So the graph looks like this, when I remove the

B --state--> C

edge everything works fine. I'm running the example on local prefect server (

prefect server start

) and LocalAgent.

A --data--> B --state--v
\-------data---------> C

Here is the code:

import os
from typing import List

import prefect
from prefect import Flow, task
from prefect.engine.results import LocalResult


@task(name="A")
def task_a() -> List[int]:
    return [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]


@task(name="B")
def task_b(foo: List) -> None:
    <http://prefect.context.logger.info|prefect.context.logger.info>(f"Task B {foo}")


@task(name="C")
def task_c(foo: List[int]) -> None:
    <http://prefect.context.logger.info|prefect.context.logger.info>(f"Task C {foo}")


result_directory = os.path.abspath(
    os.path.join(os.path.dirname(__file__), "..", "results")
)

result = LocalResult(
    dir=result_directory,
    location="{flow_name}/"
    "{scheduled_start_time:%d-%m_%H-%M-%S}/"
    "{task_full_name}-{task_run_id}.prefect_result",
)

with Flow(name="test_flow", result=result) as flow:
    a = task_a()
    b = task_b(a)
    c = task_c(a)

    # Commenting out this line removes the state dependency and the flow works
    # just fine
    flow.set_dependencies(task=c, upstream_tasks=[b])

if __name__ == "__main__":
    flow.register(project_name="default")

and here is the error:

5 November 2020,03:16:33 	prefect.LocalResult	DEBUG	Starting to upload result to test_flow/05-11_14-16-28/A-6007ec12-1de1-42c0-a9db-7cc500e017e7.prefect_result...
5 November 2020,03:16:33 	prefect.LocalResult	DEBUG	Finished uploading result to /home/some_path/results/test_flow/05-11_14-16-28/A-6007ec12-1de1-42c0-a9db-7cc500e017e7.prefect_result...
...
5 November 2020,03:16:33 	prefect.CloudTaskRunner	INFO	Task 'C': Starting task run...
5 November 2020,03:16:33 	prefect.LocalResult	DEBUG	Starting to read result from {flow_name}/{scheduled_start_time:%d-%m_%H-%M-%S}/{task_full_name}-{task_run_id}.prefect_result...

For some reason the location string doesn't get formatted 🤔 .

Just to close this end of the loop, the link to the issue -> https://github.com/PrefectHQ/prefect/issues/3618