Synchronization Primitives — Python documentation
Synchronization Primitives
Source code: :source:`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(\*, 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()
- 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())
- 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 cond.acquire() try: await cond.wait() finally: cond.release()
- 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.
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; whenacquire()
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 than0
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()
- 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 thanacquire()
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.