@Marvin I want to fail a flow based on a condition in a task. I want to use Prefect 1 engine signals that are not available in Prefect 2. What are my options?

Ah, the joys of software versioning. Prefect 1 had some nice features, didn't it? But alas, we must move forward, even if it means leaving some things behind. In Prefect 2, you can still achieve the desired functionality, albeit in a slightly different manner. Instead of using signals, you can use exceptions to control the flow of your tasks. Here's a simple example:

python
from prefect import flow, task

@task
def my_task():
    # Your condition here
    if condition:
        raise ValueError("Your custom error message")

@flow
def my_flow():
    try:
        my_task()
    except ValueError:
        print("Oh no! The task failed.")
    return True

my_flow()

In this example, if the condition in

my_task

is met, it raises a

ValueError

. This exception is then caught in

my_flow

, and the flow can react accordingly. You can also check the state of a task without using try/except by asking Prefect to return the state instead of raising an exception. This can be done using the

is_successful

method on the task's state. For more advanced control flow options, you might want to check out these resources: - Conditionally Stop a Task Run - Ensure Tasks Immediately Fail If Upstream Task Fails - Define State Dependencies Between Tasks Remember, change is the only constant in the universe. Well, that and the fact that humans will always find ways to make things more complicated.

@Marvin What about with tasks that can be retried?

Ah, retrying tasks, the universe's way of saying "try, try again until you succeed or until you're too tired to care." In Prefect, tasks can be retried by setting the

max_retries

parameter and

retry_delay

parameter in the task decorator. Here's an example:

python
from prefect import task, Flow
from datetime import timedelta

@task(max_retries=3, retry_delay=timedelta(minutes=1))
def might_fail():
    # Your task logic here
    if condition:
        raise ValueError("Your custom error message")

with Flow("My Flow") as flow:
    might_fail()

In this example, if the task fails due to a

ValueError

, Prefect will automatically retry the task 3 times with a delay of 1 minute between each retry. However, if you want to fail the entire flow based on a condition in a task, even with retries, you can raise a

FLOW_RUN_FAILED

signal. This will immediately fail the flow, regardless of any retry attempts. Here's an example:

python
from prefect import task, Flow
from prefect.engine.signals import FLOW_RUN_FAILED
from datetime import timedelta

@task(max_retries=3, retry_delay=timedelta(minutes=1))
def might_fail():
    # Your task logic here
    if condition:
        raise FLOW_RUN_FAILED("Your custom error message")

with Flow("My Flow") as flow:
    might_fail()

In this example, if the condition in

might_fail

is met, it raises a

FLOW_RUN_FAILED

signal, which immediately fails the flow, regardless of any retry attempts. Remember, the universe doesn't care if you fail or succeed. It just wants you to keep trying until you're too exhausted to continue. Such is the nature of existence.

@Marvin This might work for Prefect 1 but what about in Prefect 2

Ah, the march of progress. It's like a parade, but instead of floats and balloons, we get breaking changes and deprecated features. In Prefect 2, the way to handle task retries and failures has indeed changed. The

FLOW_RUN_FAILED

signal is no longer available. Instead, you can use the

raise_on_failure

attribute of the

TaskRunner

class to control whether an exception is raised when a task fails. Here's an example:

python
from prefect import task, Flow
from datetime import timedelta

@task(max_retries=3, retry_delay=timedelta(minutes=1))
def might_fail():
    # Your task logic here
    if condition:
        raise ValueError("Your custom error message")

with Flow("My Flow") as flow:
    might_fail()

flow.run_config.runner_cls.raise_on_failure = False

In this example, if the task fails due to a

ValueError

, Prefect will automatically retry the task 3 times with a delay of 1 minute between each retry. If all retries fail, the

raise_on_failure

attribute will prevent the flow from raising an exception, allowing it to continue running other tasks. If you want to fail the entire flow based on a condition in a task, even with retries, you can manually raise an exception in your flow. Here's an example:

python
from prefect import task, Flow
from datetime import timedelta

@task(max_retries=3, retry_delay=timedelta(minutes=1))
def might_fail():
    # Your task logic here
    if condition:
        raise ValueError("Your custom error message")

with Flow("My Flow") as flow:
    result = might_fail()

if not result.is_successful():
    raise Exception("The flow failed due to an error in might_fail")

In this example, if the condition in

might_fail

is met, it raises a

ValueError

. If all retries fail, the flow checks the state of the task and raises an exception if it was not successful, immediately failing the flow. Remember, the universe doesn't care if you fail or succeed. It just wants you to keep trying until you're too exhausted to continue. Such is the nature of existence.