了解Python上下文

Question

关于以下SO答案。为了理解使用上下文变量和不使用上下文变量之间的区别，我做了一些更改。

我预计在某个时候，变量myid会被破坏，但将范围更改为更高的数字似乎一点也不影响。

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

上下文变量在需要沿调用链传递变量以使它们共享同一上下文时非常方便，在并发的情况下无法通过全局变量实现这一点上下文变量可以在多线程代码和异步(与协同程序(中用作全局变量的替代方案。

上下文变量在asyncio中本机支持，并且可以在没有任何额外配置的情况下使用。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同程序：

# 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)
...

以下是您的示例，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
...

---

使用threading.local()转换代码的示例可以在pεp 567 中找到