[Feature Request] temporalio.CancelledError should inherit from BaseException

temporalio/exceptions.py の145行目にある CancelledError の定義付近から始め、その後、SDK 全体でキャンセルがどのように発生させられ、処理されているかを確認します。既存の Exception ハンドラーへの影響を確認し、継承を変更する前に必要な互換性の動作を定義します。完了の条件は、キャンセルが広範な Exception ハンドラーに意図せず捕捉されず、関連する動作がテストでカバーされていることです。

Cancelled Error Deriving from Exception makes it easy to inadvertently ignore Cancellation Attempts

Currently temporalio.CancelledError inherits from temporalio.FailureError which in turn inherits from Exception.

I would like to suggest that the semantics of this type of error would be more appropriate if it derived from BaseException, primarily because of expectations and patterns of how existing application code might be handling those various cases.

Basically it is very common in user/app code to have fairly broad exception cases like except Exception as e: etc. This is arguably not "best practice" (you should really catch more specific individual exception types), but it happens a lot.

My understanding is that Temporal Cancellation errors are implemented via a fairly low-level Python C-extension API, which allows them to forcibly preempt arbitrary user code. This means those cancellation errors can surface at arbitrary points in the call-graph, i.e. they pop up in random places in User code, not just in the "outer layer" of Temporal workflow code.

This creates a scenario where it's easy for error handling logic in random application code to unintentionally ignore Temporal Cancellation attempts, which leads to lots of confusion when workflows that should have been cancelled keep running.

I believe this type of semantics is why the core Python runtime typically uses BaseException for things like hard system interrupts, cancellation errors, OOMs, etc. And you can actually see that the stdlib changed the base class of asyncio.CancelledError
from Exception to BaseException, I believe for similar reasons.

I realize that while this change is simple to implement it would have meaningful consequences for existing user code, but I wanted to propose the idea and see if this is something that has come up in discussion before, or if the community has any other suggestions on how to handle temporal CancelledError when working with existing code that may be handling Exception broadly.