Jan 26, 2026 · Oct 24, 2025 · Oct 30, 2025 · Oct 30, 2025 · Oct 30, 2025 · Oct 30, 2025
diff --git a/Lib/test/test_buffer.py b/Lib/test/test_buffer.py
        with self.assertRaises(SystemError):
            obj.__buffer__(inspect.BufferFlags.WRITE)

    @support.cpython_only
    @unittest.skipIf(_testcapi is None, "requires _testcapi")
    def test_bytearray_alignment(self):
        # gh-140557: pointer alignment of buffers including empty allocation
        # should be at least to `size_t`.
        align = struct.calcsize("N")
        cases = [
            bytearray(),
            bytearray(1),
            bytearray(b"0123456789abcdef"),
            bytearray(16),
        ]
        ptrs = [_testcapi.buffer_pointer_as_int(array) for array in cases]
        self.assertEqual([ptr % align for ptr in ptrs], [0]*len(ptrs))

    @support.cpython_only
    @unittest.skipIf(_testcapi is None, "requires _testcapi")
    def test_array_alignment(self):
        # gh-140557: pointer alignment of buffers including empty allocation
        # should match the maximum array alignment.
        align = max(struct.calcsize(fmt) for fmt in ARRAY)
        cases = [array.array(fmt) for fmt in ARRAY]
        # Empty arrays
        self.assertEqual(
            [_testcapi.buffer_pointer_as_int(case) % align for case in cases],
            [0] * len(cases),
        )
        for case in cases:
            case.append(0)
        # Allocated arrays
        self.assertEqual(
            [_testcapi.buffer_pointer_as_int(case) % align for case in cases],
            [0] * len(cases),
        )

    @support.cpython_only
    def test_pybuffer_size_from_format(self):
        # basic tests
diff --git a/Misc/ACKS b/Misc/ACKS
 Eric Lindvall
 Gregor Lingl
 Everett Lipman
 Jake Lishman
 Mirko Liss
 Alexander Liu
 Hui Liu
diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2025-10-24-17-30-51.gh-issue-140557.X2GETk.rst b/Misc/NEWS.d/next/Core_and_Builtins/2025-10-24-17-30-51.gh-issue-140557.X2GETk.rst
 :class:`bytearray` buffers now have the same alignment
 when empty as when allocated. Unaligned buffers can still be created by slicing.
diff --git a/Misc/NEWS.d/next/Library/2026-01-07-11-57-59.gh-issue-140557.3P6-nW.rst b/Misc/NEWS.d/next/Library/2026-01-07-11-57-59.gh-issue-140557.3P6-nW.rst
 :class:`array.array` buffers now have the same alignment when empty as when
 allocated. Unaligned buffers can still be created by slicing.
diff --git a/Modules/_testcapi/buffer.c b/Modules/_testcapi/buffer.c
    .tp_members = testbuf_members
 };

 /* Get the pointer from a buffer-supporting object as a PyLong.
 *
 * Used to test alignment properties. */
 static PyObject *
 buffer_pointer_as_int(PyObject *Py_UNUSED(module), PyObject *obj)
 {
    PyObject *out;
    Py_buffer view;
    if (PyObject_GetBuffer(obj, &view, PyBUF_SIMPLE) != 0) {
        return NULL;
    }
    out = PyLong_FromVoidPtr(view.buf);
    PyBuffer_Release(&view);
    return out;
 }

 static PyMethodDef test_methods[] = {
    {"buffer_pointer_as_int", buffer_pointer_as_int, METH_O},
    {NULL},
 };

 int
 _PyTestCapi_Init_Buffer(PyObject *m) {
    if (PyType_Ready(&testBufType) < 0) {
    if (PyModule_AddObjectRef(m, "testBuf", (PyObject *)&testBufType)) {
        return -1;
    }
    if (PyModule_AddFunctions(m, test_methods) < 0) {
        return -1;
    }

    return 0;
 }
diff --git a/Modules/arraymodule.c b/Modules/arraymodule.c
    }
 }

 static constvoid *emptybuf = "";
 static const_Py_ALIGNED_DEF(ALIGNOF_MAX_ALIGN_T, char)emptybuf[] = "";


 static int
Original file line number	Diff line number	Diff line change
Expand Up		@@ -4447,6 +4447,41 @@ def test_bytearray_release_buffer_read_flag(self):
		with self.assertRaises(SystemError):
		obj.__buffer__(inspect.BufferFlags.WRITE)

		@support.cpython_only
		@unittest.skipIf(_testcapi is None, "requires _testcapi")
		def test_bytearray_alignment(self):
		# gh-140557: pointer alignment of buffers including empty allocation
		# should be at least to `size_t`.
		align = struct.calcsize("N")
		cases = [
		bytearray(),
		bytearray(1),
		bytearray(b"0123456789abcdef"),
		bytearray(16),
		]
		ptrs = [_testcapi.buffer_pointer_as_int(array) for array in cases]
		self.assertEqual([ptr % align for ptr in ptrs], [0]*len(ptrs))

		@support.cpython_only
		@unittest.skipIf(_testcapi is None, "requires _testcapi")
		def test_array_alignment(self):
		# gh-140557: pointer alignment of buffers including empty allocation
		# should match the maximum array alignment.
		align = max(struct.calcsize(fmt) for fmt in ARRAY)
		cases = [array.array(fmt) for fmt in ARRAY]
		# Empty arrays
		self.assertEqual(
		[_testcapi.buffer_pointer_as_int(case) % align for case in cases],
		[0] * len(cases),
		)
		for case in cases:
		case.append(0)
		# Allocated arrays
		self.assertEqual(
		[_testcapi.buffer_pointer_as_int(case) % align for case in cases],
		[0] * len(cases),
		)

		@support.cpython_only
		def test_pybuffer_size_from_format(self):
		# basic tests
Expand Down
Original file line number	Diff line number	Diff line change
Expand Up		@@ -1151,6 +1151,7 @@ Per Lindqvist
		Eric Lindvall
		Gregor Lingl
		Everett Lipman
		Jake Lishman
		Mirko Liss
		Alexander Liu
		Hui Liu
Expand Down
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1,2 @@
		:class:`bytearray` buffers now have the same alignment
		when empty as when allocated. Unaligned buffers can still be created by slicing.
jakelishman marked this conversation as resolved. Show resolvedHide resolved
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1,2 @@
		:class:`array.array` buffers now have the same alignment when empty as when
		allocated. Unaligned buffers can still be created by slicing.
Original file line number	Diff line number	Diff line change
Expand Up		@@ -98,6 +98,27 @@ static PyTypeObject testBufType = {
		.tp_members = testbuf_members
		};

		/* Get the pointer from a buffer-supporting object as a PyLong.
		*
		* Used to test alignment properties. */
		static PyObject *
		buffer_pointer_as_int(PyObject Py_UNUSED(module), PyObject obj)
		{
		PyObject *out;
		Py_buffer view;
		if (PyObject_GetBuffer(obj, &view, PyBUF_SIMPLE) != 0) {
		return NULL;
		}
		out = PyLong_FromVoidPtr(view.buf);
		PyBuffer_Release(&view);
		return out;
		}

		static PyMethodDef test_methods[] = {
		{"buffer_pointer_as_int", buffer_pointer_as_int, METH_O},
		{NULL},
		};

		int
		_PyTestCapi_Init_Buffer(PyObject *m) {
		if (PyType_Ready(&testBufType) < 0) {
Expand All		@@ -106,6 +127,9 @@ _PyTestCapi_Init_Buffer(PyObject *m) {
		if (PyModule_AddObjectRef(m, "testBuf", (PyObject *)&testBufType)) {
		return -1;
		}
		if (PyModule_AddFunctions(m, test_methods) < 0) {
		return -1;
		}

		return 0;
		}
Original file line number	Diff line number	Diff line change
Expand Up		@@ -2655,7 +2655,7 @@ array_ass_subscr(PyObject op, PyObject item, PyObject *value)
		}
		}

		static constvoid *emptybuf = "";
		static const_Py_ALIGNED_DEF(ALIGNOF_MAX_ALIGN_T, char)emptybuf[] = "";


		static int
Expand Down