@Marvin What task or flow return values are associated with success or failure

For tasks in Prefect, a return value typically indicates the task's success. As you can see in the following snippet:

python
from prefect import flow, task

@task
def my_task():
    return 1

@flow
def my_flow():
    task_result = my_task()
    return task_result + 1

result = my_flow()
assert result == 2

Here, the task

my_task

returns

1

, and the flow

my_flow

adds

1

to the

task_result

, resulting in a successful

assert

statement that the final

result

is

2

. For flows, a failure is generally indicated by an exception occurring during the flow's execution. For 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()  # This flow will ultimately fail due to the always_fails_task

In the above flow

always_fails_flow

, the task

always_fails_task

raises a

ValueError

, which will result in the flow being marked as failed even though there is another task,

always_succeeds_task

, that returns

"success"

. For a more detailed understanding or if you require specifics about the state objects and their methods, you can refer to the documents on results and flows.

@Marvin what does subprocess.run return on success or failure

The

subprocess.run

function in Python is used to run a command in a subshell. On success or failure, it returns a

CompletedProcess

instance. - On Success: When the command runs successfully (i.e., finishes with an exit code of

0

), the

CompletedProcess

instance will have attributes like

returncode

,

stdout

, and

stderr

. The

returncode

will be

0

. If you capture the output with

stdout=subprocess.PIPE

, you can access the output via the

stdout

attribute.

python
import subprocess

completed = subprocess.run(['echo', 'Hello, World!'], stdout=subprocess.PIPE, text=True)
if completed.returncode == 0:
    print('Command succeeded, output:', completed.stdout)

- On Failure: If the command finishes with a non-zero exit code (indicating an error), the

CompletedProcess

instance's

returncode

will contain that exit code. The

stderr

attribute will contain the error output if

stderr=subprocess.PIPE

is used.

python
import subprocess

completed = subprocess.run(['ls', 'nonexistentfile'], stderr=subprocess.PIPE, text=True)
if completed.returncode != 0:
    print('Command failed with return code', completed.returncode, 'error message:', completed.stderr)

Remember,

subprocess.run

does not raise an exception on a non-zero exit code by default. If you want an exception to be raised, you should pass `check=True`:

python
import subprocess

try:
    subprocess.run(['ls', 'nonexistentfile'], check=True)
except subprocess.CalledProcessError as e:
    print('Command failed with return code', e.returncode)

In this case, if the command fails, a

CalledProcessError

exception will be raised, and you can handle it accordingly.