From 92a3516ffb5265f8faf3ecf91263a04e172258d1 Mon Sep 17 00:00:00 2001 From: AN Long Date: Wed, 24 Dec 2025 04:28:32 +0900 Subject: [PATCH 1/2] gh-109263: Start process from spawn context in multiprocessing no longer have side effect (GH-135813) (cherry picked from commit c2202a7e661d40b1837cc0109cdb9ab40ec4e486) Co-authored-by: AN Long --- Lib/multiprocessing/spawn.py | 2 +- Lib/test/_test_multiprocessing.py | 20 +++++++++++++++++++ ...-06-22-18-57-19.gh-issue-109263.f92V95.rst | 2 ++ 3 files changed, 23 insertions(+), 1 deletion(-) create mode 100644 Misc/NEWS.d/next/Library/2025-06-22-18-57-19.gh-issue-109263.f92V95.rst diff --git a/Lib/multiprocessing/spawn.py b/Lib/multiprocessing/spawn.py index daac1ecc34b55e..d43864c939cb63 100644 --- a/Lib/multiprocessing/spawn.py +++ b/Lib/multiprocessing/spawn.py @@ -184,7 +184,7 @@ def get_preparation_data(name): sys_argv=sys.argv, orig_dir=process.ORIGINAL_DIR, dir=os.getcwd(), - start_method=get_start_method(), + start_method=get_start_method(allow_none=True), ) # Figure out whether to initialise main in the subprocess as a module diff --git a/Lib/test/_test_multiprocessing.py b/Lib/test/_test_multiprocessing.py index 4b437f8eb48865..a894f889beab34 100644 --- a/Lib/test/_test_multiprocessing.py +++ b/Lib/test/_test_multiprocessing.py @@ -5845,6 +5845,26 @@ def test_context(self): self.assertRaises(ValueError, ctx.set_start_method, None) self.check_context(ctx) + @staticmethod + def _dummy_func(): + pass + + @warnings_helper.ignore_fork_in_thread_deprecation_warnings() + def test_spawn_dont_set_context(self): + # Run a process with spawn or forkserver context may change + # the global start method, see gh-109263. + for method in ('fork', 'spawn', 'forkserver'): + multiprocessing.set_start_method(None, force=True) + + try: + ctx = multiprocessing.get_context(method) + except ValueError: + continue + process = ctx.Process(target=self._dummy_func) + process.start() + process.join() + self.assertIsNone(multiprocessing.get_start_method(allow_none=True)) + def test_context_check_module_types(self): try: ctx = multiprocessing.get_context('forkserver') diff --git a/Misc/NEWS.d/next/Library/2025-06-22-18-57-19.gh-issue-109263.f92V95.rst b/Misc/NEWS.d/next/Library/2025-06-22-18-57-19.gh-issue-109263.f92V95.rst new file mode 100644 index 00000000000000..6b96b5b9b2a0de --- /dev/null +++ b/Misc/NEWS.d/next/Library/2025-06-22-18-57-19.gh-issue-109263.f92V95.rst @@ -0,0 +1,2 @@ +Starting a process from spawn context in :mod:`multiprocessing` no longer +sets the start method globally. From 813b1692963128cc21e92de195c51d61c8db3382 Mon Sep 17 00:00:00 2001 From: "Gregory P. Smith" Date: Wed, 24 Dec 2025 02:39:26 +0000 Subject: [PATCH 2/2] remove 3.15 warnings_helper --- Lib/test/_test_multiprocessing.py | 1 - 1 file changed, 1 deletion(-) diff --git a/Lib/test/_test_multiprocessing.py b/Lib/test/_test_multiprocessing.py index a894f889beab34..d7f804a7c69701 100644 --- a/Lib/test/_test_multiprocessing.py +++ b/Lib/test/_test_multiprocessing.py @@ -5849,7 +5849,6 @@ def test_context(self): def _dummy_func(): pass - @warnings_helper.ignore_fork_in_thread_deprecation_warnings() def test_spawn_dont_set_context(self): # Run a process with spawn or forkserver context may change # the global start method, see gh-109263.