Hello, having an issue with passing parameters/context from flow to flow:

@task
def log_scheduled_start_time():
    """Print scheduled_start_time in logger."""
    logger = prefect.context.get("logger")
    scheduled_start_time = prefect.context.get("scheduled_start_time")
    flow_name = prefect.context.get("flow_name")
    <http://logger.info|logger.info>(f"{flow_name}: {scheduled_start_time}")


with Flow("subflow") as subflow:
    log_scheduled_start_time()

subflow.register(project_name="test_schedule", labels=["test"])

with Flow("mainflow") as mainflow:
    log_scheduled_start_time()
    StartFlowRun(
        flow_name="subflow", project_name="test_schedule", wait=True,
        scheduled_start_time=prefect.context.get("scheduled_start_time")
    )

mainflow.register(project_name="test_schedule", labels=["test"])

Expectation: mainflow's

scheduled_start_time

is passed down to subflow, both flows log the same time. Reality: subflow starts without a

scheduled_start_time

, defaults to

now()

, logged times are different. Use case: Subflow depends on

scheduled_start_time

to query some database, the agent that was supposed to run mainflow is down, mainflow and subflow are run a day later, subflow's query is then incorrect. Bonus question: I can't seem to find documentation on how to pass parameters from one flow to another/use a flow's results as parameters for another 🙇

Hi Josselin - the reason for the discrepancy is that you are providing the scheduled start time at build time, not at runtime. At build time, this value is

None

in context. To resolve this, you instead need to provide this value via a task:

with Flow("mainflow") as mainflow:
    log_scheduled_start_time()
    start_time = task(lambda: prefect.context.get("scheduled_start_time"))

    # first initialize the task, then call it to bind it to the flow
    run_task = StartFlowRun(
        flow_name="subflow", project_name="test_schedule", wait=True
    )(scheduled_start_time=start_time)

For passing results between Flows, there is no concept of a “Flow result” (only tasks produce results) but this is something that we are thinking about and exploring designs for right now.

Yep that's it, thank you.