@Marvin in prefect 3 if a task is not called using the .submit() does it still use the ThreadPoolTaskRunner

In Prefect 3, if you do not use the

.submit()

method to call a task within a flow, Prefect does not utilize the

ThreadPoolTaskRunner

for that task by default. Instead, tasks in Prefect are handled differently based on how they're executed: 1. When a task is called with

.submit()

, it uses the

ThreadPoolTaskRunner

as the default task runner. This method not only submits the task but also allows the task runner to create a

PrefectFuture

, which gives you access to the task's state and result. 2. If a task is not called with

.submit()

, it will run synchronously, directly within the context of the flow's execution, and not within a separate thread managed by the

ThreadPoolTaskRunner

. This means it won't have the same asynchronous execution benefits and handling as tasks submitted using

.submit()

. For more details, you can refer to the Prefect 3 documentation on task runners.

@Marvin how to specify upstream dependencies based on a prior task completion or failed state if the prior task doesn't return any object e.g. run a shell command

In Prefect 3, you can handle upstream dependencies based on the completion or failure state of a prior task, even if the task doesn't return an object, such as a task running a shell command. Here's how you can manage task dependencies based on their states: 1. Using Task States: Prefect allows you to dictate subsequent behavior based on the outcome of previous tasks. You can use the state of a task explicitly to manage dependencies for tasks that do not return a meaningful object (e.g., shell commands). 2. Creating Conditional Dependencies: - You can write conditional logic in your flow to check the state of a task. This allows you to decide whether to proceed with subsequent tasks based on whether the prior task succeeded or failed. 3. Using Custom Triggers: - Prefect 3 supports custom trigger functions that you can write to execute tasks only if certain conditions are met. For example, you could define a simple function to check the status of a previous task and use it as a trigger for the next task. Here’s an example to illustrate conditional logic using task state:

python
from prefect import flow, task, get_run_logger
from prefect.states import Failed

@task
def run_shell_command():
    # Example shell command
    result = os.system("echo Hello World")
    if result != 0:
        raise Exception("Shell command failed")

@task
def process_results():
    get_run_logger().info("Processing results...")

@flow
def my_flow():
    try:
        shell_task = run_shell_command()
    except Failed:
        get_run_logger().info("Shell command failed. Skipping processing.")
        return
    
    process_results()

my_flow()

In this example,

run_shell_command()

executes a system command, and if it fails, the exception is caught, and the subsequent task

process_results()

is skipped. For more information about conditional dependencies and handling task failures, you can refer to Prefect 3 documentation on writing tasks and handling dependencies.