@Marvin, is this correct to implement the on_failure usage in Prefect 2.x?

from prefect import flow, task

@task
def task1():
    raise ValueError("oh no!")

@task
def task2():
    print('Test')

@flow
def my_flow():
    task1.on_failure(task2())
    
state = my_flow(return_state=True)
result = state.result(raise_on_failure=False)
print(result)

The implementation you've provided for using an

on_failure

callback in Prefect 2.x isn't correct. Prefect 2.x doesn't support directly attaching an

on_failure

callback to a task in the way you've shown. Instead, you can use state change hooks at the flow level. Here's a more appropriate way to implement an

on_failure

hook using state change hooks in Prefect 2.x:

python
from prefect import flow, task
from prefect.states import Failed

@task
def task1():
    raise ValueError("oh no!")

@task
def task2():
    print('Task2 executed due to failure.')

def on_failure_callback(flow, flow_run, state):
    if state.is_failed():
        task2()

@flow
def my_flow():
    task1()

if __name__ == "__main__":
    my_flow().state.add_state_handler(on_failure_callback)

In this example,

on_failure_callback

is defined to execute

task2

when the flow enters a failed state. You can find more about state change hooks in the Prefect documentation.

@Marvin, actually, if look for "on_failure" in Prefect 2.x, you will find. So, this exists.

I couldn't find specific documentation directly mentioning an

on_failure

method or attribute for tasks in Prefect 2.x. However, Prefect does support state change hooks and notifications, which can be configured to respond to task or flow failures. It seems that the

on_failure

you're referring to might be part of a different mechanism or a misunderstanding. If you have seen

on_failure

used in Prefect 2.x elsewhere, it might be a custom implementation or part of a community extension. For built-in functionality, using state change hooks or Prefect's notification system is the recommended approach. For more details on state change hooks, you can refer to the Prefect documentation on state change hooks. If you have a specific example or context where you've seen

on_failure

, feel free to share it for further clarification.