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# 18.5.9. Synchronization primitives

Locks:

- `Lock`
- `Event`
- `Condition`
- `Semaphore`
- `BoundedSemaphore`

Queues:

- `Queue`
- `PriorityQueue`
- `LifoQueue`
- `JoinableQueue`

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

## 18.5.9.1. Locks

### 18.5.9.1.1. Lock

class `asyncio.``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, `acquire()` and `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 `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.

  `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
          ...

  `locked`()

    Return `True` if the lock is acquired.

  `acquire`()

    Acquire a lock.

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

    This method is a coroutine.

  `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 `RuntimeError` is raised.

    There is no return value.

### 18.5.9.1.2. Event

class `asyncio.``Event`(*, loop=None)

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

  Class implementing event objects. An event manages a 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 true. The flag is initially false.

  `clear`()

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

  `is_set`()

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

  `set`()

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

  `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 `set()` to set the flag to true, then return `True`.

    This method is a coroutine.

### 18.5.9.1.3. Condition

class `asyncio.``Condition`(lock=None, *, loop=None)

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

  `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 coroutine.

  `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 `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 `wait()` call until it can reacquire the lock. Since `notify()` does not release the lock, its caller should.

  `locked`()

    Return `True` if the underlying lock is acquired.

  `notify_all`()

    Wake up all threads waiting on this condition. This method acts like `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 `RuntimeError` is raised.

  `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 `RuntimeError` is raised.

    There is no return value.

  `wait`()

    Wait until notified.

    If the calling coroutine 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 for the same condition variable in another coroutine. Once awakened, it re-acquires the lock and returns `True`.

    This method is a coroutine.

  `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 coroutine.

## 18.5.9.2. Semaphores

### 18.5.9.2.1. Semaphore

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

  A Semaphore implementation.

  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 other thread calls `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`, `ValueError` is raised.

  `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 `release()` to make it larger than `0`, and then return `True`.

    This method is a coroutine.

  `locked`()

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

  `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.

### 18.5.9.2.2. BoundedSemaphore

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

  A bounded semaphore implementation. Inherit from `Semaphore`.

  This raises `ValueError` in `release()` if it would increase the value above the initial value.

## 18.5.9.3. Queues

### 18.5.9.3.1. Queue

class `asyncio.``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 `get()`.

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

  `empty`()

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

  `full`()

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

    Note

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

  `get`()

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

    This method is a coroutine.

    See also

    The `empty()` method.

  `get_nowait`()

    Remove and return an item from the queue.

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

  `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 coroutine.

    See also

    The `full()` method.

  `put_nowait`(item)

    Put an item into the queue without blocking.

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

  `qsize`()

    Number of items in the queue.

  `maxsize`

    Number of items allowed in the queue.

### 18.5.9.3.2. PriorityQueue

class `asyncio.``PriorityQueue`

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

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

### 18.5.9.3.3. LifoQueue

class `asyncio.``LifoQueue`

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

### 18.5.9.3.4. JoinableQueue

class `asyncio.``JoinableQueue`

  A subclass of `Queue` with `task_done()` and `join()` methods.

  `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 `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, `join()` unblocks.

    This method is a coroutine.

  `task_done`()

    Indicate that a formerly enqueued task is complete.

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

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

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

### 18.5.9.3.5. Exceptions

exception `asyncio.``QueueEmpty`

  Exception raised when the `get_nowait()` method is called on a `Queue` object which is empty.

exception `asyncio.``QueueFull`

  Exception raised when the `put_nowait()` method is called on a `Queue` object which is full.

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