Synchronization Primitives — Python 3.7.2 documentation

# Synchronization Primitives

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 sychronization primitives:

- `Lock`
- `Event`
- `Condition`
- `Semaphore`
- `BoundedSemaphore`

## Lock

class `asyncio.` `Lock`(*, loop=None)

  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()

  coroutine `acquire`()

    Acquire the lock.

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

  `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`(*, loop=None)

  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.

  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())

  coroutine `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, *, loop=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.

  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 lock.acquire()
      try:
          await cond.wait()
      finally:
          lock.release()

  coroutine `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.

  coroutine `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`.

  coroutine `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 final value is the return value.

## Semaphore

class `asyncio.` `Semaphore`(value=1, *, loop=None)

  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.

  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()

  coroutine `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, *, loop=None)

  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.

Deprecated since version 3.7: Acquiring a lock using `await lock` or `yield from lock` and/or `with` statement (`with await lock`, `with (yield from lock)`) is deprecated. Use `async with lock` instead.