Yes — subflows can wait on upstream task futures before they start. In Prefect 3.x, a subflow call supports

wait_for

much like a task does:

python
from prefect import flow, task

@task
def step_a():
    print("A done")

@task
def step_b():
    print("B done")

@flow
def child_flow():
    print("child flow started")

@flow
def parent_flow():
    a = step_a.submit()
    b = step_b.submit()

    child_flow(wait_for=[a, b])

parent_flow()

Here,

child_flow

won't begin until both

a

and

b

have finished. A useful distinction: -

wait_for=[...]

= ordering only - passing a future as an argument = ordering + pass the resolved result Example of passing data instead:

python
from prefect import flow, task

@task
def make_value():
    return 42

@flow
def child_flow(x):
    print(f"got {x}")

@flow
def parent_flow():
    x = make_value.submit()
    child_flow(x)

parent_flow()

In that case, Prefect waits for

x

and passes its result into the subflow. Rule of thumb: - Use

wait_for

when you just want the subflow to start after something else - Pass futures as parameters when the subflow actually needs the data One caveat: if something in

wait_for

fails, the subflow generally won’t start unless you explicitly handle failure cases. If you want, I can also show: - subflow waiting on another subflow - using

wait_for

with

.submit()

on flows - how this appears in the UI/dependency graph