1.. currentmodule:: asyncio
2
3.. _asyncio-sync:
4
5==========================
6Synchronization Primitives
7==========================
8
9**Source code:** :source:`Lib/asyncio/locks.py`
10
11-----------------------------------------------
12
13asyncio synchronization primitives are designed to be similar to
14those of the :mod:`threading` module with two important caveats:
15
16* asyncio primitives are not thread-safe, therefore they should not
17  be used for OS thread synchronization (use :mod:`threading` for
18  that);
19
20* methods of these synchronization primitives do not accept the *timeout*
21  argument; use the :func:`asyncio.wait_for` function to perform
22  operations with timeouts.
23
24asyncio has the following basic synchronization primitives:
25
26* :class:`Lock`
27* :class:`Event`
28* :class:`Condition`
29* :class:`Semaphore`
30* :class:`BoundedSemaphore`
31
32
33---------
34
35
36Lock
37====
38
39.. class:: Lock(\*, loop=None)
40
41   Implements a mutex lock for asyncio tasks.  Not thread-safe.
42
43   An asyncio lock can be used to guarantee exclusive access to a
44   shared resource.
45
46   The preferred way to use a Lock is an :keyword:`async with`
47   statement::
48
49       lock = asyncio.Lock()
50
51       # ... later
52       async with lock:
53           # access shared state
54
55   which is equivalent to::
56
57       lock = asyncio.Lock()
58
59       # ... later
60       await lock.acquire()
61       try:
62           # access shared state
63       finally:
64           lock.release()
65
66   .. deprecated-removed:: 3.8 3.10
67      The *loop* parameter.
68
69   .. coroutinemethod:: acquire()
70
71      Acquire the lock.
72
73      This method waits until the lock is *unlocked*, sets it to
74      *locked* and returns ``True``.
75
76      When more than one coroutine is blocked in :meth:`acquire`
77      waiting for the lock to be unlocked, only one coroutine
78      eventually proceeds.
79
80      Acquiring a lock is *fair*: the coroutine that proceeds will be
81      the first coroutine that started waiting on the lock.
82
83   .. method:: release()
84
85      Release the lock.
86
87      When the lock is *locked*, reset it to *unlocked* and return.
88
89      If the lock is *unlocked*, a :exc:`RuntimeError` is raised.
90
91   .. method:: locked()
92
93      Return ``True`` if the lock is *locked*.
94
95
96Event
97=====
98
99.. class:: Event(\*, loop=None)
100
101   An event object.  Not thread-safe.
102
103   An asyncio event can be used to notify multiple asyncio tasks
104   that some event has happened.
105
106   An Event object manages an internal flag that can be set to *true*
107   with the :meth:`~Event.set` method and reset to *false* with the
108   :meth:`clear` method.  The :meth:`~Event.wait` method blocks until the
109   flag is set to *true*.  The flag is set to *false* initially.
110
111
112   .. deprecated-removed:: 3.8 3.10
113      The *loop* parameter.
114
115   .. _asyncio_example_sync_event:
116
117   Example::
118
119      async def waiter(event):
120          print('waiting for it ...')
121          await event.wait()
122          print('... got it!')
123
124      async def main():
125          # Create an Event object.
126          event = asyncio.Event()
127
128          # Spawn a Task to wait until 'event' is set.
129          waiter_task = asyncio.create_task(waiter(event))
130
131          # Sleep for 1 second and set the event.
132          await asyncio.sleep(1)
133          event.set()
134
135          # Wait until the waiter task is finished.
136          await waiter_task
137
138      asyncio.run(main())
139
140   .. coroutinemethod:: wait()
141
142      Wait until the event is set.
143
144      If the event is set, return ``True`` immediately.
145      Otherwise block until another task calls :meth:`~Event.set`.
146
147   .. method:: set()
148
149      Set the event.
150
151      All tasks waiting for event to be set will be immediately
152      awakened.
153
154   .. method:: clear()
155
156      Clear (unset) the event.
157
158      Tasks awaiting on :meth:`~Event.wait` will now block until the
159      :meth:`~Event.set` method is called again.
160
161   .. method:: is_set()
162
163      Return ``True`` if the event is set.
164
165
166Condition
167=========
168
169.. class:: Condition(lock=None, \*, loop=None)
170
171   A Condition object.  Not thread-safe.
172
173   An asyncio condition primitive can be used by a task to wait for
174   some event to happen and then get exclusive access to a shared
175   resource.
176
177   In essence, a Condition object combines the functionality
178   of an :class:`Event` and a :class:`Lock`.  It is possible to have
179   multiple Condition objects share one Lock, which allows coordinating
180   exclusive access to a shared resource between different tasks
181   interested in particular states of that shared resource.
182
183   The optional *lock* argument must be a :class:`Lock` object or
184   ``None``.  In the latter case a new Lock object is created
185   automatically.
186
187
188   .. deprecated-removed:: 3.8 3.10
189      The *loop* parameter.
190
191   The preferred way to use a Condition is an :keyword:`async with`
192   statement::
193
194       cond = asyncio.Condition()
195
196       # ... later
197       async with cond:
198           await cond.wait()
199
200   which is equivalent to::
201
202       cond = asyncio.Condition()
203
204       # ... later
205       await cond.acquire()
206       try:
207           await cond.wait()
208       finally:
209           cond.release()
210
211   .. coroutinemethod:: acquire()
212
213      Acquire the underlying lock.
214
215      This method waits until the underlying lock is *unlocked*,
216      sets it to *locked* and returns ``True``.
217
218   .. method:: notify(n=1)
219
220      Wake up at most *n* tasks (1 by default) waiting on this
221      condition.  The method is no-op if no tasks are waiting.
222
223      The lock must be acquired before this method is called and
224      released shortly after.  If called with an *unlocked* lock
225      a :exc:`RuntimeError` error is raised.
226
227   .. method:: locked()
228
229      Return ``True`` if the underlying lock is acquired.
230
231   .. method:: notify_all()
232
233      Wake up all tasks waiting on this condition.
234
235      This method acts like :meth:`notify`, but wakes up all waiting
236      tasks.
237
238      The lock must be acquired before this method is called and
239      released shortly after.  If called with an *unlocked* lock
240      a :exc:`RuntimeError` error is raised.
241
242   .. method:: release()
243
244      Release the underlying lock.
245
246      When invoked on an unlocked lock, a :exc:`RuntimeError` is
247      raised.
248
249   .. coroutinemethod:: wait()
250
251      Wait until notified.
252
253      If the calling task has not acquired the lock when this method is
254      called, a :exc:`RuntimeError` is raised.
255
256      This method releases the underlying lock, and then blocks until
257      it is awakened by a :meth:`notify` or :meth:`notify_all` call.
258      Once awakened, the Condition re-acquires its lock and this method
259      returns ``True``.
260
261   .. coroutinemethod:: wait_for(predicate)
262
263      Wait until a predicate becomes *true*.
264
265      The predicate must be a callable which result will be
266      interpreted as a boolean value.  The final value is the
267      return value.
268
269
270Semaphore
271=========
272
273.. class:: Semaphore(value=1, \*, loop=None)
274
275   A Semaphore object.  Not thread-safe.
276
277   A semaphore manages an internal counter which is decremented by each
278   :meth:`acquire` call and incremented by each :meth:`release` call.
279   The counter can never go below zero; when :meth:`acquire` finds
280   that it is zero, it blocks, waiting until some task calls
281   :meth:`release`.
282
283   The optional *value* argument gives the initial value for the
284   internal counter (``1`` by default). If the given value is
285   less than ``0`` a :exc:`ValueError` is raised.
286
287
288   .. deprecated-removed:: 3.8 3.10
289      The *loop* parameter.
290
291   The preferred way to use a Semaphore is an :keyword:`async with`
292   statement::
293
294       sem = asyncio.Semaphore(10)
295
296       # ... later
297       async with sem:
298           # work with shared resource
299
300   which is equivalent to::
301
302       sem = asyncio.Semaphore(10)
303
304       # ... later
305       await sem.acquire()
306       try:
307           # work with shared resource
308       finally:
309           sem.release()
310
311   .. coroutinemethod:: acquire()
312
313      Acquire a semaphore.
314
315      If the internal counter is greater than zero, decrement
316      it by one and return ``True`` immediately.  If it is zero, wait
317      until a :meth:`release` is called and return ``True``.
318
319   .. method:: locked()
320
321      Returns ``True`` if semaphore can not be acquired immediately.
322
323   .. method:: release()
324
325      Release a semaphore, incrementing the internal counter by one.
326      Can wake up a task waiting to acquire the semaphore.
327
328      Unlike :class:`BoundedSemaphore`, :class:`Semaphore` allows
329      making more ``release()`` calls than ``acquire()`` calls.
330
331
332BoundedSemaphore
333================
334
335.. class:: BoundedSemaphore(value=1, \*, loop=None)
336
337   A bounded semaphore object.  Not thread-safe.
338
339   Bounded Semaphore is a version of :class:`Semaphore` that raises
340   a :exc:`ValueError` in :meth:`~Semaphore.release` if it
341   increases the internal counter above the initial *value*.
342
343
344   .. deprecated-removed:: 3.8 3.10
345      The *loop* parameter.
346
347---------
348
349
350.. deprecated:: 3.7
351
352   Acquiring a lock using ``await lock`` or ``yield from lock`` and/or
353   :keyword:`with` statement (``with await lock``, ``with (yield from
354   lock)``) is deprecated.  Use ``async with lock`` instead.
355