@Marvin prefect 2 PicklingError import of module '__prefect_loader__' failed

The error you're encountering,

PicklingError import of module '__prefect_loader__' failed

, seems to be related to the use of the Python multiprocessing library within Prefect, specifically when using

multiprocessing.Pool.apply_async

in a flow that's deployed remotely. This issue has been reported in the Prefect GitHub repository. From the relevant issue discussions, here are some important points: 1. Error Context: The error occurs when the flow is executed via a deployment and an agent, rather than directly through a Python script. The function

bar

in the example can't be pickled due to the

__prefect_loader__

module import failure. 2. Workaround: Direct invocation of the flow does not trigger this issue, indicating that it likely stems from the environment or the deployment method used. 3. Related Issues: A somewhat related issue is seen with deadlocks when using the

multiprocessing

library in Docker containers. This points to possible threading or multiprocessing management differences when Prefect agents handle flow execution versus standalone script execution. There's no direct solution, but investigating alternative concurrency models (like using Prefect's concurrency capabilities) might help. For more details and potential updates, you can check these discussions: - PicklingError with `__prefect_loader__` - Multiprocessing deadlock It's also advisable to review Prefect's release notes and documentation for updates or fixes on this topic as they become available.