.. currentmodule:: asyncio

.. _asyncio-sync:

========================== Synchronization Primitives

Source code: :source:Lib/asyncio/locks.py

asyncio synchronization primitives are designed to be similar to those of the :mod:threading module with two important caveats:

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

asyncio has the following basic synchronization primitives:

:class:Lock
:class:Event
:class:Condition
:class:Semaphore
:class:BoundedSemaphore
:class:Barrier

Lock

.. class:: 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 :keyword:async with 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:: acquire() :async:

  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 :meth:`acquire`
  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 :exc:`RuntimeError` is raised.

.. method:: locked()

  Return ``True`` if the lock is *locked*.

Event

.. class:: 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 :meth:~Event.set method and reset to false with the :meth:clear method. The :meth:~Event.wait 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:: wait() :async:

  Wait until the event is set.

  If the event is set, return ``True`` immediately.
  Otherwise block until another task calls :meth:`~Event.set`.

.. 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 :meth:`~Event.wait` will now block until the
  :meth:`~Event.set` method is called again.

.. method:: is_set()

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

Condition

.. class:: 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 :class:Event and a :class:Lock. 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 :class:Lock 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 :keyword:async with 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:: acquire() :async:

  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 :exc:`RuntimeError` 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 :meth:`notify`, 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 :exc:`RuntimeError` error is raised.

.. method:: release()

  Release the underlying lock.

  When invoked on an unlocked lock, a :exc:`RuntimeError` is
  raised.

.. method:: wait() :async:

  Wait until notified.

  If the calling task has not acquired the lock when this method is
  called, a :exc:`RuntimeError` is raised.

  This method releases the underlying lock, and then blocks until
  it is awakened by a :meth:`notify` or :meth:`notify_all` 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 :meth:`~Condition.wait` again. For this reason, you may
  prefer to use :meth:`~Condition.wait_for` instead.

.. method:: wait_for(predicate) :async:

  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
  :meth:`~Condition.wait` until the predicate evaluates to *true*. The final value is the
  return value.

Semaphore

.. class:: Semaphore(value=1)

A Semaphore object. Not thread-safe.

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

The optional value argument gives the initial value for the internal counter (1 by default). If the given value is less than 0 a :exc:ValueError is raised.

.. versionchanged:: 3.10 Removed the loop parameter.

The preferred way to use a Semaphore is an :keyword:async with 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:: acquire() :async:

  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 :meth:`release` 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 :class:`BoundedSemaphore`, :class:`Semaphore` allows
  making more ``release()`` calls than ``acquire()`` calls.

BoundedSemaphore

.. class:: BoundedSemaphore(value=1)

A bounded semaphore object. Not thread-safe.

Bounded Semaphore is a version of :class:Semaphore that raises a :exc:ValueError in :meth:~Semaphore.release if it increases the internal counter above the initial value.

.. versionchanged:: 3.10 Removed the loop parameter.

Barrier

.. class:: 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 :meth:~Barrier.wait method and would be blocked until the specified number of tasks end up waiting on :meth:~Barrier.wait. At that point all of the waiting tasks would unblock simultaneously.

:keyword:async with can be used as an alternative to awaiting on :meth:~Barrier.wait.

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:: wait() :async:

  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 :class:`BrokenBarrierError` exception if the
  barrier is broken or reset while a task is waiting.
  It could raise a :exc:`CancelledError` if a task is cancelled.

.. method:: reset() :async:

  Return the barrier to the default, empty state.  Any tasks waiting on it
  will receive the :class:`BrokenBarrierError` exception.

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

.. method:: abort() :async:

  Put the barrier into a broken state.  This causes any active or future
  calls to :meth:`~Barrier.wait` to fail with the :class:`BrokenBarrierError`.
  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 :exc:RuntimeError, is raised when the :class:Barrier object is reset or broken.

.. versionchanged:: 3.9

Acquiring a lock using await lock or yield from lock and/or :keyword:with statement (with await lock, with (yield from lock)) was removed. Use async with lock instead.