cpython/Doc/library/asyncio-sync.rst

.. currentmodule:: asyncio
.. _asyncio-sync:

Synchronization primitives
==========================

Locks:

* :class:`Lock`
* :class:`Event`
* :class:`Condition`
* :class:`Semaphore`
* :class:`BoundedSemaphore`

Queues:

* :class:`Queue`
* :class:`PriorityQueue`
* :class:`LifoQueue`
* :class:`JoinableQueue`

asyncio locks and queues API were designed to be close to classes of the
:mod:`threading` module (:class:`~threading.Lock`, :class:`~threading.Event`,
:class:`~threading.Condition`, :class:`~threading.Semaphore`,
:class:`~threading.BoundedSemaphore`) and the :mod:`queue` module
(:class:`~queue.Queue`, :class:`~queue.PriorityQueue`,
:class:`~queue.LifoQueue`), but they have no *timeout* parameter. The
:func:`asyncio.wait_for` function can be used to cancel a task after a timeout.

Locks
-----

Lock
^^^^

.. class:: 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, :meth:`acquire`
   and :meth:`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 :exc:`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.

   :meth:`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 context manager expression.

   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
           ...

   .. method:: locked()

      Return ``True`` if the lock is acquired.

   .. method:: acquire()

      Acquire a lock.

      This method blocks until the lock is unlocked, then sets it to locked and
      returns ``True``.

      This method is a :ref:`coroutine <coroutine>`.

   .. method:: 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 :exc:`RuntimeError` is raised.

      There is no return value.


Event
^^^^^

.. class:: Event(\*, loop=None)

   An Event implementation, asynchronous equivalent to :class:`threading.Event`.

   Class implementing event objects. An event manages a flag that can be set to
   true with the :meth:`set` method and reset to false with the :meth:`clear`
   method.  The :meth:`wait` method blocks until the flag is true. The flag is
   initially false.

   .. method:: clear()

      Reset the internal flag to false. Subsequently, coroutines calling
      :meth:`wait` will block until :meth:`set` is called to set the internal
      flag to true again.

   .. method:: is_set()

      Return ``True`` if and only if the internal flag is true.

   .. method:: set()

      Set the internal flag to true. All coroutines waiting for it to become
      true are awakened. Coroutine that call :meth:`wait` once the flag is true
      will not block at all.

   .. method:: wait()

      Block until the internal flag is true.

      If the internal flag is true on entry, return ``True`` immediately.
      Otherwise, block until another coroutine calls :meth:`set` to set the
      flag to true, then return ``True``.

      This method is a :ref:`coroutine <coroutine>`.


Condition
^^^^^^^^^

.. class:: Condition(lock=None, \*, loop=None)

   A Condition implementation, asynchronous equivalent to
   :class:`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 :class:`Lock`
   object, and it is used as the underlying lock.  Otherwise,
   a new :class:`Lock` object is created and used as the underlying lock.

   .. method:: 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 :ref:`coroutine <coroutine>`.

   .. method:: 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 :exc:`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 :meth:`wait`
         call until it can reacquire the lock. Since :meth:`notify` does not
         release the lock, its caller should.

   .. method:: locked()

      Return ``True`` if the underlying lock is acquired.

   .. method:: notify_all()

      Wake up all threads waiting on this condition. This method acts like
      :meth:`notify`, but wakes up all waiting threads instead of one. If the
      calling thread has not acquired the lock when this method is called, a
      :exc:`RuntimeError` is raised.

   .. method:: 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 :exc:`RuntimeError` is raised.

      There is no return value.

   .. method:: wait()

      Wait until notified.

      If the calling coroutine has not acquired the lock when this method is
      called, a :exc:`RuntimeError` is raised.

      This method releases the underlying lock, and then blocks until it is
      awakened by a :meth:`notify` or :meth:`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 :ref:`coroutine <coroutine>`.

   .. method:: wait_for(predicate)

      Wait until a predicate becomes true.

      The predicate should be a callable which result will be interpreted as a
      boolean value. The final predicate value is the return value.

      This method is a :ref:`coroutine <coroutine>`.


Semaphores
----------

Semaphore
^^^^^^^^^

.. class:: Semaphore(value=1, \*, loop=None)

   A Semaphore implementation.

   A semaphore manages an internal counter which is decremented by each
   :meth:`acquire` call and incremented by each :meth:`release` call. The
   counter can never go below zero; when :meth:`acquire` finds that it is zero,
   it blocks, waiting until some other thread calls :meth:`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``, :exc:`ValueError`
   is raised.

   .. method:: 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 :meth:`release` to make it larger
      than ``0``, and then return ``True``.

      This method is a :ref:`coroutine <coroutine>`.

   .. method:: locked()

      Returns ``True`` if semaphore can not be acquired immediately.

   .. method:: 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.


BoundedSemaphore
^^^^^^^^^^^^^^^^

.. class:: BoundedSemaphore(value=1, \*, loop=None)

    A bounded semaphore implementation. Inherit from :class:`Semaphore`.

    This raises :exc:`ValueError` in :meth:`~Semaphore.release` if it would
    increase the value above the initial value.


Queues
------

Queue
^^^^^

.. class:: Queue(maxsize=0, \*, loop=None)

   A queue, useful for coordinating producer and consumer coroutines.

   If *maxsize* is less than or equal to zero, the queue size is infinite. If
   it is an integer greater than ``0``, then ``yield from put()`` will block
   when the queue reaches *maxsize*, until an item is removed by :meth:`get`.

   Unlike the standard library :mod:`queue`, you can reliably know this Queue's
   size with :meth:`qsize`, since your single-threaded asyncio application won't
   be interrupted between calling :meth:`qsize` and doing an operation on the
   Queue.

   .. method:: empty()

      Return ``True`` if the queue is empty, ``False`` otherwise.

   .. method:: full()

      Return ``True`` if there are :attr:`maxsize` items in the queue.

      .. note::

         If the Queue was initialized with ``maxsize=0`` (the default), then
         :meth:`full()` is never ``True``.

   .. method:: get()

      Remove and return an item from the queue. If queue is empty, wait until
      an item is available.

      This method is a :ref:`coroutine <coroutine>`.

      .. seealso::

         The :meth:`empty` method.

   .. method:: get_nowait()

      Remove and return an item from the queue.

      Return an item if one is immediately available, else raise
      :exc:`QueueEmpty`.

   .. method:: put(item)

      Put an item into the queue. If the queue is full, wait until a free slot
      is available before adding item.

      This method is a :ref:`coroutine <coroutine>`.

      .. seealso::

         The :meth:`full` method.

   .. method:: put_nowait(item)

      Put an item into the queue without blocking.

      If no free slot is immediately available, raise :exc:`QueueFull`.

   .. method:: qsize()

      Number of items in the queue.

   .. attribute:: maxsize

      Number of items allowed in the queue.


PriorityQueue
^^^^^^^^^^^^^

.. class:: PriorityQueue

   A subclass of :class:`Queue`; retrieves entries in priority order (lowest
   first).

   Entries are typically tuples of the form: (priority number, data).


LifoQueue
^^^^^^^^^

.. class:: LifoQueue

    A subclass of :class:`Queue` that retrieves most recently added entries
    first.


JoinableQueue
^^^^^^^^^^^^^

.. class:: JoinableQueue

   A subclass of :class:`Queue` with :meth:`task_done` and :meth:`join`
   methods.

   .. method:: join()

      Block until all items in the queue have been gotten and processed.

      The count of unfinished tasks goes up whenever an item is added to the
      queue. The count goes down whenever a consumer thread calls
      :meth:`task_done` to indicate that the item was retrieved and all work on
      it is complete.  When the count of unfinished tasks drops to zero,
      :meth:`join` unblocks.

      This method is a :ref:`coroutine <coroutine>`.

   .. method:: task_done()

      Indicate that a formerly enqueued task is complete.

      Used by queue consumers. For each :meth:`~Queue.get` used to fetch a task, a
      subsequent call to :meth:`task_done` tells the queue that the processing
      on the task is complete.

      If a :meth:`join` is currently blocking, it will resume when all items
      have been processed (meaning that a :meth:`task_done` call was received
      for every item that had been :meth:`~Queue.put` into the queue).

      Raises :exc:`ValueError` if called more times than there were items
      placed in the queue.


Exceptions
^^^^^^^^^^

.. exception:: QueueEmpty

   Exception raised when the :meth:`~Queue.get_nowait` method is called on a
   :class:`Queue` object which is empty.


.. exception:: QueueFull

   Exception raised when the :meth:`~Queue.put_nowait` method is called on a
   :class:`Queue` object which is full.