How to pass data from a child flow to another task in a parent flow?

It’s not directly possible to pass data in-memory from create_flow_run and StartFlowRun tasks to the parent flow because the execution of the respective flow runs happens on different executors. Therefore, intermediate persistence of data is needed.

The code below is for Prefect 1.0. In Prefect 2.0 subflows is a first-class feature and you can pass data dependencies in the same way you do it using tasks.

Here is how you can pass data between flows in Prefect 1.0 (example from this article):

from typing import List

from prefect import Flow, Parameter, task
from prefect.engine.results import LocalResult
from prefect.tasks.prefect import create_flow_run, get_task_run_result

def create_some_data(length: int):
    return list(range(length))

with Flow("child") as child_flow:
    data_size = Parameter("data_size", default=5)
    data = create_some_data(data_size)

def transform_and_show(data: List[int]) -> List[int]:
    print(f"Got: {data!r}")
    new_data = [x + 1 for x in data]
    print(f"Created: {new_data!r}")
    return new_data

with Flow("parent") as parent_flow:
    child_run_id = create_flow_run(, parameters=dict(data_size=10)
    child_data = get_task_run_result(child_run_id, "create_some_data-1")

The transform_and_show task could be anything (including downstream tasks of the parent flow).

Note: create_flow_run , wait_for_flow_run , get_task_run_result have been implemented as a more consistent interface with flow runs (instead of StartFlowRun of which return value depended on the wait argument). The get_task_run_result task retrieves the result based on a task-slug.