了解 Python 上下文变量

Question

关于以下。我做了一些更改，以了解使用和不使用 Contextvars 之间的区别。

我预计在某些时候变量 myid 会损坏，但将范围更改为更高的数字似乎根本不会影响。

import asyncio
import contextvars

# declare context var
request_id = contextvars.ContextVar('Id of request.')


async def some_inner_coroutine(myid):
    # get value
    print('Processed inner coroutine of myid   : {}'.format(myid))
    print('Processed inner coroutine of request: {}'.format(request_id.get()))
    if myid != request_id.get():
        print("ERROR")


async def some_outer_coroutine(req_id):
    # set value
    request_id.set(req_id)

    await some_inner_coroutine(req_id)

    # get value
    print('Processed outer coroutine of request: {}'.format(request_id.get()))


async def main():
    tasks = []
    for req_id in range(1, 1250):
        tasks.append(asyncio.create_task(some_outer_coroutine(req_id)))

    await asyncio.gather(*tasks)


if __name__ == '__main__':
    asyncio.run(main())

结果

Processed inner coroutine of myid   : 1
Processed inner coroutine of request: 1
Processed outer coroutine of request: 1
Processed inner coroutine of myid   : 2
Processed inner coroutine of request: 2
Processed outer coroutine of request: 2
Processed inner coroutine of myid   : 3
Processed inner coroutine of request: 3
Processed outer coroutine of request: 3
Processed inner coroutine of myid   : 4
Processed inner coroutine of request: 4
Processed outer coroutine of request: 4
...
...
Processed inner coroutine of myid   : 1244
Processed inner coroutine of request: 1244
Processed outer coroutine of request: 1244
Processed inner coroutine of myid   : 1245
Processed inner coroutine of request: 1245
Processed outer coroutine of request: 1245
Processed inner coroutine of myid   : 1246
Processed inner coroutine of request: 1246
Processed outer coroutine of request: 1246
Processed inner coroutine of myid   : 1247
Processed inner coroutine of request: 1247
Processed outer coroutine of request: 1247
Processed inner coroutine of myid   : 1248
Processed inner coroutine of request: 1248
Processed outer coroutine of request: 1248
Processed inner coroutine of myid   : 1249
Processed inner coroutine of request: 1249
Processed outer coroutine of request: 1249

我应该更改什么才能看到变量 myid 的意外行为？

Answer 1

上下文变量 当您需要沿调用链传递变量以便它们共享相同的上下文时，在无法通过全局变量完成的情况下，这很方便并发情况下的变量。 上下文变量 可以在多线程代码和异步（协同程序）中用作全局变量的替代方法。

asyncio 原生支持上下文变量，无需任何额外配置即可使用。因为当创建 Task 时，它会复制当前上下文，然后在复制的上下文中运行其协程：

# asyncio/task.py
class Task:
    def __init__(self, coro):
        ...
        # Get the current context snapshot.
        self._context = contextvars.copy_context()
        self._loop.call_soon(self._step, context=self._context)

    def _step(self, exc=None):
        ...
        # Every advance of the wrapped coroutine is done in
        # the task's context.
        self._loop.call_soon(self._step, context=self._context)
        ...

下面是您的示例，显示了与上下文变量相比全局变量的损坏：

import asyncio
import contextvars

# declare context var
current_request_id_ctx = contextvars.ContextVar('')
current_request_id_global = ''


async def some_inner_coroutine():
    global current_request_id_global

    # simulate some async work
    await asyncio.sleep(0.1)

    # get value
    print('Processed inner coroutine of request: {}'.format(current_request_id_ctx.get()))
    if current_request_id_global != current_request_id_ctx.get():
        print(f"ERROR! global var={current_request_id_global}")


async def some_outer_coroutine(req_id):
    global current_request_id_global

    # set value
    current_request_id_ctx.set(req_id)
    current_request_id_global = req_id

    await some_inner_coroutine()

    # get value
    print('Processed outer coroutine of request: {}\n'.format(current_request_id_ctx.get()))


async def main():
    tasks = []
    for req_id in range(1, 10000):
        tasks.append(asyncio.create_task(some_outer_coroutine(req_id)))

    await asyncio.gather(*tasks)


if __name__ == '__main__':
    asyncio.run(main())

输出：

...
Processed inner coroutine of request: 458
ERROR! global var=9999
Processed outer coroutine of request: 458

Processed inner coroutine of request: 459
ERROR! global var=9999
Processed outer coroutine of request: 459
...

可以在 PЕP 567

中找到使用 threading.local() 转换代码的示例

了解 Python 上下文变量

Understanding Python contextvars

python

multithreading

python-asyncio

python-contextvars