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

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"

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


Lock
====

class asyncio.Lock(*, loop=None)

   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()

   coroutine 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(*, loop=None)

   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.

   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())

   coroutine 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.

      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, *, loop=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.

   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()

   coroutine 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 at most *n* tasks (1 by default) waiting on this
      condition.  The method is no-op if no tasks are waiting.

      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.

   coroutine 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".

   coroutine 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 final value is the return
      value.


Semaphore
=========

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

   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.

   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()

   coroutine 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, *, loop=None)

   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*.

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

3.7 版後已棄用: Acquiring a lock using "await lock" or "yield from
lock" and/or "with" statement ("with await lock", "with (yield from
lock)") is deprecated.  Use "async with lock" instead.
