bpo-33649: Cleanup asyncio/streams and asyncio/synchronization docs (GH-9192)
diff --git a/Doc/library/asyncio-sync.rst b/Doc/library/asyncio-sync.rst
index 574f70f..8e01ca9 100644
--- a/Doc/library/asyncio-sync.rst
+++ b/Doc/library/asyncio-sync.rst
@@ -1,172 +1,204 @@
.. currentmodule:: asyncio
+
.. _asyncio-sync:
-Synchronization primitives
+==========================
+Synchronization Primitives
==========================
-**Source code:** :source:`Lib/asyncio/locks.py`
+asyncio synchronization primitives are designed to be similar to
+those of the :mod:`threading` module with two important caveats:
-Locks:
+* asyncio primitives are not thread-safe, therefore they should not
+ be used for OS threads synchronization (use :mod:`threading` for
+ that);
+
+* methods of synchronization objects do not accept the *timeout*
+ argument; use the :func:`asyncio.wait_for` function to perform
+ operations with timeouts.
+
+asyncio has the following basic primitives:
* :class:`Lock`
* :class:`Event`
* :class:`Condition`
-
-Semaphores:
-
* :class:`Semaphore`
* :class:`BoundedSemaphore`
-asyncio lock API was 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`), but it has no *timeout* parameter. The
-:func:`asyncio.wait_for` function can be used to cancel a task after a timeout.
+
+---------
Lock
-----
+====
.. class:: Lock(\*, loop=None)
- Primitive lock objects.
+ Implements a mutex lock for asyncio tasks. Not thread-safe.
- 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'.
+ An asyncio lock can be used to guarantee exclusive access to a
+ shared resource.
- The lock 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.
+ The preferred way to use a Lock is an :keyword:`async with`
+ statement::
- 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.
+ lock = asyncio.Lock()
- :meth:`acquire` is a coroutine and should be called with ``await``.
+ # ... later
+ async with lock:
+ # access shared state
- Locks support the :ref:`context management protocol <async-with-locks>`.
+ which is equivalent to::
- This class is :ref:`not thread safe <asyncio-multithreading>`.
+ lock = asyncio.Lock()
- .. method:: locked()
-
- Return ``True`` if the lock is acquired.
+ # ... later
+ await lock.acquire()
+ try:
+ # access shared state
+ finally:
+ lock.release()
.. coroutinemethod:: acquire()
- Acquire a lock.
+ Acquire the lock.
- This method blocks until the lock is unlocked, then sets it to locked and
- returns ``True``.
-
- This method is a :ref:`coroutine <coroutine>`.
+ This method waits until the lock is *unlocked*, sets it to
+ *locked* and returns ``True``.
.. method:: release()
- Release a lock.
+ Release the 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 the lock is *locked*, reset it to *unlocked* and return.
- When invoked on an unlocked lock, a :exc:`RuntimeError` is raised.
+ If the lock is *unlocked* a :exc:`RuntimeError` is raised.
- There is no return value.
+ .. method:: locked()
+
+ Return ``True`` if the lock is *locked*.
Event
------
+=====
.. class:: Event(\*, loop=None)
- An Event implementation, asynchronous equivalent to :class:`threading.Event`.
+ An event object. Not thread-safe.
- 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.
+ An asyncio event can be used to notify multiple asyncio tasks
+ that some event has happened.
- This class is :ref:`not thread safe <asyncio-multithreading>`.
+ An Event object manages an internal 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 set to *true*. The flag is set to *false* initially.
- .. method:: clear()
+ Example::
- 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.
+ async def waiter(event):
+ print('waiting ...')
+ await event.wait()
+ print('... got it!')
- .. method:: is_set()
+ async def main():
+ # Create an Event object.
+ event = asyncio.Event()
- Return ``True`` if and only if the internal flag is true.
+ # Spawn a Task to wait until 'event' is set.
+ waiter_task = asyncio.create_task(waiter(event))
- .. method:: set()
+ # Sleep for 1 second and set the event.
+ await asyncio.sleep(1)
+ event.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.
+ # Wait until the waiter task is finished.
+ await waiter_task
+
+ asyncio.run(main())
.. coroutinemethod:: wait()
- Block until the internal flag is true.
+ Wait until the event is set.
- 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``.
+ If the event is set, return ``True`` immediately.
+ Otherwise block until another task calls :meth:`set`.
- This method is a :ref:`coroutine <coroutine>`.
+ .. method:: set()
+
+ Set the event.
+
+ All tasks waiting for event to be set will be immediately
+ awakened.
+
+ .. method:: clear()
+
+ Clear (unset) the event.
+
+ Tasks awaiting on :meth:`wait` will now block until the
+ :meth:`set` method is called again.
+
+ .. method:: is_set()
+
+ Return ``True`` if the event is set.
Condition
----------
+=========
.. class:: Condition(lock=None, \*, loop=None)
- A Condition implementation, asynchronous equivalent to
- :class:`threading.Condition`.
+ A Condition object. Not thread-safe.
- This class implements condition variable objects. A condition variable
- allows one or more coroutines to wait until they are notified by another
- coroutine.
+ An asyncio condition primitive can be used by a task to wait for
+ some event to happen and then get an exclusive access to a shared
+ resource.
- 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.
+ In essence, a Condition object combines the functionality
+ of :class:`Event` and :class:`Lock`. It is possible to have many
+ Condition objects sharing one Lock, which allows to coordinate
+ exclusive access to a shared resource between different tasks
+ interested in particular states of that shared resource.
- Conditions support the :ref:`context management protocol
- <async-with-locks>`.
+ The optional *lock* argument must be a :class:`Lock` object or
+ ``None``. In the latter case a new Lock object is created
+ automatically.
- This class is :ref:`not thread safe <asyncio-multithreading>`.
+ The preferred way to use a Condition is an :keyword:`async with`
+ statement::
+
+ cond = asyncio.Condition()
+
+ # ... later
+ async with cond:
+ await cond.wait()
+
+ which is equivalent to::
+
+ cond = asyncio.Condition()
+
+ # ... later
+ await lock.acquire()
+ try:
+ await cond.wait()
+ finally:
+ lock.release()
.. coroutinemethod:: 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>`.
+ This method waits until the underlying lock is *unlocked*,
+ sets it to *locked* and returns ``True``.
.. 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.
+ Wake up at most *n* tasks (1 by default) waiting on this
+ condition. The method is no-op if no tasks are waiting.
- 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.
+ The lock must be acquired before this method is called and
+ released shortly after. If called with an *unlocked* lock
+ a :exc:`RuntimeError` error is raised.
.. method:: locked()
@@ -174,78 +206,87 @@
.. method:: notify_all()
- Wake up all coroutines waiting on this condition. This method acts like
- :meth:`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
- :exc:`RuntimeError` is raised.
+ Wake up all tasks waiting on this condition.
+
+ This method acts like :meth:`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 :exc:`RuntimeError` error 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.
+ When invoked on an unlocked lock, a :exc:`RuntimeError` is
+ raised.
.. coroutinemethod:: wait()
Wait until notified.
- If the calling coroutine has not acquired the lock when this method is
+ If the calling task 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>`.
+ This method releases the underlying lock, and then blocks until
+ it is awakened by a :meth:`notify` or :meth:`notify_all` call.
+ Once awakened, the Condition re-acquires its lock and this method
+ returns ``True``.
.. coroutinemethod:: wait_for(predicate)
- Wait until a predicate becomes true.
+ 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>`.
+ The predicate must be a callable which result will be
+ interpreted as a boolean value. The final value is the
+ return value.
Semaphore
----------
+=========
.. class:: Semaphore(value=1, \*, loop=None)
- A Semaphore implementation.
+ A Semaphore object. Not thread-safe.
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 coroutine calls :meth:`release`.
+ :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 task calls
+ :meth:`release`.
- 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.
+ The optional *value* argument gives the initial value for the
+ internal counter (``1`` by default). If the given value is
+ less than ``0`` a :exc:`ValueError` is raised.
- Semaphores support the :ref:`context management protocol
- <async-with-locks>`.
+ The preferred way to use a Semaphore is an :keyword:`async with`
+ statement::
- This class is :ref:`not thread safe <asyncio-multithreading>`.
+ 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()
.. coroutinemethod:: 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>`.
+ If the internal counter is greater than zero, decrement
+ it by one and return ``True`` immediately. If it is zero wait
+ until a :meth:`release` is called and return ``True``.
.. method:: locked()
@@ -253,53 +294,30 @@
.. 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.
+ Release a semaphore, incrementing the internal counter by one.
+ Can wake up a task waiting to acquire the semaphore.
+
+ Unlike :class:`BoundedSemaphore`, :class:`Semaphore` allows
+ to make more ``release()`` calls than ``acquire()`` calls.
BoundedSemaphore
-----------------
+================
.. class:: BoundedSemaphore(value=1, \*, loop=None)
- A bounded semaphore implementation. Inherit from :class:`Semaphore`.
+ A bounded semaphore object. Not thread-safe.
- This raises :exc:`ValueError` in :meth:`~Semaphore.release` if it would
- increase the value above the initial value.
-
- Bounded semaphores support the :ref:`context management
- protocol <async-with-locks>`.
-
- This class is :ref:`not thread safe <asyncio-multithreading>`.
+ Bounded Semaphore is a version of :class:`Semaphore` that raises
+ a :exc:`ValueError` in :meth:`~Semaphore.release` if it
+ increases the internal counter above the initial *value*.
-.. _async-with-locks:
+---------
-Using locks, conditions and semaphores in the :keyword:`async with` statement
------------------------------------------------------------------------------
-
-:class:`Lock`, :class:`Condition`, :class:`Semaphore`, and
-:class:`BoundedSemaphore` objects can be used in :keyword:`async with`
-statements.
-
-The :meth:`acquire` method will be called when the block is entered,
-and :meth:`release` will be called when the block is exited. Hence,
-the following snippet::
-
- async with lock:
- # do something...
-
-is equivalent to::
-
- await lock.acquire()
- try:
- # do something...
- finally:
- lock.release()
.. deprecated:: 3.7
- Lock acquiring using ``await lock`` or ``yield from lock`` and
+ Acquiring a lock using ``await lock`` or ``yield from lock`` and/or
:keyword:`with` statement (``with await lock``, ``with (yield from
- lock)``) are deprecated.
+ lock)``) is deprecated. Use ``async with lock`` instead.