8000 gh-140557: Force alignment of empty `bytearray` and `array.array` buf… · python/cpython@19de10d · GitHub
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gh-140557: Force alignment of empty bytearray and array.array buffers (GH-140559)
This ensures the buffers used by the empty `bytearray` and `array.array` are aligned the same as a pointer returned by the allocator. This is a more convenient default for interop with other languages that have stricter requirements of type-safe buffers (e.g. Rust's `&[T]` type) even when empty.
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‎Lib/test/test_buffer.py‎

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@@ -4447,6 +4447,41 @@ def test_bytearray_release_buffer_read_flag(self):
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with self.assertRaises(SystemError):
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obj.__buffer__(inspect.BufferFlags.WRITE)
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@support.cpython_only
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@unittest.skipIf(_testcapi is None, "requires _testcapi")
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def test_bytearray_alignment(self):
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# gh-140557: pointer alignment of buffers including empty allocation
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# should be at least to `size_t`.
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align = struct.calcsize("N")
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cases = [
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bytearray(),
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bytearray(1),
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bytearray(b"0123456789abcdef"),
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bytearray(16),
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]
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ptrs = [_testcapi.buffer_pointer_as_int(array) for array in cases]
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self.assertEqual([ptr % align for ptr in ptrs], [0]*len(ptrs))
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@support.cpython_only
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@unittest.skipIf(_testcapi is None, "requires _testcapi")
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def test_array_alignment(self):
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# gh-140557: pointer alignment of buffers including empty allocation
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# should match the maximum array alignment.
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align = max(struct.calcsize(fmt) for fmt in ARRAY)
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cases = [array.array(fmt) for fmt in ARRAY]
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# Empty arrays
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self.assertEqual(
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[_testcapi.buffer_pointer_as_int(case) % align for case in cases],
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[0] * len(cases),
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)
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for case in cases:
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case.append(0)
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# Allocated arrays
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self.assertEqual(
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[_testcapi.buffer_pointer_as_int(case) % align for case in cases],
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[0] * len(cases),
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)
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@support.cpython_only
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def test_pybuffer_size_from_format(self):
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# basic tests

‎Misc/ACKS‎

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@@ -1151,6 +1151,7 @@ Per Lindqvist
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Eric Lindvall
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Gregor Lingl
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Everett Lipman
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Jake Lishman
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Mirko Liss
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Alexander Liu
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Hui Liu

‎Misc/NEWS.d/next/Core_and_Builtins/2025-10-24-17-30-51.gh-issue-140557.X2GETk.rst‎

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@@ -0,0 +1,2 @@
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:class:`bytearray` buffers now have the same alignment
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when empty as when allocated. Unaligned buffers can still be created by slicing.

‎Misc/NEWS.d/next/Library/2026-01-07-11-57-59.gh-issue-140557.3P6-nW.rst‎

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@@ -0,0 +1,2 @@
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:class:`array.array` buffers now have the same alignment when empty as when
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allocated. Unaligned buffers can still be created by slicing.

‎Modules/_testcapi/buffer.c‎

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@@ -98,6 +98,27 @@ static PyTypeObject testBufType = {
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.tp_members = testbuf_members
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};
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/* Get the pointer from a buffer-supporting object as a PyLong.
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*
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* Used to test alignment properties. */
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static PyObject *
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buffer_pointer_as_int(PyObject *Py_UNUSED(module), PyObject *obj)
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{
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PyObject *out;
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Py_buffer view;
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if (PyObject_GetBuffer(obj, &view, PyBUF_SIMPLE) != 0) {
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return NULL;
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}
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out = PyLong_FromVoidPtr(view.buf);
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PyBuffer_Release(&view);
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return out;
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}
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static PyMethodDef test_methods[] = {
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{"buffer_pointer_as_int", buffer_pointer_as_int, METH_O},
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{NULL},
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};
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int
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_PyTestCapi_Init_Buffer(PyObject *m) {
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if (PyType_Ready(&testBufType) < 0) {
@@ -106,6 +127,9 @@ _PyTestCapi_Init_Buffer(PyObject *m) {
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if (PyModule_AddObjectRef(m, "testBuf", (PyObject *)&testBufType)) {
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return -1;
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}
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if (PyModule_AddFunctions(m, test_methods) < 0) {
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return -1;
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}
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return 0;
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}

‎Modules/arraymodule.c‎

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@@ -2664,7 +2664,7 @@ array_ass_subscr(PyObject *op, PyObject *item, PyObject *value)
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}
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}
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static const void *emptybuf = "";
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static const _Py_ALIGNED_DEF(ALIGNOF_MAX_ALIGN_T, char) emptybuf[] = "";
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static int

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