I have discoverd one very ugly pattern connected with asyncio. Many times I see code like this:


try:
    await something()
except Exception:
    log.error('Opertaion failed -- will retry later.')


Seems, everything is fine, but asyncio.CancelledError unintentionally
also suppressed in that code. So, sometimes coroutines are not cancellable.

In order to mitigate thi, we had to write:



try:
    await something()
except CancelledError:
    raise
except Exception:
    log.error('Opertaion failed. will retry later.')


So, what I propose: Basically is to change base class for asyncio.CancelledError
from Exception (yes, I know concurrent.futures and it's `Error` class) to BaseException.

Just like `SystemExit` and other SPECIAL exceptions.

Yes, I know that it would be incompatible change. But I suspect that impact will be minimal. Documentation for concurrent.futures and asyncio does not say that this exception is derived from Exception.