Jan 13, 2026 · Jan 12, 2026
diff --git a/Lib/multiprocessing/forkserver.py b/Lib/multiprocessing/forkserver.py
                    main_kws['sys_path'] = data['sys_path']
                if 'init_main_from_path' in data:
                    main_kws['main_path'] = data['init_main_from_path']
                if 'sys_argv' in data:
                    main_kws['sys_argv'] = data['sys_argv']

            with socket.socket(socket.AF_UNIX) as listener:
                address = connection.arbitrary_address('AF_UNIX')
 #

 def main(listener_fd, alive_r, preload, main_path=None, sys_path=None,
         *, authkey_r=None):
         *,sys_argv=None,authkey_r=None):
    """Run forkserver."""
    if authkey_r is not None:
        try:
        authkey = b''

    if preload:
        if sys_argv is not None:
            sys.argv[:] = sys_argv
        if sys_path is not None:
            sys.path[:] = sys_path
        if '__main__' in preload and main_path is not None:
diff --git a/Lib/test/_test_multiprocessing.py b/Lib/test/_test_multiprocessing.py
        out = out.decode().split("\n")
        self.assertEqual(out, ['__main__', '__mp_main__', 'f', 'f', ''])

    def test_preload_main_sys_argv(self):
        # gh-143706: Check that sys.argv is set before __main__ is pre-loaded
        if multiprocessing.get_start_method() != "forkserver":
            self.skipTest("forkserver specific test")

        name = os.path.join(os.path.dirname(__file__), 'mp_preload_sysargv.py')
        _, out, err = test.support.script_helper.assert_python_ok(
            name, 'foo', 'bar')
        self.assertEqual(err, b'')

        out = out.decode().split("\n")
        expected_argv = "['foo', 'bar']"
        self.assertEqual(out, [
            f"module:{expected_argv}",
            f"fun:{expected_argv}",
            f"module:{expected_argv}",
            f"fun:{expected_argv}",
            '',
        ])

 #
 # Mixins
 #
diff --git a/Lib/test/mp_preload_sysargv.py b/Lib/test/mp_preload_sysargv.py
 # gh-143706: Test that sys.argv is correctly set during main module import
 # when using forkserver with __main__ preloading.

 import multiprocessing
 import sys

 # This will be printed during module import - sys.argv should be correct here
 print(f"module:{sys.argv[1:]}")

 def fun():
    # This will be printed when the function is called
    print(f"fun:{sys.argv[1:]}")

 if __name__ == "__main__":
    ctx = multiprocessing.get_context("forkserver")
    ctx.set_forkserver_preload(['__main__'])

    fun()

    p = ctx.Process(target=fun)
    p.start()
    p.join()
diff --git a/Misc/NEWS.d/next/Library/2026-01-12-07-17-38.gh-issue-143706.sysArgv.rst b/Misc/NEWS.d/next/Library/2026-01-12-07-17-38.gh-issue-143706.sysArgv.rst
 Fix :mod:`multiprocessing` forkserver so that :data:`sys.argv` is correctly
 set before ``__main__`` is preloaded. Previously, :data:`sys.argv` was empty
 during main module import in forkserver child processes. This fixes a
 regression introduced in 3.13.8 and 3.14.1. Root caused by Aaron Wieczorek,
 test provided by Thomas Watson, thanks!
Original file line number	Diff line number	Diff line change
Expand Up		@@ -152,6 +152,8 @@ def ensure_running(self):
		main_kws['sys_path'] = data['sys_path']
		if 'init_main_from_path' in data:
		main_kws['main_path'] = data['init_main_from_path']
		if 'sys_argv' in data:
		main_kws['sys_argv'] = data['sys_argv']

		with socket.socket(socket.AF_UNIX) as listener:
		address = connection.arbitrary_address('AF_UNIX')
Expand DownExpand Up		@@ -197,7 +199,7 @@ def ensure_running(self):
		#

		def main(listener_fd, alive_r, preload, main_path=None, sys_path=None,
		*, authkey_r=None):
		*,sys_argv=None,authkey_r=None):
		"""Run forkserver."""
		if authkey_r is not None:
		try:
Expand All		@@ -209,6 +211,8 @@ def main(listener_fd, alive_r, preload, main_path=None, sys_path=None,
		authkey = b''

		if preload:
		if sys_argv is not None:
		sys.argv[:] = sys_argv
		if sys_path is not None:
		sys.path[:] = sys_path
		if '__main__' in preload and main_path is not None:
Expand Down
Original file line number	Diff line number	Diff line change
Expand Up		@@ -6939,6 +6939,26 @@ def test_preload_main(self):
		out = out.decode().split("\n")
		self.assertEqual(out, ['__main__', '__mp_main__', 'f', 'f', ''])

		def test_preload_main_sys_argv(self):
		# gh-143706: Check that sys.argv is set before __main__ is pre-loaded
		if multiprocessing.get_start_method() != "forkserver":
		self.skipTest("forkserver specific test")

		name = os.path.join(os.path.dirname(__file__), 'mp_preload_sysargv.py')
		_, out, err = test.support.script_helper.assert_python_ok(
		name, 'foo', 'bar')
		self.assertEqual(err, b'')

		out = out.decode().split("\n")
		expected_argv = "['foo', 'bar']"
		self.assertEqual(out, [
		f"module:{expected_argv}",
		f"fun:{expected_argv}",
		f"module:{expected_argv}",
		f"fun:{expected_argv}",
		'',
		])

		#
		# Mixins
		#
Expand Down
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1,22 @@
		# gh-143706: Test that sys.argv is correctly set during main module import
		# when using forkserver with __main__ preloading.

		import multiprocessing
		import sys

		# This will be printed during module import - sys.argv should be correct here
		print(f"module:{sys.argv[1:]}")

		def fun():
		# This will be printed when the function is called
		print(f"fun:{sys.argv[1:]}")

		if __name__ == "__main__":
		ctx = multiprocessing.get_context("forkserver")
		ctx.set_forkserver_preload(['__main__'])

		fun()

		p = ctx.Process(target=fun)
		p.start()
		p.join()
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1,5 @@
		Fix :mod:`multiprocessing` forkserver so that :data:`sys.argv` is correctly
		set before ``__main__`` is preloaded. Previously, :data:`sys.argv` was empty
		during main module import in forkserver child processes. This fixes a
		regression introduced in 3.13.8 and 3.14.1. Root caused by Aaron Wieczorek,
		test provided by Thomas Watson, thanks!