18.5.7. Synchronization primitives — Python documentation
18.5.7. Synchronization primitives
Source code: :source:`Lib/asyncio/locks.py`
Locks:
Semaphores:
asyncio lock API was designed to be close to classes of the threading module (Lock, Event, Condition, Semaphore, BoundedSemaphore), but it has no timeout parameter. The asyncio.wait_for()
function can be used to cancel a task after a timeout.
18.5.7.1. Locks
18.5.7.1.1. 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 withyield from
.Locks also support the context management protocol.
(yield from lock)
should be used as the context manager expression.This class is not thread safe.
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.
- 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.
18.5.7.1.2. 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.This class is not thread safe.
- 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.
18.5.7.1.3. 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.This class is not thread safe.
- 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 coroutines waiting on this condition. This method acts like notify(), but wakes up all waiting coroutines instead of one. If the calling coroutine 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.
18.5.7.2. Semaphores
18.5.7.2.1. 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; whenacquire()
finds that it is zero, it blocks, waiting until some other coroutine 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 than0
, ValueError is raised.This class is not thread safe.
- 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.
18.5.7.2.2. 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.