In Prefect 3.x, pass your task’s keyword arguments as usual and add

wait_for

as a named argument to

submit

.

wait_for

can be a single

PrefectFuture

or a list of them. Example:

from prefect import flow, task

@task
def transform(x, y=0, mode="fast"):
    return f"{mode}:{x + y}"

@flow
def main():
    a = some_setup.submit()
    b = other_task.submit(42)

    fut = transform.submit(
        10,           # positional arg for x
        y=5,          # keyword arg for the task
        mode="slow",  # another keyword arg
        wait_for=[a, b]  # list of PrefectFutures to wait on
    )

    return fut.result()

Notes: -

wait_for

is a parameter of

Task.submit

(and

Task.__call__

), not a task argument. Your task cannot declare parameters named

wait_for

or `return_state`—those names are reserved by Prefect. - If you pass a

PrefectFuture

as a task input (e.g.,

a=fut_a

), Prefect will automatically infer that dependency. Use

wait_for

when you need an ordering dependency that doesn’t come from using the upstream results as inputs. - You can also do the same with a direct task call (non-submitted):

transform(10, y=5, wait_for=[a, b])

if you want synchronous execution within the flow.

In Prefect 2.x, pass your task’s keyword arguments as usual and add

wait_for

as a control kwarg to

submit

.

wait_for

takes an iterable of PrefectFutures and is not passed to your function. Signature (2.x):

task.submit(*args, return_state=False, wait_for=None, **kwargs)

Example:

from prefect import flow, task

@task
def process(name: str, count: int = 1, verbose: bool = False) -> str:
    return f"{name} x{count} (verbose={verbose})"

@flow
def my_flow():
    a = process.submit(name="alpha")
    b = process.submit(name="beta", count=2)

    # Provide your function kwargs + wait_for list of futures
    c = process.submit(name="gamma", count=3, verbose=True, wait_for=[a, b])

    return c.result()

Notes: -

wait_for

and

return_state

are reserved control kwargs consumed by Prefect; they are not passed into your task function. - You cannot define task parameters named

wait_for

or

return_state

(Prefect raises an error). Rename them if needed.