Hello, everyone! I'm new to Prefect and Python as well. Correct me if I'm wrong: tasks inside a flow will run in sequence. If a task fails the downstream ones will not be executed. On my tests, the execution flow is not deterministic. In the code below I expect the following output:

==== START ====
=== starting the flow
== generate called
= status: [1, 2, 3]
== task runner called with 1
== task runner called with 2
== task runner called with 3
==== END ====

Code:

@task
def generate_task_list() -> list:
    print("== generate called")
    return [1, 2, 3]


@task(max_retries=3, retry_delay=timedelta(seconds=0))
def task_runner(x: int) -> None:
    print('== task runner called with {}'.format(x))


@task
def print_status(text: any) -> None:
    print('= status: {}'.format(text))


with Flow("random-mapping") as f:
    print_status('=== starting the flow')
    values = generate_task_list()
    print_status(values)
    final = task_runner.map(x=values)

print('==== START ====')
f.run()
print('==== END ====')

By running this multiple times I get different results but never what I expected. What am I missing? Thank you!

Hi @Argemiro Neto! I assume

== generate called
= status: [1, 2, 3]
== task runner called with 1
== task runner called with 2
== task runner called with 3

is always printed in the correct order, but

=== starting the flow

shows up randomly, is that accurate?

that's true! Is that only because of the value dependency on the task?

if you run

f.visualize()

you’ll see how Prefect views your Flow: the

print_status

task doesn’t have any dependencies (upstream or downstream), and so can run at anytime; if you want to ensure it always runs prior to

generate_task_list

, you can update your code to:

status = print_status('=== starting the flow')
values = generate_task_list(upstream_tasks=[status])

oh, I see. Thank you so much!

@Marvin archive “Tasks are not running in the order that I expect”

👍 anytime!