Which multiprocessing start method ensures compatible behavior across Linux, macOS, and Windows in Python 3.8+?

Goal

Identify a start method that allows a library using multiprocessing to run reliably on Linux (default fork), macOS (default spawn since 3.8), and Windows (only spawn) without requiring application‑level changes.

Constraints and uncertainty

• Changing the start method after any process creation raises RuntimeError.
• Third‑party code may implicitly depend on fork semantics such as inherited file descriptors or thread‑local state.
• The forkserver method offers a compromise but is not the default anywhere.
• No standardized environment variable (e.g., MP_START_METHOD) exists across implementations.

Open questions

Which start method should a library document as required? Can a library safely enforce a context via get_context() without breaking host applications? How can CI pipelines detect start‑method mismatches before deployment?