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.
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 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.
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.
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.
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.
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.
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.
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.
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 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 than 0, ValueError
is raised.
This class is not thread safe.
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.
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.