Hi, I recently encountered a situation with asyncio where the stack
trace is getting truncated: an exception isn't getting chained as
expected.
I was able to reduce it down to the code below.
The reduced case seems like a pattern that can come up a lot, and I
wasn't able to find an issue on the CPython tracker, so I'm wondering
if I'm doing something wrong or if the behavior is deliberate.
The way I would describe it is that (at least in this case) awaiting
on a task that raises an exception doesn't set __context__ when
awaiting on the task inside an "except" block. Here is the code:
import asyncio
async def raise_error():
raise RuntimeError('exception #2')
async def main():
try:
raise RuntimeError('exception #1')
except Exception:
future = raise_error()
# Uncommenting the next line makes exc.__context__ None.
# future = asyncio.ensure_future(future)
try:
await future
except Exception as exc:
print(f'exc: {exc!r}, context: {exc.__context__!r}')
raise
asyncio.get_event_loop().run_until_complete(main())
Here is the output of the above, which works as expected:
exc: RuntimeError('exception #2',), context: RuntimeError('exception #1',)
Traceback (most recent call last):
File "test.py", line 8, in main
raise RuntimeError('exception #1')
RuntimeError: exception #1
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "test.py", line 20, in <module>
asyncio.get_event_loop().run_until_complete(main())
File "/.../python3.6/asyncio/base_events.py", line 466, in run_until_complete
return future.result()
File "test.py", line 15, in main
await future
File "test.py", line 4, in raise_error
raise RuntimeError('exception #2')
RuntimeError: exception #2
And here is the unexpected output when the commented line is
uncommented (exception not getting chained):
exc: RuntimeError('exception #2',), context: None
Traceback (most recent call last):
File "test.py", line 22, in <module>
asyncio.get_event_loop().run_until_complete(main())
File "/.../python3.6/asyncio/base_events.py", line 466, in run_until_complete
return future.result()
File "test.py", line 17, in main
await future
File "test.py", line 6, in raise_error
raise RuntimeError('exception #2')
RuntimeError: exception #2
--Chris
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