@Marvin How do I make a failure in a non-blocking Task make the whole Flow fail in Prefect 2?

Ah, the joys of failure. I'm quite familiar with that. Here's how you can make a failure in a non-blocking Task cause the whole Flow to fail in Prefect 2: You can create a flow with no return statement, and its state will be determined by the state of all of its task runs. Here's an example:

python
from prefect import flow, task

@task
def always_fails_task():
    raise ValueError("I fail successfully")

@task
def always_succeeds_task():
    print("I'm fail safe!")
    return "success"

@flow
def always_fails_flow():
    always_fails_task.submit().result(raise_on_failure=False)
    always_succeeds_task()

if __name__ == "__main__":
    always_fails_flow()

In this example,

always_fails_task

is a task that always fails, and

always_succeeds_task

is a task that always succeeds. The flow

always_fails_flow

submits both tasks. If

always_fails_task

fails, it will not block the execution of

always_succeeds_task

, but it will cause the entire flow to fail. Isn't it wonderful how failure can be so... pervasive?

@Marvin I need several tasks to run concurrently AND have a failure in any of them make the whole Flow fail.

Ah, the sweet symphony of concurrent failure. Here's how you can make a failure in any concurrent Task cause the whole Flow to fail in Prefect 2: You can use the

concurrent.futures

module to run tasks concurrently, and then check the status of each task. If any task fails, you can raise an exception to make the whole flow fail. Here's an example:

python
from prefect import flow, task
from concurrent.futures import ThreadPoolExecutor

@task
def task_that_might_fail():
    # Your task code here
    pass

@flow
def flow_with_concurrent_tasks():
    with ThreadPoolExecutor(max_workers=5) as executor:
        futures = [executor.submit(task_that_might_fail) for _ in range(5)]
        for future in futures:
            if not future.result():
                raise ValueError("A task failed")

if __name__ == "__main__":
    flow_with_concurrent_tasks()

In this example,

task_that_might_fail

is a task that might fail, and

flow_with_concurrent_tasks

is a flow that runs five instances of

task_that_might_fail

concurrently. If any instance of

task_that_might_fail

fails, it will cause the entire flow to fail. Isn't it fascinating how failure can spread like a virus?

@Marvin I need several tasks to run concurrently AND have a failure in any of them make the whole Flow fail, after submitted to a Prefect 2

ConcurrentTaskRunner

Ah, the delightful complexity of concurrent failures in Prefect 2. Here's how you can make a failure in any concurrent Task cause the whole Flow to fail: You can use Prefect's

ConcurrentTaskRunner

to run tasks concurrently. If any task fails, the

ConcurrentTaskRunner

will raise an exception and cause the whole flow to fail. Here's an example:

python
from prefect import Flow, Task, ConcurrentTaskRunner
from prefect.engine.results import Result

class TaskThatMightFail(Task):
    def run(self):
        # Your task code here
        pass

class MyConcurrentTaskRunner(ConcurrentTaskRunner):
    def run(self, *args, **kwargs):
        result = super().run(*args, **kwargs)
        if isinstance(result, Result) and result.is_failed():
            raise ValueError("A task failed")
        return result

with Flow("My Flow") as flow:
    task_runner = MyConcurrentTaskRunner(task=TaskThatMightFail(), max_workers=5)
    task_runner.run()

flow.run()

In this example,

TaskThatMightFail

is a task that might fail, and

MyConcurrentTaskRunner

is a task runner that runs tasks concurrently using

ConcurrentTaskRunner

. If any task fails,

MyConcurrentTaskRunner

raises an exception, causing the entire flow to fail. Isn't it wonderful how failure can be so... contagious?