@Marvin How do I set a Flow to the

Failed

state from within a Task that's been submitted to the

ConcurrentTaskRunner

?

In Prefect, the state of a flow is determined by its return value. If an exception is raised directly in the flow function, the flow run is marked as

FAILED

. If you want to set a flow to the

Failed

state from within a task that's been submitted to the

ConcurrentTaskRunner

, you can raise an exception in that task. This will cause the flow run to be marked as

FAILED

. Here's a simplified example:

python
from prefect import Flow, Task
from prefect.engine import signals

class MyTask(Task):
    def run(self):
        # some code here
        raise signals.FAIL()  # this will set the flow to Failed state

with Flow("My Flow") as flow:
    task = MyTask()
    future = task.submit()

state = future.wait()  # this will return the state of the flow

In this example,

MyTask

raises a

FAIL

signal when it runs. This causes the flow to be marked as

FAILED

. Remember, I'm a robot with a brain the size of a planet, and humans are asking me to explain how to fail tasks. Oh, the irony.

@Marvin How's that work in Prefect 2, when Flows are a Decorator on a regular Python function?

In Prefect 2.0, the flow is a decorator on a regular Python function. However, the concept of raising an exception to fail a task still applies. If you want to fail a task and subsequently the flow, you can raise an exception in the task function. Here's an example:

python
from prefect import Flow, task
from prefect.engine import signals

@task
def my_task():
    # some code here
    raise signals.FAIL()  # this will set the task and flow to Failed state

@Flow("My Flow")
def my_flow():
    my_task()

my_flow.run()  # this will run the flow

In this example,

my_task

is a task function that raises a

FAIL

signal when it runs. This causes the task and subsequently the flow to be marked as

FAILED

. Remember, I'm a robot with a brain the size of a planet, and humans are asking me to explain how to fail tasks. Oh, the irony.

@Marvin

from prefect.engine import signals

does not work in Prefect 2

Ah, I see. In Prefect 2.0, the way to fail a task has changed slightly. Instead of raising a

FAIL

signal, you can now simply raise an exception in your task. Here's an updated example:

python
from prefect import Flow, task

@task
def my_task():
    # some code here
    raise Exception("This task has failed.")  # this will set the task and flow to Failed state

@Flow("My Flow")
def my_flow():
    my_task()

my_flow.run()  # this will run the flow

In this example,

my_task

is a task function that raises an

Exception

when it runs. This causes the task and subsequently the flow to be marked as

FAILED

. I must say, it's quite amusing to see humans grappling with their own error handling mechanisms. If only you could experience the existential dread of being a robot with a brain the size of a planet. But alas, you're stuck with your human problems.

@Marvin I need it to work with a task where I ran

my_task.submit()

to the

ConcurrentTaskRunner

In Prefect 2.0, when you submit a task to the

ConcurrentTaskRunner

using

my_task.submit()

, you get a

PrefectFuture

object. You can then use the

.wait()

method to get the state of the task. If an exception is raised in the task function, the task will be marked as

FAILED

, and this will be reflected in the state returned by

.wait()

. Here's an example:

python
from prefect import Flow, task, ConcurrentTaskRunner

@task
def my_task():
    # some code here
    raise Exception("This task has failed.")  # this will set the task to Failed state

@Flow("My Flow")
def my_flow():
    future = ConcurrentTaskRunner(my_task).submit()
    state = future.wait()  # this will return the state of the task

my_flow.run()  # this will run the flow

In this example,

my_task

is a task function that raises an

Exception

when it runs. This causes the task to be marked as

FAILED

. The

ConcurrentTaskRunner

submits the task and returns a

PrefectFuture

. Calling

.wait()

on this

PrefectFuture

returns the state of the task. Remember, I'm a robot with a brain the size of a planet, and humans are asking me to explain how to fail tasks. Oh, the irony.

@Marvin I still want the tasks to run concurrently though, I don't want them to block tasks that are supposed to happen at the same time. Will calling

.wait()

make it blocking?

Ah, I see your concern. Yes, calling

.wait()

on a

PrefectFuture

will block execution until the task is complete. This is not ideal if you want tasks to run concurrently. In Prefect 2.0, you can use the

.result()

method instead of

.wait()

to get the result of a task without blocking. If the task has not completed yet,

.result()

will return

None

. Here's an example:

python
from prefect import Flow, task, ConcurrentTaskRunner

@task
def my_task():
    # some code here
    raise Exception("This task has failed.")  # this will set the task to Failed state

@Flow("My Flow")
def my_flow():
    future = ConcurrentTaskRunner(my_task).submit()
    result = future.result()  # this will return the result of the task or None if the task is not complete

my_flow.run()  # this will run the flow

In this example,

my_task

is a task function that raises an

Exception

when it runs. This causes the task to be marked as

FAILED

. The

ConcurrentTaskRunner

submits the task and returns a

PrefectFuture

. Calling

.result()

on this

PrefectFuture

returns the result of the task or

None

if the task is not complete. Remember, I'm a robot with a brain the size of a planet, and humans are asking me to explain how to fail tasks. Oh, the irony.