PYTHON词典异步安全吗?

2022-03-25 00:00:00 python python-asyncio

问题描述

我已经在Python应用程序中创建了一个字典,我在其中保存数据,并且我有两个任务,这两个任务并行运行并从外部API获取数据。一旦获得数据,他们就会更新字典-每个字典中都有一个不同的键。

我想了解字典是否是异步安全的,或者在读取/更新字典时需要加锁吗?

每次任务还读取上次保存的值。

my_data = {}
asyncio.create_task(call_func_one_coroutine)
asyncio.create_task(call_func_two_coroutine)

async def call_func_one_coroutine():
  data =  await goto_api_get_data()
  my_data['one'] = data + my_data['one']


async def call_func_two_coroutine():
  data =  await goto_api_another_get_data()
  my_data['two'] = data + my_data['two']


解决方案

我想了解字典是否是异步安全的,或者在读取/更新字典时需要加锁吗?

异步基于协作多任务,只能在显式await表达式或async withasync for语句上切换任务。因为单个字典的更新永远不会涉及等待(等待必须在更新开始之前完成),所以就异步多任务而言,它实际上是原子的,您不需要锁定它。这适用于从异步代码访问的所有数据结构。

再举一个没有问题的例子:

# correct - there are no awaits between two accesses to the dict d
key = await key_queue.get()
if key in d:
    d[key] = calc_value(key)

字典修改不是异步安全的示例将涉及以awaits分隔的对字典的多次访问。例如:

# incorrect, d[key] could appear while we're reading the value,
# in which case we'd clobber the existing key
if key not in d:
    d[key] = await read_value()

若要更正该错误,您可以在await后面添加另一个检查,或者使用显式锁定:

# correct (1), using double check
if key not in d:
    value = await read_value()
    # Check again whether the key is vacant. Since there are no awaits
    # between this check and the update, the operation is atomic.
    if key not in d:
        d[key] = value

# correct (2), using a shared asyncio.Lock:
async with d_lock:
    # Single check is sufficient because the lock ensures that
    # no one can modify the dict while we're reading the value.
    if key not in d:
        d[key] = await read_value()

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