wait-for2 0.1.0

Asyncio wait_for that can handle simultaneous cancellation and future completion.

wait_for2

Alternate implementation of asyncio.wait_for() based on the version from Python 3.8. It handles simultaneous cancellation of wait and completion of future differently and consistently across Python versions 3.6+.

Details

Builtin asyncio.wait_for() behaviours:

• Python 3.6 and 3.7: Cancellation of wait_for could lose the completed future's result.
• Python 3.8+ and PyPy3: Cancellation of wait_for could lose the cancellation request.

Whenever waiting for a future's result the user expects to either have the future completed or cancelled. Unfortunately due to technical details there is a chance that both will happen simultaneously. The builtin version of asyncio.wait_for() shipped with Python either handles one or the other only. If losing the future's result or ignoring the cancellation is critical to the application it may not be suitable for use.

Using this implementation, in case both conditions occur at the same time a subclassed CancelledError will be raised which also contains the result of the future. The caller code must catch this exception and handle the result if it is important. Otherwise it can be used the same way as the builtin wait_for.

NOTE: CancelledWithResultError is limited to the coroutine wait_for is invoked from! If this wait_for is wrapped in tasks those will not propagate the special exception, but raise their own CancelledError instances.

Install & usage

A source distribution is available on PyPI:

$ python -m pip install wait_for2

import asyncio
import wait_for2

task = asyncio.create_task(...)

async def process_result(r):
    print("processed:", r)

...

try:
    await process_result(await wait_for2.wait_for(task, 5.0))
except wait_for2.CancelledWithResultError as e:
    # NOTE: e.result could be an exception raised by the task; handling or ignoring it is up to the user code here
    await process_result(e.result)
    raise asyncio.CancelledError()