Jan 9, 2026 · Jan 9, 2026
diff --git a/Lib/test/test_thread.py b/Lib/test/test_thread.py
        thread.stack_size(0)
        self.assertEqual(thread.stack_size(), 0, "stack_size not reset to default")

        with self.assertRaises(ValueError):
            # 123 bytes is too small
            thread.stack_size(123)

        with self.assertRaises(ValueError):
            # size must be positive
            thread.stack_size(-4096)

    @unittest.skipIf(os.name not in ("nt", "posix"), 'test meant for nt and posix')
    def test_nt_and_posix_stack_size(self):
        try:
diff --git a/Misc/NEWS.d/next/Library/2026-01-09-13-07-22.gh-issue-143191.PPR_vW.rst b/Misc/NEWS.d/next/Library/2026-01-09-13-07-22.gh-issue-143191.PPR_vW.rst
 :func:`_thread.stack_size` now raises :exc:`ValueError` if the stack size is
 too small. Patch by Victor Stinner.
diff --git a/Modules/_threadmodule.c b/Modules/_threadmodule.c
    if (!PyArg_ParseTuple(args, "|n:stack_size", &new_size))
        return NULL;

    if (new_size < 0) {
        PyErr_SetString(PyExc_ValueError,
                        "size must be 0 or a positive value");
    Py_ssize_t min_size = _PyOS_MIN_STACK_SIZE + SYSTEM_PAGE_SIZE;
    if (new_size != 0 && new_size < min_size) {
        PyErr_Format(PyExc_ValueError,
                     "size must be at least %zi bytes", min_size);
        return NULL;
    }
Original file line number	Diff line number	Diff line change
Expand Up		@@ -76,6 +76,14 @@ def test_stack_size(self):
		thread.stack_size(0)
		self.assertEqual(thread.stack_size(), 0, "stack_size not reset to default")

		with self.assertRaises(ValueError):
		# 123 bytes is too small
		thread.stack_size(123)

		with self.assertRaises(ValueError):
		# size must be positive
		thread.stack_size(-4096)

		@unittest.skipIf(os.name not in ("nt", "posix"), 'test meant for nt and posix')
		def test_nt_and_posix_stack_size(self):
		try:
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Original file line number	Diff line number	Diff line change
		@@ -0,0 +1,2 @@
		:func:`_thread.stack_size` now raises :exc:`ValueError` if the stack size is
		too small. Patch by Victor Stinner.
Original file line number	Diff line number	Diff line change
Expand Up		@@ -2200,9 +2200,10 @@ thread_stack_size(PyObject self, PyObject args)
		if (!PyArg_ParseTuple(args, "\|n:stack_size", &new_size))
		return NULL;

		if (new_size < 0) {
		PyErr_SetString(PyExc_ValueError,
		"size must be 0 or a positive value");
		Py_ssize_t min_size = _PyOS_MIN_STACK_SIZE + SYSTEM_PAGE_SIZE;
		if (new_size != 0 && new_size < min_size) {
		PyErr_Format(PyExc_ValueError,
		"size must be at least %zi bytes", min_size);
		return NULL;
		}

Expand Down