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335 lines
8.5 KiB
ReStructuredText
335 lines
8.5 KiB
ReStructuredText
.. currentmodule:: asyncio
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.. _asyncio-sync:
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==========================
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Synchronization Primitives
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==========================
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**Source code:** :source:`Lib/asyncio/locks.py`
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-----------------------------------------------
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asyncio synchronization primitives are designed to be similar to
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those of the :mod:`threading` module with two important caveats:
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* asyncio primitives are not thread-safe, therefore they should not
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be used for OS thread synchronization (use :mod:`threading` for
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that);
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* methods of these synchronization primitives do not accept the *timeout*
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argument; use the :func:`asyncio.wait_for` function to perform
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operations with timeouts.
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asyncio has the following basic synchronization primitives:
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* :class:`Lock`
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* :class:`Event`
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* :class:`Condition`
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* :class:`Semaphore`
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* :class:`BoundedSemaphore`
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---------
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Lock
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====
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.. class:: Lock()
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Implements a mutex lock for asyncio tasks. Not thread-safe.
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An asyncio lock can be used to guarantee exclusive access to a
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shared resource.
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The preferred way to use a Lock is an :keyword:`async with`
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statement::
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lock = asyncio.Lock()
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# ... later
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async with lock:
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# access shared state
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which is equivalent to::
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lock = asyncio.Lock()
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# ... later
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await lock.acquire()
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try:
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# access shared state
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finally:
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lock.release()
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.. coroutinemethod:: acquire()
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Acquire the lock.
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This method waits until the lock is *unlocked*, sets it to
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*locked* and returns ``True``.
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When more than one coroutine is blocked in :meth:`acquire`
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waiting for the lock to be unlocked, only one coroutine
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eventually proceeds.
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Acquiring a lock is *fair*: the coroutine that proceeds will be
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the first coroutine that started waiting on the lock.
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.. method:: release()
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Release the lock.
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When the lock is *locked*, reset it to *unlocked* and return.
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If the lock is *unlocked*, a :exc:`RuntimeError` is raised.
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.. method:: locked()
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Return ``True`` if the lock is *locked*.
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Event
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=====
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.. class:: Event()
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An event object. Not thread-safe.
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An asyncio event can be used to notify multiple asyncio tasks
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that some event has happened.
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An Event object manages an internal flag that can be set to *true*
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with the :meth:`set` method and reset to *false* with the
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:meth:`clear` method. The :meth:`wait` method blocks until the
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flag is set to *true*. The flag is set to *false* initially.
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.. _asyncio_example_sync_event:
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Example::
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async def waiter(event):
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print('waiting for it ...')
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await event.wait()
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print('... got it!')
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async def main():
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# Create an Event object.
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event = asyncio.Event()
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# Spawn a Task to wait until 'event' is set.
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waiter_task = asyncio.create_task(waiter(event))
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# Sleep for 1 second and set the event.
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await asyncio.sleep(1)
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event.set()
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# Wait until the waiter task is finished.
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await waiter_task
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asyncio.run(main())
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.. coroutinemethod:: wait()
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Wait until the event is set.
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If the event is set, return ``True`` immediately.
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Otherwise block until another task calls :meth:`set`.
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.. method:: set()
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Set the event.
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All tasks waiting for event to be set will be immediately
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awakened.
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.. method:: clear()
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Clear (unset) the event.
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Tasks awaiting on :meth:`wait` will now block until the
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:meth:`set` method is called again.
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.. method:: is_set()
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Return ``True`` if the event is set.
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Condition
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=========
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.. class:: Condition(lock=None)
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A Condition object. Not thread-safe.
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An asyncio condition primitive can be used by a task to wait for
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some event to happen and then get exclusive access to a shared
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resource.
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In essence, a Condition object combines the functionality
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of an :class:`Event` and a :class:`Lock`. It is possible to have
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multiple Condition objects share one Lock, which allows coordinating
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exclusive access to a shared resource between different tasks
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interested in particular states of that shared resource.
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The optional *lock* argument must be a :class:`Lock` object or
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``None``. In the latter case a new Lock object is created
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automatically.
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The preferred way to use a Condition is an :keyword:`async with`
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statement::
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cond = asyncio.Condition()
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# ... later
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async with cond:
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await cond.wait()
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which is equivalent to::
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cond = asyncio.Condition()
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# ... later
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await cond.acquire()
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try:
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await cond.wait()
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finally:
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cond.release()
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.. coroutinemethod:: acquire()
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Acquire the underlying lock.
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This method waits until the underlying lock is *unlocked*,
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sets it to *locked* and returns ``True``.
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.. method:: notify(n=1)
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Wake up at most *n* tasks (1 by default) waiting on this
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condition. The method is no-op if no tasks are waiting.
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The lock must be acquired before this method is called and
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released shortly after. If called with an *unlocked* lock
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a :exc:`RuntimeError` error is raised.
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.. method:: locked()
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Return ``True`` if the underlying lock is acquired.
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.. method:: notify_all()
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Wake up all tasks waiting on this condition.
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This method acts like :meth:`notify`, but wakes up all waiting
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tasks.
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The lock must be acquired before this method is called and
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released shortly after. If called with an *unlocked* lock
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a :exc:`RuntimeError` error is raised.
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.. method:: release()
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Release the underlying lock.
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When invoked on an unlocked lock, a :exc:`RuntimeError` is
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raised.
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.. coroutinemethod:: wait()
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Wait until notified.
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If the calling task has not acquired the lock when this method is
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called, a :exc:`RuntimeError` is raised.
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This method releases the underlying lock, and then blocks until
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it is awakened by a :meth:`notify` or :meth:`notify_all` call.
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Once awakened, the Condition re-acquires its lock and this method
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returns ``True``.
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.. coroutinemethod:: wait_for(predicate)
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Wait until a predicate becomes *true*.
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The predicate must be a callable which result will be
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interpreted as a boolean value. The final value is the
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return value.
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Semaphore
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=========
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.. class:: Semaphore(value=1)
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A Semaphore object. Not thread-safe.
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A semaphore manages an internal counter which is decremented by each
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:meth:`acquire` call and incremented by each :meth:`release` call.
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The counter can never go below zero; when :meth:`acquire` finds
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that it is zero, it blocks, waiting until some task calls
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:meth:`release`.
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The optional *value* argument gives the initial value for the
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internal counter (``1`` by default). If the given value is
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less than ``0`` a :exc:`ValueError` is raised.
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The preferred way to use a Semaphore is an :keyword:`async with`
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statement::
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sem = asyncio.Semaphore(10)
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# ... later
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async with sem:
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# work with shared resource
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which is equivalent to::
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sem = asyncio.Semaphore(10)
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# ... later
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await sem.acquire()
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try:
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# work with shared resource
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finally:
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sem.release()
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.. coroutinemethod:: acquire()
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Acquire a semaphore.
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If the internal counter is greater than zero, decrement
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it by one and return ``True`` immediately. If it is zero, wait
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until a :meth:`release` is called and return ``True``.
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.. method:: locked()
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Returns ``True`` if semaphore can not be acquired immediately.
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.. method:: release()
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Release a semaphore, incrementing the internal counter by one.
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Can wake up a task waiting to acquire the semaphore.
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Unlike :class:`BoundedSemaphore`, :class:`Semaphore` allows
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making more ``release()`` calls than ``acquire()`` calls.
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BoundedSemaphore
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================
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.. class:: BoundedSemaphore(value=1)
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A bounded semaphore object. Not thread-safe.
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Bounded Semaphore is a version of :class:`Semaphore` that raises
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a :exc:`ValueError` in :meth:`~Semaphore.release` if it
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increases the internal counter above the initial *value*.
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---------
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.. versionchanged:: 3.9
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Acquiring a lock using ``await lock`` or ``yield from lock`` and/or
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:keyword:`with` statement (``with await lock``, ``with (yield from
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lock)``) was removed. Use ``async with lock`` instead.
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