::: BoundedSemaphore(value=1) A bounded semaphore object. Not thread-safe.

Bounded Semaphore is a version of Semaphore{.interpreted-text role="class"} that raises a ValueError{.interpreted-text role="exc"} in ~Semaphore.release{.interpreted-text role="meth"} if it increases the internal counter above the initial value.

::: versionchanged 3.10 Removed the loop parameter. ::: ::::

Barrier

::::::::::: Barrier(parties) A barrier object. Not thread-safe.

A barrier is a simple synchronization primitive that allows to block until parties number of tasks are waiting on it. Tasks can wait on the ~Barrier.wait{.interpreted-text role="meth"} method and would be blocked until the specified number of tasks end up waiting on ~Barrier.wait{.interpreted-text role="meth"}. At that point all of the waiting tasks would unblock simultaneously.

async with{.interpreted-text role="keyword"} can be used as an alternative to awaiting on ~Barrier.wait{.interpreted-text role="meth"}.

The barrier can be reused any number of times.

::: {#asyncio_example_barrier} Example:

async def example_barrier():
   # barrier with 3 parties
   b = asyncio.Barrier(3)

   # create 2 new waiting tasks
   asyncio.create_task(b.wait())
   asyncio.create_task(b.wait())

   await asyncio.sleep(0)
   print(b)

   # The third .wait() call passes the barrier
   await b.wait()
   print(b)
   print("barrier passed")

   await asyncio.sleep(0)
   print(b)

asyncio.run(example_barrier())

:::

Result of this example is:

<asyncio.locks.Barrier object at 0x... [filling, waiters:2/3]>
<asyncio.locks.Barrier object at 0x... [draining, waiters:0/3]>
barrier passed
<asyncio.locks.Barrier object at 0x... [filling, waiters:0/3]>

::: versionadded 3.11 :::

::: {.method async=""} wait()

Pass the barrier. When all the tasks party to the barrier have called this function, they are all unblocked simultaneously.

When a waiting or blocked task in the barrier is cancelled, this task exits the barrier which stays in the same state. If the state of the barrier is "filling", the number of waiting task decreases by 1.

The return value is an integer in the range of 0 to parties-1, different for each task. This can be used to select a task to do some special housekeeping, e.g.:

...
async with barrier as position:
   if position == 0:
      # Only one task prints this
      print('End of *draining phase*')

This method may raise a BrokenBarrierError{.interpreted-text role="class"} exception if the barrier is broken or reset while a task is waiting. It could raise a CancelledError{.interpreted-text role="exc"} if a task is cancelled. :::

::: {.method async=""} reset()

Return the barrier to the default, empty state. Any tasks waiting on it will receive the BrokenBarrierError{.interpreted-text role="class"} exception.

If a barrier is broken it may be better to just leave it and create a new one. :::

::: {.method async=""} abort()

Put the barrier into a broken state. This causes any active or future calls to ~Barrier.wait{.interpreted-text role="meth"} to fail with the BrokenBarrierError{.interpreted-text role="class"}. Use this for example if one of the tasks needs to abort, to avoid infinite waiting tasks. :::

::: attribute parties

The number of tasks required to pass the barrier. :::

::: attribute n_waiting

The number of tasks currently waiting in the barrier while filling. :::

::: attribute broken

A boolean that is True if the barrier is in the broken state. ::: :::::::::::

::: exception BrokenBarrierError

This exception, a subclass of RuntimeError{.interpreted-text role="exc"}, is raised when the Barrier{.interpreted-text role="class"} object is reset or broken. :::

---

::: versionchanged 3.9

Acquiring a lock using await lock or yield from lock and/or with{.interpreted-text role="keyword"} statement (with await lock, with (yield from lock)) was removed. Use async with lock instead. :::