如果任务对象存储在实例变量中，Starlette + asyncio.create_task() 不会记录错误

Question

好吧，这很奇怪，但是-

import asyncio

from starlette.applications import Starlette


class MyTasks:
    def __init__(self):
        self.task = None

    async def main(self):
        self.task = asyncio.create_task(self.hello())

    async def hello(self):
        raise ValueError


async def main():
    await MyTasks().main()


app = Starlette(on_startup=[main])

$ uvicorn test:app
INFO:     Started server process [26622]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
INFO:     Uvicorn running on http://127.0.0.1:8000 (Press CTRL+C to quit)

嗯，这里没有 ValueError...

现在，删除 MyTasks.main() 中对 self.task 的分配。

    async def main(self):
        asyncio.create_task(self.hello())
    ...

$ uvicorn test:app
INFO:     Started server process [29083]
INFO:     Waiting for application startup.
ERROR:    Task exception was never retrieved
future: <Task finished name='Task-3' coro=<MyTasks.hello() done, defined at ./test.py:13> exception=ValueError()>
Traceback (most recent call last):
  File "./test.py", line 14, in hello
    raise ValueError
ValueError
INFO:     Application startup complete.
INFO:     Uvicorn running on http://127.0.0.1:8000 (Press CTRL+C to quit)

... 瞧。

这是怎么回事？该分配如何建立或破坏异常记录！？

Answer 1

任务是future的子类，也就是说它有结果的概念。在任务的情况下，结果是它驱动的协程返回的值。如果协程引发异常，则异常会封装在任务对象中。正确编写的代码应该最终等待任务或访问其结果，以便异常不会静默传递。

为了帮助调试忘记访问任务的代码，如果任务引发异常但从未等待，则任务的析构函数会记录错误。此错误无法在在运行析构函数之前记录，因为只要您的代码持有任务对象，就可以想象它随时等待它。析构函数运行的时间点是 Python 可以可靠地 "prove" 未等待任务的第一个实例。

但是这种最后的日志记录不是您应该依赖的东西，它是在尽力而为的基础上提供的。例如，运行析构函数可以因 GC 而推迟。我希望将任务分配给一个实例，该实例的绑定方法由该任务驱动，从而使该任务成为引用循环的一部分。这会将运行析构函数推迟到完全 GC 之前，并且您看不到日志。

要解决此问题，您应该在协程中捕获异常并自己记录它们而不是让它们传播，或者在代码中的某个时刻实际等待任务。

Answer 2

抱歉，这是 this other question 的骗局。

asyncio.create_task() has been posted there.

的直接替代品

这是使用替换 create_task()

后的输出

案例 1：使用 await self.task

$ uvicorn test:app
INFO:     Started server process [33213]
INFO:     Waiting for application startup.
ERROR:    Traceback (most recent call last):
  File "/Users/dev/.virtualenvs/server-99338def/lib/python3.8/site-packages/starlette/routing.py", line 517, in lifespan
    await self.startup()
  File "/Users/dev/.virtualenvs/server-99338def/lib/python3.8/site-packages/starlette/routing.py", line 494, in startup
    await handler()
  File "./test.py", line 35, in main
    await MyTasks().main()
  File "./test.py", line 27, in main
    print(await self.task)
  File "./test.py", line 13, in wrapper
    return await task
  File "./test.py", line 30, in hello
    raise ValueError
ValueError

ERROR:    Application startup failed. Exiting.

情况 2：没有 await self.task

$ uvicorn test:app
INFO:     Started server process [32627]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
ERROR:    Exception in callback <function create_task.<locals>.on_done at 0x10c519550>
handle: <Handle create_task.<locals>.on_done created at /Users/dev/.virtualenvs/server-99338def/lib/python3.8/site-packages/uvicorn/main.py:382>
source_traceback: Object created at (most recent call last):
  File "/Users/dev/.virtualenvs/server-99338def/bin/uvicorn", line 8, in <module>
    sys.exit(main())
  File "/Users/dev/.virtualenvs/server-99338def/lib/python3.8/site-packages/click/core.py", line 764, in __call__
    return self.main(*args, **kwargs)
  File "/Users/dev/.virtualenvs/server-99338def/lib/python3.8/site-packages/click/core.py", line 717, in main
    rv = self.invoke(ctx)
  File "/Users/dev/.virtualenvs/server-99338def/lib/python3.8/site-packages/click/core.py", line 956, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/Users/dev/.virtualenvs/server-99338def/lib/python3.8/site-packages/click/core.py", line 555, in invoke
    return callback(*args, **kwargs)
  File "/Users/dev/.virtualenvs/server-99338def/lib/python3.8/site-packages/uvicorn/main.py", line 331, in main
    run(**kwargs)
  File "/Users/dev/.virtualenvs/server-99338def/lib/python3.8/site-packages/uvicorn/main.py", line 354, in run
    server.run()
  File "/Users/dev/.virtualenvs/server-99338def/lib/python3.8/site-packages/uvicorn/main.py", line 382, in run
    loop.run_until_complete(self.serve(sockets=sockets))
Traceback (most recent call last):
  File "uvloop/cbhandles.pyx", line 70, in uvloop.loop.Handle._run
  File "./test.py", line 9, in on_done
    fut.result()
  File "./test.py", line 30, in hello
    raise ValueError
ValueError

如果任务对象存储在实例变量中，Starlette + asyncio.create_task() 不会记录错误

Starlette + asyncio.create_task() doesn't log error if task object is stored in instance variable

python

python-asyncio

asgi

starlette

uvicorn