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¶
- 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()
Zmienione w wersji 3.10: Removed the loop parameter.
- coroutine 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.
Zdarzenie¶
- 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 theclear()
method. Thewait()
method blocks until the flag is set to true. The flag is set to false initially.Zmienione w wersji 3.10: Removed the loop parameter.
Przykład:
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 callsset()
.
- 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 theset()
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 aLock
. 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 orNone
. In the latter case a new Lock object is created automatically.Zmienione w wersji 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()
- 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 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
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()
ornotify_all()
call. Once awakened, the Condition re-acquires its lock and this method returnsTrue
.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
wait()
again. For this reason, you may prefer to usewait_for()
instead.
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 eachrelease()
call. The counter can never go below zero; whenacquire()
finds that it is zero, it blocks, waiting until some task callsrelease()
.The optional value argument gives the initial value for the internal counter (
1
by default). If the given value is less than0
aValueError
is raised.Zmienione w wersji 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()
- 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 arelease()
is called and returnTrue
.
- 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 morerelease()
calls thanacquire()
calls.
BoundedSemaphore¶
- class asyncio.BoundedSemaphore(value=1)¶
A bounded semaphore object. Not thread-safe.
Bounded Semaphore is a version of
Semaphore
that raises aValueError
inrelease()
if it increases the internal counter above the initial value.Zmienione w wersji 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 onwait()
. At that point all of the waiting tasks would unblock simultaneously.async with
can be used as an alternative to awaiting onwait()
.The barrier can be reused any number of times.
Przykład:
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.
- coroutine 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 aCancelledError
if a task is cancelled.
- coroutine 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.
- coroutine abort()¶
Put the barrier into a broken state. This causes any active or future calls to
wait()
to fail with theBrokenBarrierError
. 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 theBarrier
object is reset or broken.
Zmienione w wersji 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.