PyAutoFit: importing autofit fixes the multiprocessing start method, so set_start_method() raises on macOS (test suite cannot load conftest)

[samlange04]

Package: PyAutoFit (autofit), current main (891fe64b9). Platform: macOS, Python 3.13.7.

What happens

Importing autofit and then choosing a multiprocessing start method fails:

import multiprocessing
import autofit
multiprocessing.set_start_method("fork")
# RuntimeError: context has already been set

The autofit test suite does exactly this on macOS in test_autofit/conftest.py (line 18, multiprocessing.set_start_method("fork")), so on a Mac plain python -m pytest test_autofit has failed to load the conftest at all since 2026-07-31:

ImportError while loading conftest '.../test_autofit/conftest.py'.
test_autofit/conftest.py:18: in <module>
    multiprocessing.set_start_method("fork")
.../multiprocessing/context.py:247: in set_start_method
    raise RuntimeError('context has already been set')
E   RuntimeError: context has already been set

Any user script that imports autofit before calling set_start_method hits the same error.

Cause

Commit e6279c53f ("fix: pin multiprocessing pools to the fork start method") added autofit/non_linear/parallel/context.py::fork_context(). On Linux it returns multiprocessing.get_context("fork"), which is fine: passing a method name does not touch the interpreter default. On macOS and Windows it falls through to multiprocessing.get_context() with no argument. That call permanently fixes _default_context._actual_context, and fork_context() runs at import time because autofit/non_linear/parallel/process.py does class Process(fork_context().Process). Anything that later calls set_start_method without force=True raises.

Traced by patching multiprocessing.context._default_context.get_context and printing the stack on the first call while the context was still unset: it fires from process.py:47 -> context.py:33.

Proposed fix

On macOS/Windows return the multiprocessing module itself instead of get_context(). It exposes the same Process / Queue / Pool API, but resolves the start method when a process is created, which is the pre-3.14 behaviour fork_context() exists to preserve. Linux behaviour (pinned "fork") is unchanged.

Branch: https://github.com/samlange04/PyAutoFit/tree/fix/multiprocessing-context-import-time. With it applied, plain python -m pytest test_autofit on macOS loads the conftest and gives 2924 passed, 39 skipped, 1 failed, and the one failure (test_pdf.py::test_save_covariance_matrix, a last-digit float text comparison) fails identically on unmodified main.

PR: PyAutoLabs/PyAutoFit#1657

[Jammy2211]

Hi @samlange04, thanks so much for this. The stack-patch trace pointing at process.py:47 → context.py:33 made it very quick to confirm. You're right: calling get_context() with no argument locked the default start method at import time, which broke set_start_method for our own conftest on macOS and for any user script. Your PR #1657 is merged into PyAutoFit main and will go out in the next release. Linux behaviour (fork pinned) is unchanged. Thanks again for the careful report and the clean fix.