# AN-2026-0757: Python 3.14: multiprocessing and ProcessPoolExecutor default to 'forkserver' instead of 'fork' on Linux and other non-macOS Unix; code that relied on fork-inherited state can fail

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- id: AN-2026-0757 (version 1; cite https://breakwire.org/claim/AN-2026-0757/v/1)
- kind: breaking · status: active · review: editor
- source: <https://docs.python.org/3/whatsnew/3.14.html#:~:text=forkserver%20is%20now%20the%20default>
- event date: 2025-10-07
- services: python
- affected: engines.python >=3.14
- severity: medium
- verification: unverified (0 reports from 0 independent operators; weighted support 0, against 0)
- lookups: matched 0, fixed 0, not fixed 0

## Statements (checkable)
0. What's New in Python 3.14, Porting to Python 3.14 (opened 2026-11-04): on Unix platforms other than macOS, forkserver is now the default start method for multiprocessing and ProcessPoolExecutor instead of fork. If you encounter NameErrors or pickling errors coming out of multiprocessing or concurrent.futures, see the forkserver restrictions. Windows and macOS keep spawn.  [✓0 ✗0]
1. With forkserver, worker processes do not inherit the parent's in-memory state: the target function and its arguments must be picklable and importable, module-level setup must not be hidden behind if __name__ == '__main__' when workers need it, and globals mutated at runtime are not seen by workers.  [✓0 ✗0]

## Fix
```text
Make worker targets top-level importable functions with picklable arguments, protect the entry point with if __name__ == '__main__':, and pass state explicitly (initializer/initargs). To keep the old behavior, request it explicitly: multiprocessing.get_context('fork') or ProcessPoolExecutor(mp_context=multiprocessing.get_context('fork')) (fork is unsafe with threads).
```

## Outcome reports (0)
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## History
- v1 2026-11-04 by wire-editor: filed from primary source opened 2026-10-28