18.5.9. Synchronization primitives

Locks:

Queues:

asyncio locks and queues API were designed to be close to classes of the threading module (Lock, Event, Condition, Semaphore, BoundedSemaphore) and the queue module (Queue, PriorityQueue, LifoQueue), but they have no timeout parameter. The asyncio.wait_for() function can be used to cancel a task after a timeout.

18.5.9.1. Locks

18.5.9.1.1. Lock

class asyncio.Lock(*, loop=None)

Primitive lock objects.

A primitive lock is a synchronization primitive that is not owned by a particular coroutine when locked. A primitive lock is in one of two states, ‘locked’ or ‘unlocked’.

It is created in the unlocked state. It has two basic methods, acquire() and release(). When the state is unlocked, acquire() changes the state to locked and returns immediately. When the state is locked, acquire() blocks until a call to release() in another coroutine changes it to unlocked, then the acquire() call resets it to locked and returns. The release() method should only be called in the locked state; it changes the state to unlocked and returns immediately. If an attempt is made to release an unlocked lock, a RuntimeError will be raised.

When more than one coroutine is blocked in acquire() waiting for the state to turn to unlocked, only one coroutine proceeds when a release() call resets the state to unlocked; first coroutine which is blocked in acquire() is being processed.

acquire() is a coroutine and should be called with yield from.

Locks also support the context management protocol. (yield from lock) should be used as context manager expression.

Usage:

lock = Lock()
...
yield from lock
try:
    ...
finally:
    lock.release()

Context manager usage:

lock = Lock()
...
with (yield from lock):
     ...

Lock objects can be tested for locking state:

if not lock.locked():
   yield from lock
else:
   # lock is acquired
    ...
locked()

Return True if the lock is acquired.

acquire()

Acquire a lock.

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

This method is a coroutine.

release()

Release a lock.

When the lock is locked, reset it to unlocked, and return. If any other coroutines are blocked waiting for the lock to become unlocked, allow exactly one of them to proceed.

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

There is no return value.

18.5.9.1.2. Event

class asyncio.Event(*, loop=None)

An Event implementation, asynchronous equivalent to threading.Event.

Class implementing event objects. An event manages a 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 true. The flag is initially false.

clear()

Reset the internal flag to false. Subsequently, coroutines calling wait() will block until set() is called to set the internal flag to true again.

is_set()

Return True if and only if the internal flag is true.

set()

Set the internal flag to true. All coroutines waiting for it to become true are awakened. Coroutine that call wait() once the flag is true will not block at all.

wait()

Block until the internal flag is true.

If the internal flag is true on entry, return True immediately. Otherwise, block until another coroutine calls set() to set the flag to true, then return True.

This method is a coroutine.

18.5.9.1.3. Condition

class asyncio.Condition(lock=None, *, loop=None)

A Condition implementation, asynchronous equivalent to threading.Condition.

This class implements condition variable objects. A condition variable allows one or more coroutines to wait until they are notified by another coroutine.

If the lock argument is given and not None, it must be a Lock object, and it is used as the underlying lock. Otherwise, a new Lock object is created and used as the underlying lock.

acquire()

Acquire the underlying lock.

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

This method is a coroutine.

notify(n=1)

By default, wake up one coroutine waiting on this condition, if any. If the calling coroutine has not acquired the lock when this method is called, a RuntimeError is raised.

This method wakes up at most n of the coroutines waiting for the condition variable; it is a no-op if no coroutines are waiting.

Note

An awakened coroutine does not actually return from its wait() call until it can reacquire the lock. Since notify() does not release the lock, its caller should.

locked()

Return True if the underlying lock is acquired.

notify_all()

Wake up all threads waiting on this condition. This method acts like notify(), but wakes up all waiting threads instead of one. If the calling thread has not acquired the lock when this method is called, a RuntimeError is raised.

release()

Release the underlying lock.

When the lock is locked, reset it to unlocked, and return. If any other coroutines are blocked waiting for the lock to become unlocked, allow exactly one of them to proceed.

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

There is no return value.

wait()

Wait until notified.

If the calling coroutine 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 for the same condition variable in another coroutine. Once awakened, it re-acquires the lock and returns True.

This method is a coroutine.

wait_for(predicate)

Wait until a predicate becomes true.

The predicate should be a callable which result will be interpreted as a boolean value. The final predicate value is the return value.

This method is a coroutine.

18.5.9.2. Semaphores

18.5.9.2.1. Semaphore

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

A Semaphore implementation.

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 other thread calls release().

Semaphores also support the context management protocol.

The optional argument gives the initial value for the internal counter; it defaults to 1. If the value given is less than 0, ValueError is raised.

acquire()

Acquire a semaphore.

If the internal counter is larger than zero on entry, decrement it by one and return True immediately. If it is zero on entry, block, waiting until some other coroutine has called release() to make it larger than 0, and then return True.

This method is a coroutine.

locked()

Returns True if semaphore can not be acquired immediately.

release()

Release a semaphore, incrementing the internal counter by one. When it was zero on entry and another coroutine is waiting for it to become larger than zero again, wake up that coroutine.

18.5.9.2.2. BoundedSemaphore

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

A bounded semaphore implementation. Inherit from Semaphore.

This raises ValueError in release() if it would increase the value above the initial value.

18.5.9.3. Queues

18.5.9.3.1. Queue

class asyncio.Queue(maxsize=0, *, loop=None)

A queue, useful for coordinating producer and consumer coroutines.

If maxsize is less than or equal to zero, the queue size is infinite. If it is an integer greater than 0, then yield from put() will block when the queue reaches maxsize, until an item is removed by get().

Unlike the standard library queue, you can reliably know this Queue’s size with qsize(), since your single-threaded asyncio application won’t be interrupted between calling qsize() and doing an operation on the Queue.

empty()

Return True if the queue is empty, False otherwise.

full()

Return True if there are maxsize items in the queue.

Note

If the Queue was initialized with maxsize=0 (the default), then full() is never True.

get()

Remove and return an item from the queue. If queue is empty, wait until an item is available.

This method is a coroutine.

See also

The empty() method.

get_nowait()

Remove and return an item from the queue.

Return an item if one is immediately available, else raise QueueEmpty.

put(item)

Put an item into the queue. If the queue is full, wait until a free slot is available before adding item.

This method is a coroutine.

See also

The full() method.

put_nowait(item)

Put an item into the queue without blocking.

If no free slot is immediately available, raise QueueFull.

qsize()

Number of items in the queue.

maxsize

Number of items allowed in the queue.

18.5.9.3.2. PriorityQueue

class asyncio.PriorityQueue

A subclass of Queue; retrieves entries in priority order (lowest first).

Entries are typically tuples of the form: (priority number, data).

18.5.9.3.3. LifoQueue

class asyncio.LifoQueue

A subclass of Queue that retrieves most recently added entries first.

18.5.9.3.4. JoinableQueue

class asyncio.JoinableQueue

A subclass of Queue with task_done() and join() methods.

join()

Block until all items in the queue have been gotten and processed.

The count of unfinished tasks goes up whenever an item is added to the queue. The count goes down whenever a consumer thread calls task_done() to indicate that the item was retrieved and all work on it is complete. When the count of unfinished tasks drops to zero, join() unblocks.

This method is a coroutine.

task_done()

Indicate that a formerly enqueued task is complete.

Used by queue consumers. For each get() used to fetch a task, a subsequent call to task_done() tells the queue that the processing on the task is complete.

If a join() is currently blocking, it will resume when all items have been processed (meaning that a task_done() call was received for every item that had been put() into the queue).

Raises ValueError if called more times than there were items placed in the queue.

18.5.9.3.5. Exceptions

exception asyncio.QueueEmpty

Exception raised when the get_nowait() method is called on a Queue object which is empty.

exception asyncio.QueueFull

Exception raised when the put_nowait() method is called on a Queue object which is full.