Original error not present in traceback

[davidbrochart]

I don't think it's specifically an AnyIO issue, but I find it strange that the following code crashes with no mention to the original error (that bar is undefined):

from contextlib import AsyncExitStack

from anyio import Event, create_task_group, run


class Foo:
    def __init__(self):
        self.started = Event()

    async def __aenter__(self):
        async with AsyncExitStack() as exit_stack:
            self.task_group = await exit_stack.enter_async_context(create_task_group())
            self._exit_stack = exit_stack.pop_all()
            self.task_group.start_soon(self.task)
            await self.started.wait()
        return self

    async def __aexit__(self, exc_type, exc_value, exc_tb):
        return await self._exit_stack.__aexit__(exc_type, exc_value, exc_tb)

    async def task(self):
        bar  # should raise a NameError
        self.started.set()


async def main():
    async with Foo():
        pass

run(main)

It just raises a CancelledError.
Catching the CancelledError while waiting for the started event gives a traceback which leads to the original NameError:

from contextlib import AsyncExitStack

from anyio import Event, create_task_group, get_cancelled_exc_class, run


class Foo:
    def __init__(self):
        self.started = Event()

    async def __aenter__(self):
        async with AsyncExitStack() as exit_stack:
            self.task_group = await exit_stack.enter_async_context(create_task_group())
            self._exit_stack = exit_stack.pop_all()
            self.task_group.start_soon(self.task)
            try:
                await self.started.wait()
            except get_cancelled_exc_class():
                pass
        return self

    async def __aexit__(self, exc_type, exc_value, exc_tb):
        return await self._exit_stack.__aexit__(exc_type, exc_value, exc_tb)

    async def task(self):
        bar
        self.started.set()


async def main():
    async with Foo():
        pass

run(main)

So this means that CancelledError is raised before the original error, but shouldn't it be the opposite?

[graingert]

There's a mistake in how you are running your AsyncExitStack:

--- before.py	2024-11-30 07:16:46.519537586 +0000
+++ after.py	2024-11-30 07:17:41.005645305 +0000
@@ -10,9 +10,9 @@
     async def __aenter__(self):
         async with AsyncExitStack() as exit_stack:
             self.task_group = await exit_stack.enter_async_context(create_task_group())
-            self._exit_stack = exit_stack.pop_all()
             self.task_group.start_soon(self.task)
             await self.started.wait()
+            self._exit_stack = exit_stack.pop_all()
         return self
 
     async def __aexit__(self, exc_type, exc_value, exc_tb):

the pop all needs to be the absolute last operation before the return

[davidbrochart]

Oh, thanks so much @graingert !