Cancellation - Python SDK feature guide

How to cancel an Activity from a Workflow using the Temporal Python SDK.

Canceling an Activity from within a Workflow requires that the Activity Execution sends Heartbeats and sets a Heartbeat Timeout. If the Heartbeat is not invoked, the Activity cannot receive a cancellation request. When any non-immediate Activity is executed, the Activity Execution should send Heartbeats and set a Heartbeat Timeout to ensure that the server knows it is still working.

When an Activity is canceled, an error is raised in the Activity at the next available opportunity. If cleanup logic needs to be performed, it can be done in a finally clause or inside a caught cancel error. However, for the Activity to appear canceled the exception needs to be re-raised.

note

Unlike regular Activities, Local Activities can be canceled if they don't send Heartbeats. Local Activities are handled locally, and all the information needed to handle the cancellation logic is available in the same Worker process.

To cancel an Activity from a Workflow Execution, call the cancel() method on the Activity handle that is returned from start_activity().

@activity.defn
async def cancellable_activity(input: ComposeArgsInput) -> NoReturn:
    try:
        while True:
            print("Heartbeating cancel activity")
            await asyncio.sleep(0.5)
            activity.heartbeat("some details")
    except asyncio.CancelledError:
        print("Activity cancelled")
        raise


@activity.defn
async def run_activity(input: ComposeArgsInput):
    print("Executing activity")
    return input.arg1 + input.arg2

@workflow.defn
 class GreetingWorkflow:
     @workflow.run
     async def run(self, input: ComposeArgsInput) -> None:
        activity_handle = workflow.start_activity(
            cancellable_activity,
            ComposeArgsInput(input.arg1, input.arg2),
            start_to_close_timeout=timedelta(minutes=5),
            heartbeat_timeout=timedelta(seconds=30),
        )

        await asyncio.sleep(3)
        activity_handle.cancel()

note

The Activity handle is a Python task. By calling cancel(), you're essentially requesting the task to be canceled.