# Synchronization Primitives

Source code: Lib/asyncio/locks.py

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

- asyncio primitives are not thread-safe, therefore they should not be used for OS thread synchronization (use `threading` for that);

- methods of these synchronization primitives do not accept the timeout argument; use the `asyncio.wait_for()` function to perform operations with timeouts.

asyncio has the following basic synchronization primitives:

- `Lock`

- `Event`

- `Condition`

- `Semaphore`

- `BoundedSemaphore`

- `Barrier`

## Lock

class asyncio.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` 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()

  Changed in version 3.10: Removed the loop parameter.

  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()` 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.

  release()

    Release the lock.

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

    If the lock is unlocked, a `RuntimeError` is raised.

  locked()

    Return `True` if the lock is locked.

## Event

class asyncio.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 `set()` method and reset to false with the `clear()` method. The `wait()` method blocks until the flag is set to true. The flag is set to false initially.

  Changed in version 3.10: Removed the loop parameter.

  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())

  async wait()

    Wait until the event is set.

    If the event is set, return `True` immediately. Otherwise block until another task calls `set()`.

  set()

    Set the event.

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

  clear()

    Clear (unset) the event.

    Tasks awaiting on `wait()` will now block until the `set()` method is called again.

  is_set()

    Return `True` if the event is set.

## Condition

class asyncio.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` and a `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 `Lock` object or `None`. In the latter case a new Lock object is created automatically.

  Changed in version 3.10: Removed the loop parameter.

  The preferred way to use a Condition is an `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()

  async acquire()

    Acquire the underlying lock.

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

  notify(n=1)

    Wake up at most n tasks (1 by default) waiting on this condition. The method is no-op if no tasks are waiting.

    The lock must be acquired before this method is called and released shortly after. If called with an unlocked lock a `RuntimeError` error is raised.

  locked()

    Return `True` if the underlying lock is acquired.

  notify_all()

    Wake up all tasks waiting on this condition.

    This method acts like `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 `RuntimeError` error is raised.

  release()

    Release the underlying lock.

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

  async wait()

    Wait until notified.

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

    This method releases the underlying lock, and then blocks until it is awakened by a `notify()` or `notify_all()` call. Once awakened, the Condition re-acquires its lock and this method returns `True`.

  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 `wait()` until the predicate evaluates to true. The final value is the return value.

## Semaphore

class asyncio.Semaphore(value=1)

  A Semaphore object. Not thread-safe.

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

  Changed in version 3.10: Removed the loop parameter.

  The preferred way to use a Semaphore is an `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()

  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()` is called and return `True`.

  locked()

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

  release()

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

    Unlike `BoundedSemaphore`, `Semaphore` allows making more `release()` calls than `acquire()` calls.

## BoundedSemaphore

class asyncio.BoundedSemaphore(value=1)

  A bounded semaphore object. Not thread-safe.

  Bounded Semaphore is a version of `Semaphore` that raises a `ValueError` in `release()` if it increases the internal counter above the initial value.

  Changed in version 3.10: Removed the loop parameter.

## Barrier

class asyncio.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 `wait()` method and would be blocked until the specified number of tasks end up waiting on `wait()`. At that point all of the waiting tasks would unblock simultaneously.

  `async with` can be used as an alternative to awaiting on `wait()`.

  The barrier can be reused any number of times.

  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]>

  Added in version 3.11.

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

  async reset()

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

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

  async abort()

    Put the barrier into a broken state. This causes any active or future calls to `wait()` to fail with the `BrokenBarrierError`. Use this for example if one of the tasks needs to abort, to avoid infinite waiting tasks.

  parties

    The number of tasks required to pass the barrier.

  n_waiting

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

  broken

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

exception asyncio.BrokenBarrierError

  This exception, a subclass of `RuntimeError`, is raised when the `Barrier` object is reset or broken.

Changed in version 3.9: Acquiring a lock using `await lock` or `yield from lock` and/or `with` statement (`with await lock`, `with (yield from lock)`) was removed. Use `async with lock` instead.