Synchronization Primitives
**************************

**Source code:** 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"

* "Event"

* "Condition"

* "Semaphore"

* "BoundedSemaphore"

* "Barrier"

======================================================================


Lock
====

class asyncio.Lock

   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()

   Changed in version 3.10: Removed the *loop* parameter.

   async acquire()

      Acquire the lock.

      This method waits until the lock is *unlocked*, sets it to
      *locked* and returns "True".

      When more than one coroutine is blocked in "acquire()" waiting
      for the lock to be unlocked, only one coroutine eventually
      proceeds.

      Acquiring a lock is *fair*: the coroutine that proceeds will be
      the first coroutine that started waiting on the lock.

   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

   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.

   Changed in version 3.10: Removed the *loop* parameter.

   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())

   async wait()

      Wait until the event is set.

      If the event is set, return "True" immediately. Otherwise block
      until another task calls "set()".

   set()

      Set the event.

      All tasks waiting for event to be set will be immediately
      awakened.

   clear()

      Clear (unset) the event.

      Subsequent 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)

   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.

   Changed in version 3.10: Removed the *loop* parameter.

   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()

   async acquire()

      Acquire the underlying lock.

      This method waits until the underlying lock is *unlocked*, sets
      it to *locked* and returns "True".

   notify(n=1)

      Wake up *n* tasks (1 by default) waiting on this condition.  If
      fewer than *n* tasks are waiting they are all awakened.

      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.

   async wait()

      Wait until notified.

      If the calling task has not acquired the lock when this method
      is called, a "RuntimeError" is raised.

      This method releases the underlying lock, and then blocks until
      it is awakened by a "notify()" or "notify_all()" call. Once
      awakened, the Condition re-acquires its lock and this method
      returns "True".

      Note that a task *may* return from this call spuriously, which
      is why the caller should always re-check the state and be
      prepared to "wait()" again. For this reason, you may prefer to
      use "wait_for()" instead.

   async wait_for(predicate)

      Wait until a predicate becomes *true*.

      The predicate must be a callable which result will be
      interpreted as a boolean value.  The method will repeatedly
      "wait()" until the predicate evaluates to *true*. The final
      value is the return value.


Semaphore
=========

class asyncio.Semaphore(value=1)

   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; when "acquire()" 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 than
   "0" a "ValueError" is raised.

   Changed in version 3.10: Removed the *loop* parameter.

   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()

   async acquire()

      Acquire a semaphore.

      If the internal counter is greater than zero, decrement it by
      one and return "True" immediately.  If it is zero, wait until a
      "release()" is called and return "True".

   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 than "acquire()" calls.


BoundedSemaphore
================

class asyncio.BoundedSemaphore(value=1)

   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*.

   Changed in version 3.10: Removed the *loop* parameter.


Barrier
=======

class asyncio.Barrier(parties)

   A barrier object.  Not thread-safe.

   A barrier is a simple synchronization primitive that allows to
   block until *parties* number of tasks are waiting on it. Tasks can
   wait on the "wait()" method and would be blocked until the
   specified number of tasks end up waiting on "wait()". At that point
   all of the waiting tasks would unblock simultaneously.

   "async with" can be used as an alternative to awaiting on "wait()".

   The barrier can be reused any number of times.

   Example:

      async def example_barrier():
         # barrier with 3 parties
         b = asyncio.Barrier(3)

         # create 2 new waiting tasks
         asyncio.create_task(b.wait())
         asyncio.create_task(b.wait())

         await asyncio.sleep(0)
         print(b)

         # The third .wait() call passes the barrier
         await b.wait()
         print(b)
         print("barrier passed")

         await asyncio.sleep(0)
         print(b)

      asyncio.run(example_barrier())

   Result of this example is:

      <asyncio.locks.Barrier object at 0x... [filling, waiters:2/3]>
      <asyncio.locks.Barrier object at 0x... [draining, waiters:0/3]>
      barrier passed
      <asyncio.locks.Barrier object at 0x... [filling, waiters:0/3]>

   Added in version 3.11.

   async wait()

      Pass the barrier. When all the tasks party to the barrier have
      called this function, they are all unblocked simultaneously.

      When a waiting or blocked task in the barrier is cancelled, this
      task exits the barrier which stays in the same state. If the
      state of the barrier is “filling”, the number of waiting task
      decreases by 1.

      The return value is an integer in the range of 0 to "parties-1",
      different for each task. This can be used to select a task to do
      some special housekeeping, e.g.:

         ...
         async with barrier as position:
            if position == 0:
               # Only one task prints this
               print('End of *draining phase*')

      This method may raise a "BrokenBarrierError" exception if the
      barrier is broken or reset while a task is waiting. It could
      raise a "CancelledError" if a task is cancelled.

   async reset()

      Return the barrier to the default, empty state.  Any tasks
      waiting on it will receive the "BrokenBarrierError" exception.

      If a barrier is broken it may be better to just leave it and
      create a new one.

   async abort()

      Put the barrier into a broken state.  This causes any active or
      future calls to "wait()" to fail with the "BrokenBarrierError".
      Use this for example if one of the tasks needs to abort, to
      avoid infinite waiting tasks.

   parties

      The number of tasks required to pass the barrier.

   n_waiting

      The number of tasks currently waiting in the barrier while
      filling.

   broken

      A boolean that is "True" if the barrier is in the broken state.

exception asyncio.BrokenBarrierError

   This exception, a subclass of "RuntimeError", is raised when the
   "Barrier" object is reset or broken.

======================================================================

Changed in version 3.9: Acquiring a lock using "await lock" or "yield
from lock" and/or "with" statement ("with await lock", "with (yield
from lock)") was removed.  Use "async with lock" instead.
