How to find the cause of CancelledError in asyncio?

Question

I have a big project which depends some third-party libraries, and sometimes its execution gets interrupted by a CancelledError.

To demonstrate the issue, let’s look at a small example:

import asyncio


async def main():
    task = asyncio.create_task(foo())

    # Cancel the task in 1 second.
    loop = asyncio.get_event_loop()
    loop.call_later(1.0, lambda: task.cancel())

    await task


async def foo():
    await asyncio.sleep(999)


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

Traceback:

Traceback (most recent call last):
  File "/Users/ss/Library/Application Support/JetBrains/PyCharm2021.2/scratches/async.py", line 19, in <module>
    asyncio.run(main())
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/asyncio/runners.py", line 43, in run
    return loop.run_until_complete(main)
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/asyncio/base_events.py", line 579, in run_until_complete
    return future.result()
concurrent.futures._base.CancelledError

As you can see, there’s no information about the place the CancelledError originates from. How do I find out the exact cause of it?

One approach that I came up with is to place a lot of try/except blocks which would catch the CancelledError and narrow down the place where it comes from. But that’s quite tedious.

Asked By: MMM

||

Source

Answer 1

It is not possible to handle an async exception of a task if it does not call the call_exception_handler inside of it. That provides a context to the exception handler which can be customized using set_exception_handler.

If you are creating an async task you have to use try/except in the coroutine where possibly can occur an exception. I have demonstrated some minimal implementation to catch exceptions of async tasks using set_exception_handler

import asyncio
import logging
from asyncio import CancelledError


def async_error_handler(loop, context):
    logger = logging.Logger(__name__)
    logger.error(context.get("message"))


async def main():
    loop = asyncio.get_running_loop()
    
    # Set custom exception handler
    loop.set_exception_handler(async_error_handler)
    task = loop.create_task(foo())

    # Cancel the task after 1 second
    loop.call_later(1, task.cancel)

    await task


async def foo():
    try:
        await asyncio.sleep(999)
    except CancelledError:
        # Catch the case when the coroutine has been canceled
        loop = asyncio.get_running_loop()

        # Emit an event to exception handler with custom context
        loop.call_exception_handler(context={
            "message": "Task has been canceled"
        })


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

The context has more attributes that can also be customized. Read more about error handling here.

Answered By: Artyom Vancyan

Answer 2

I’ve solved it by applyting a decorator to every async function in the project. The decorator’s job is simple – log a message when a CancelledError is raised from the function. This way we will see which functions (and more importantly, in which order) get cancelled.

Here’s the decorator code:

def log_cancellation(f):
    async def wrapper(*args, **kwargs):
        try:
            return await f(*args, **kwargs)
        except asyncio.CancelledError:
            print(f"Cancelled {f}")
            raise
    return wrapper

In order to add this decorator everywhere I used regex. Find: (.*)(async def). Replace with: $1@log_cancellationn$1$2.

Also to avoid importing log_cancellation in every file I modified the builtins:
builtins.log_cancellation = log_cancellation

Answered By: MMM

Answer 3

The rich package has helped us to identify the cause of CancelledError, without much code change required.

from rich.console import Console

console = Console()

if __name__ == "__main__":
    try:
        asyncio.run(main())  # replace main() with your entrypoint
    except BaseException as e:
        console.print_exception(show_locals=True)

Answered By: Higgs

How to find the cause of CancelledError in asyncio?

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