gh-102500: Implement PEP 688 (#102521)

Co-authored-by: Kumar Aditya <59607654+kumaraditya303@users.noreply.github.com>

Include/internal/pycore_global_objects_fini_generated.h

@@ -593,6 +593,7 @@ _PyStaticObjects_CheckRefcnt(PyInterpreterState *interp) {
     _PyStaticObject_CheckRefcnt((PyObject *)&_Py_ID(__await__));
     _PyStaticObject_CheckRefcnt((PyObject *)&_Py_ID(__bases__));
     _PyStaticObject_CheckRefcnt((PyObject *)&_Py_ID(__bool__));
+    _PyStaticObject_CheckRefcnt((PyObject *)&_Py_ID(__buffer__));
     _PyStaticObject_CheckRefcnt((PyObject *)&_Py_ID(__build_class__));
     _PyStaticObject_CheckRefcnt((PyObject *)&_Py_ID(__builtins__));
     _PyStaticObject_CheckRefcnt((PyObject *)&_Py_ID(__bytes__));
@@ -692,6 +693,7 @@ _PyStaticObjects_CheckRefcnt(PyInterpreterState *interp) {
     _PyStaticObject_CheckRefcnt((PyObject *)&_Py_ID(__rdivmod__));
     _PyStaticObject_CheckRefcnt((PyObject *)&_Py_ID(__reduce__));
     _PyStaticObject_CheckRefcnt((PyObject *)&_Py_ID(__reduce_ex__));
+    _PyStaticObject_CheckRefcnt((PyObject *)&_Py_ID(__release_buffer__));
     _PyStaticObject_CheckRefcnt((PyObject *)&_Py_ID(__repr__));
     _PyStaticObject_CheckRefcnt((PyObject *)&_Py_ID(__reversed__));
     _PyStaticObject_CheckRefcnt((PyObject *)&_Py_ID(__rfloordiv__));
@@ -1122,6 +1124,7 @@ _PyStaticObjects_CheckRefcnt(PyInterpreterState *interp) {
     _PyStaticObject_CheckRefcnt((PyObject *)&_Py_ID(reducer_override));
     _PyStaticObject_CheckRefcnt((PyObject *)&_Py_ID(registry));
     _PyStaticObject_CheckRefcnt((PyObject *)&_Py_ID(rel_tol));
+    _PyStaticObject_CheckRefcnt((PyObject *)&_Py_ID(release));
     _PyStaticObject_CheckRefcnt((PyObject *)&_Py_ID(reload));
     _PyStaticObject_CheckRefcnt((PyObject *)&_Py_ID(repl));
     _PyStaticObject_CheckRefcnt((PyObject *)&_Py_ID(replace));

Include/internal/pycore_global_strings.h

@@ -81,6 +81,7 @@ struct _Py_global_strings {
         STRUCT_FOR_ID(__await__)
         STRUCT_FOR_ID(__bases__)
         STRUCT_FOR_ID(__bool__)
+        STRUCT_FOR_ID(__buffer__)
         STRUCT_FOR_ID(__build_class__)
         STRUCT_FOR_ID(__builtins__)
         STRUCT_FOR_ID(__bytes__)
@@ -180,6 +181,7 @@ struct _Py_global_strings {
         STRUCT_FOR_ID(__rdivmod__)
         STRUCT_FOR_ID(__reduce__)
         STRUCT_FOR_ID(__reduce_ex__)
+        STRUCT_FOR_ID(__release_buffer__)
         STRUCT_FOR_ID(__repr__)
         STRUCT_FOR_ID(__reversed__)
         STRUCT_FOR_ID(__rfloordiv__)
@@ -610,6 +612,7 @@ struct _Py_global_strings {
         STRUCT_FOR_ID(reducer_override)
         STRUCT_FOR_ID(registry)
         STRUCT_FOR_ID(rel_tol)
+        STRUCT_FOR_ID(release)
         STRUCT_FOR_ID(reload)
         STRUCT_FOR_ID(repl)
         STRUCT_FOR_ID(replace)

Include/internal/pycore_memoryobject.h

@@ -0,0 +1,17 @@
+#ifndef Py_INTERNAL_MEMORYOBJECT_H
+#define Py_INTERNAL_MEMORYOBJECT_H
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#ifndef Py_BUILD_CORE
+#  error "this header requires Py_BUILD_CORE define"
+#endif
+
+PyObject *
+PyMemoryView_FromObjectAndFlags(PyObject *v, int flags);
+
+#ifdef __cplusplus
+}
+#endif
+#endif /* !Py_INTERNAL_MEMORYOBJECT_H */

Include/internal/pycore_runtime_init_generated.h

@@ -587,6 +587,7 @@ extern "C" {
     INIT_ID(__await__), \
     INIT_ID(__bases__), \
     INIT_ID(__bool__), \
+    INIT_ID(__buffer__), \
     INIT_ID(__build_class__), \
     INIT_ID(__builtins__), \
     INIT_ID(__bytes__), \
@@ -686,6 +687,7 @@ extern "C" {
     INIT_ID(__rdivmod__), \
     INIT_ID(__reduce__), \
     INIT_ID(__reduce_ex__), \
+    INIT_ID(__release_buffer__), \
     INIT_ID(__repr__), \
     INIT_ID(__reversed__), \
     INIT_ID(__rfloordiv__), \
@@ -1116,6 +1118,7 @@ extern "C" {
     INIT_ID(reducer_override), \
     INIT_ID(registry), \
     INIT_ID(rel_tol), \
+    INIT_ID(release), \
     INIT_ID(reload), \
     INIT_ID(repl), \
     INIT_ID(replace), \

Include/internal/pycore_typeobject.h

@@ -138,6 +138,8 @@ _Py_type_getattro(PyTypeObject *type, PyObject *name);
 PyObject *_Py_slot_tp_getattro(PyObject *self, PyObject *name);
 PyObject *_Py_slot_tp_getattr_hook(PyObject *self, PyObject *name);
 
+PyAPI_DATA(PyTypeObject) _PyBufferWrapper_Type;
+
 PyObject *
 _PySuper_Lookup(PyTypeObject *su_type, PyObject *su_obj, PyObject *name, int *meth_found);
 PyObject *

Include/internal/pycore_unicodeobject_generated.h

@@ -96,6 +96,9 @@ _PyUnicode_InitStaticStrings(PyInterpreterState *interp) {
     string = &_Py_ID(__bool__);
     assert(_PyUnicode_CheckConsistency(string, 1));
     _PyUnicode_InternInPlace(interp, &string);
+    string = &_Py_ID(__buffer__);
+    assert(_PyUnicode_CheckConsistency(string, 1));
+    _PyUnicode_InternInPlace(interp, &string);
     string = &_Py_ID(__build_class__);
     assert(_PyUnicode_CheckConsistency(string, 1));
     _PyUnicode_InternInPlace(interp, &string);
@@ -393,6 +396,9 @@ _PyUnicode_InitStaticStrings(PyInterpreterState *interp) {
     string = &_Py_ID(__reduce_ex__);
     assert(_PyUnicode_CheckConsistency(string, 1));
     _PyUnicode_InternInPlace(interp, &string);
+    string = &_Py_ID(__release_buffer__);
+    assert(_PyUnicode_CheckConsistency(string, 1));
+    _PyUnicode_InternInPlace(interp, &string);
     string = &_Py_ID(__repr__);
     assert(_PyUnicode_CheckConsistency(string, 1));
     _PyUnicode_InternInPlace(interp, &string);
@@ -1683,6 +1689,9 @@ _PyUnicode_InitStaticStrings(PyInterpreterState *interp) {
     string = &_Py_ID(rel_tol);
     assert(_PyUnicode_CheckConsistency(string, 1));
     _PyUnicode_InternInPlace(interp, &string);
+    string = &_Py_ID(release);
+    assert(_PyUnicode_CheckConsistency(string, 1));
+    _PyUnicode_InternInPlace(interp, &string);
     string = &_Py_ID(reload);
     assert(_PyUnicode_CheckConsistency(string, 1));
     _PyUnicode_InternInPlace(interp, &string);

Include/pybuffer.h

@@ -104,7 +104,7 @@ PyAPI_FUNC(void) PyBuffer_Release(Py_buffer *view);
 /* Maximum number of dimensions */
 #define PyBUF_MAX_NDIM 64
 
-/* Flags for getting buffers */
+/* Flags for getting buffers. Keep these in sync with inspect.BufferFlags. */
 #define PyBUF_SIMPLE 0
 #define PyBUF_WRITABLE 0x0001
 

Lib/_collections_abc.py

@@ -49,7 +49,7 @@ __all__ = ["Awaitable", "Coroutine",
            "Mapping", "MutableMapping",
            "MappingView", "KeysView", "ItemsView", "ValuesView",
            "Sequence", "MutableSequence",
-           "ByteString",
+           "ByteString", "Buffer",
            ]
 
 # This module has been renamed from collections.abc to _collections_abc to
@@ -439,6 +439,21 @@ class Collection(Sized, Iterable, Container):
         return NotImplemented
 
 
+class Buffer(metaclass=ABCMeta):
+
+    __slots__ = ()
+
+    @abstractmethod
+    def __buffer__(self, flags: int, /) -> memoryview:
+        raise NotImplementedError
+
+    @classmethod
+    def __subclasshook__(cls, C):
+        if cls is Buffer:
+            return _check_methods(C, "__buffer__")
+        return NotImplemented
+
+
 class _CallableGenericAlias(GenericAlias):
     """ Represent `Callable[argtypes, resulttype]`.
 

Lib/inspect.py

@@ -43,6 +43,7 @@ __all__ = [
     "Attribute",
     "BlockFinder",
     "BoundArguments",
+    "BufferFlags",
     "CORO_CLOSED",
     "CORO_CREATED",
     "CORO_RUNNING",
@@ -3312,6 +3313,28 @@ def signature(obj, *, follow_wrapped=True, globals=None, locals=None, eval_str=F
                                    globals=globals, locals=locals, eval_str=eval_str)
 
 
+class BufferFlags(enum.IntFlag):
+    SIMPLE = 0x0
+    WRITABLE = 0x1
+    FORMAT = 0x4
+    ND = 0x8
+    STRIDES = 0x10 | ND
+    C_CONTIGUOUS = 0x20 | STRIDES
+    F_CONTIGUOUS = 0x40 | STRIDES
+    ANY_CONTIGUOUS = 0x80 | STRIDES
+    INDIRECT = 0x100 | STRIDES
+    CONTIG = ND | WRITABLE
+    CONTIG_RO = ND
+    STRIDED = STRIDES | WRITABLE
+    STRIDED_RO = STRIDES
+    RECORDS = STRIDES | WRITABLE | FORMAT
+    RECORDS_RO = STRIDES | FORMAT
+    FULL = INDIRECT | WRITABLE | FORMAT
+    FULL_RO = INDIRECT | FORMAT
+    READ = 0x100
+    WRITE = 0x200
+
+
 def _main():
     """ Logic for inspecting an object given at command line """
     import argparse

Lib/test/test_buffer.py

@@ -17,6 +17,7 @@ import contextlib
 import unittest
 from test import support
 from test.support import os_helper
+import inspect
 from itertools import permutations, product
 from random import randrange, sample, choice
 import warnings
@@ -4438,5 +4439,146 @@ class TestBufferProtocol(unittest.TestCase):
                              struct.calcsize(format))
 
 
+class TestPythonBufferProtocol(unittest.TestCase):
+    def test_basic(self):
+        class MyBuffer:
+            def __buffer__(self, flags):
+                return memoryview(b"hello")
+
+        mv = memoryview(MyBuffer())
+        self.assertEqual(mv.tobytes(), b"hello")
+        self.assertEqual(bytes(MyBuffer()), b"hello")
+
+    def test_bad_buffer_method(self):
+        class MustReturnMV:
+            def __buffer__(self, flags):
+                return 42
+
+        self.assertRaises(TypeError, memoryview, MustReturnMV())
+
+        class NoBytesEither:
+            def __buffer__(self, flags):
+                return b"hello"
+
+        self.assertRaises(TypeError, memoryview, NoBytesEither())
+
+        class WrongArity:
+            def __buffer__(self):
+                return memoryview(b"hello")
+
+        self.assertRaises(TypeError, memoryview, WrongArity())
+
+    def test_release_buffer(self):
+        class WhatToRelease:
+            def __init__(self):
+                self.held = False
+                self.ba = bytearray(b"hello")
+
+            def __buffer__(self, flags):
+                if self.held:
+                    raise TypeError("already held")
+                self.held = True
+                return memoryview(self.ba)
+
+            def __release_buffer__(self, buffer):
+                self.held = False
+
+        wr = WhatToRelease()
+        self.assertFalse(wr.held)
+        with memoryview(wr) as mv:
+            self.assertTrue(wr.held)
+            self.assertEqual(mv.tobytes(), b"hello")
+        self.assertFalse(wr.held)
+
+    def test_same_buffer_returned(self):
+        class WhatToRelease:
+            def __init__(self):
+                self.held = False
+                self.ba = bytearray(b"hello")
+                self.created_mv = None
+
+            def __buffer__(self, flags):
+                if self.held:
+                    raise TypeError("already held")
+                self.held = True
+                self.created_mv = memoryview(self.ba)
+                return self.created_mv
+
+            def __release_buffer__(self, buffer):
+                assert buffer is self.created_mv
+                self.held = False
+
+        wr = WhatToRelease()
+        self.assertFalse(wr.held)
+        with memoryview(wr) as mv:
+            self.assertTrue(wr.held)
+            self.assertEqual(mv.tobytes(), b"hello")
+        self.assertFalse(wr.held)
+
+    def test_buffer_flags(self):
+        class PossiblyMutable:
+            def __init__(self, data, mutable) -> None:
+                self._data = bytearray(data)
+                self._mutable = mutable
+
+            def __buffer__(self, flags):
+                if flags & inspect.BufferFlags.WRITABLE:
+                    if not self._mutable:
+                        raise RuntimeError("not mutable")
+                    return memoryview(self._data)
+                else:
+                    return memoryview(bytes(self._data))
+
+        mutable = PossiblyMutable(b"hello", True)
+        immutable = PossiblyMutable(b"hello", False)
+        with memoryview._from_flags(mutable, inspect.BufferFlags.WRITABLE) as mv:
+            self.assertEqual(mv.tobytes(), b"hello")
+            mv[0] = ord(b'x')
+            self.assertEqual(mv.tobytes(), b"xello")
+        with memoryview._from_flags(mutable, inspect.BufferFlags.SIMPLE) as mv:
+            self.assertEqual(mv.tobytes(), b"xello")
+            with self.assertRaises(TypeError):
+                mv[0] = ord(b'h')
+            self.assertEqual(mv.tobytes(), b"xello")
+        with memoryview._from_flags(immutable, inspect.BufferFlags.SIMPLE) as mv:
+            self.assertEqual(mv.tobytes(), b"hello")
+            with self.assertRaises(TypeError):
+                mv[0] = ord(b'x')
+            self.assertEqual(mv.tobytes(), b"hello")
+
+        with self.assertRaises(RuntimeError):
+            memoryview._from_flags(immutable, inspect.BufferFlags.WRITABLE)
+        with memoryview(immutable) as mv:
+            self.assertEqual(mv.tobytes(), b"hello")
+            with self.assertRaises(TypeError):
+                mv[0] = ord(b'x')
+            self.assertEqual(mv.tobytes(), b"hello")
+
+    def test_call_builtins(self):
+        ba = bytearray(b"hello")
+        mv = ba.__buffer__(0)
+        self.assertEqual(mv.tobytes(), b"hello")
+        ba.__release_buffer__(mv)
+        with self.assertRaises(OverflowError):
+            ba.__buffer__(sys.maxsize + 1)
+
+    @unittest.skipIf(_testcapi is None, "requires _testcapi")
+    def test_c_buffer(self):
+        buf = _testcapi.testBuf()
+        self.assertEqual(buf.references, 0)
+        mv = buf.__buffer__(0)
+        self.assertIsInstance(mv, memoryview)
+        self.assertEqual(mv.tobytes(), b"test")
+        self.assertEqual(buf.references, 1)
+        buf.__release_buffer__(mv)
+        self.assertEqual(buf.references, 0)
+        with self.assertRaises(ValueError):
+            mv.tobytes()
+        # Calling it again doesn't cause issues
+        with self.assertRaises(ValueError):
+            buf.__release_buffer__(mv)
+        self.assertEqual(buf.references, 0)
+
+
 if __name__ == "__main__":
     unittest.main()

Lib/test/test_collections.py

@@ -25,7 +25,7 @@ from collections.abc import Sized, Container, Callable, Collection
 from collections.abc import Set, MutableSet
 from collections.abc import Mapping, MutableMapping, KeysView, ItemsView, ValuesView
 from collections.abc import Sequence, MutableSequence
-from collections.abc import ByteString
+from collections.abc import ByteString, Buffer
 
 
 class TestUserObjects(unittest.TestCase):
@@ -1949,6 +1949,15 @@ class TestCollectionABCs(ABCTestCase):
         self.assertFalse(issubclass(memoryview, ByteString))
         self.validate_abstract_methods(ByteString, '__getitem__', '__len__')
 
+    def test_Buffer(self):
+        for sample in [bytes, bytearray, memoryview]:
+            self.assertIsInstance(sample(b"x"), Buffer)
+            self.assertTrue(issubclass(sample, Buffer))
+        for sample in [str, list, tuple]:
+            self.assertNotIsInstance(sample(), Buffer)
+            self.assertFalse(issubclass(sample, Buffer))
+        self.validate_abstract_methods(Buffer, '__buffer__')
+
     def test_MutableSequence(self):
         for sample in [tuple, str, bytes]:
             self.assertNotIsInstance(sample(), MutableSequence)

Lib/test/test_doctest.py

@@ -707,7 +707,7 @@ plain ol' Python and is guaranteed to be available.
 
     >>> import builtins
     >>> tests = doctest.DocTestFinder().find(builtins)
-    >>> 830 < len(tests) < 850 # approximate number of objects with docstrings
+    >>> 830 < len(tests) < 860 # approximate number of objects with docstrings
     True
     >>> real_tests = [t for t in tests if len(t.examples) > 0]
     >>> len(real_tests) # objects that actually have doctests

Misc/NEWS.d/next/Core and Builtins/2023-03-07-17-37-00.gh-issue-102500.RUSQhz.rst

@@ -0,0 +1,3 @@
+Make the buffer protocol accessible in Python code using the new
+``__buffer__`` and ``__release_buffer__`` magic methods. See :pep:`688` for
+details. Patch by Jelle Zijlstra.

Modules/Setup.stdlib.in

@@ -169,7 +169,7 @@
 @MODULE__XXTESTFUZZ_TRUE@_xxtestfuzz _xxtestfuzz/_xxtestfuzz.c _xxtestfuzz/fuzzer.c
 @MODULE__TESTBUFFER_TRUE@_testbuffer _testbuffer.c
 @MODULE__TESTINTERNALCAPI_TRUE@_testinternalcapi _testinternalcapi.c
-@MODULE__TESTCAPI_TRUE@_testcapi _testcapimodule.c _testcapi/vectorcall.c _testcapi/vectorcall_limited.c _testcapi/heaptype.c _testcapi/unicode.c _testcapi/getargs.c _testcapi/pytime.c _testcapi/datetime.c _testcapi/docstring.c _testcapi/mem.c _testcapi/watchers.c _testcapi/long.c _testcapi/float.c _testcapi/structmember.c _testcapi/exceptions.c _testcapi/code.c _testcapi/pyos.c _testcapi/immortal.c _testcapi/heaptype_relative.c
+@MODULE__TESTCAPI_TRUE@_testcapi _testcapimodule.c _testcapi/vectorcall.c _testcapi/vectorcall_limited.c _testcapi/heaptype.c _testcapi/unicode.c _testcapi/getargs.c _testcapi/pytime.c _testcapi/datetime.c _testcapi/docstring.c _testcapi/mem.c _testcapi/watchers.c _testcapi/long.c _testcapi/float.c _testcapi/structmember.c _testcapi/exceptions.c _testcapi/code.c _testcapi/buffer.c _testcapi/pyos.c _testcapi/immortal.c _testcapi/heaptype_relative.c
 @MODULE__TESTCLINIC_TRUE@_testclinic _testclinic.c
 
 # Some testing modules MUST be built as shared libraries.

Modules/_testcapi/buffer.c

@@ -0,0 +1,102 @@
+/* Test PEP 688 - Buffers */
+
+#include "parts.h"
+
+#include "structmember.h"           // PyMemberDef
+#include <stddef.h>                 // offsetof
+
+typedef struct {
+    PyObject_HEAD
+    PyObject *obj;
+    Py_ssize_t references;
+} testBufObject;
+
+static PyObject *
+testbuf_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
+{
+    PyObject *obj = PyBytes_FromString("test");
+    if (obj == NULL) {
+        return NULL;
+    }
+    testBufObject *self = (testBufObject *)type->tp_alloc(type, 0);
+    if (self == NULL) {
+        Py_DECREF(obj);
+        return NULL;
+    }
+    self->obj = obj;
+    self->references = 0;
+    return (PyObject *)self;
+}
+
+static int
+testbuf_traverse(testBufObject *self, visitproc visit, void *arg)
+{
+    Py_VISIT(self->obj);
+    return 0;
+}
+
+static int
+testbuf_clear(testBufObject *self)
+{
+    Py_CLEAR(self->obj);
+    return 0;
+}
+
+static void
+testbuf_dealloc(testBufObject *self)
+{
+    PyObject_GC_UnTrack(self);
+    Py_XDECREF(self->obj);
+    Py_TYPE(self)->tp_free((PyObject *) self);
+}
+
+static int
+testbuf_getbuf(testBufObject *self, Py_buffer *view, int flags)
+{
+    int buf = PyObject_GetBuffer(self->obj, view, flags);
+    Py_SETREF(view->obj, Py_NewRef(self));
+    self->references++;
+    return buf;
+}
+
+static void
+testbuf_releasebuf(testBufObject *self, Py_buffer *view)
+{
+    self->references--;
+    assert(self->references >= 0);
+}
+
+static PyBufferProcs testbuf_as_buffer = {
+    .bf_getbuffer = (getbufferproc) testbuf_getbuf,
+    .bf_releasebuffer = (releasebufferproc) testbuf_releasebuf,
+};
+
+static struct PyMemberDef testbuf_members[] = {
+    {"references", T_PYSSIZET, offsetof(testBufObject, references), READONLY},
+    {NULL},
+};
+
+static PyTypeObject testBufType = {
+    PyVarObject_HEAD_INIT(NULL, 0)
+    .tp_name = "testBufType",
+    .tp_basicsize = sizeof(testBufObject),
+    .tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC,
+    .tp_new = testbuf_new,
+    .tp_dealloc = (destructor) testbuf_dealloc,
+    .tp_traverse = (traverseproc) testbuf_traverse,
+    .tp_clear = (inquiry) testbuf_clear,
+    .tp_as_buffer = &testbuf_as_buffer,
+    .tp_members = testbuf_members
+};
+
+int
+_PyTestCapi_Init_Buffer(PyObject *m) {
+    if (PyType_Ready(&testBufType) < 0) {
+        return -1;
+    }
+    if (PyModule_AddObjectRef(m, "testBuf", (PyObject *)&testBufType)) {
+        return -1;
+    }
+
+    return 0;
+}

Modules/_testcapi/parts.h

@@ -38,6 +38,7 @@ int _PyTestCapi_Init_Float(PyObject *module);
 int _PyTestCapi_Init_Structmember(PyObject *module);
 int _PyTestCapi_Init_Exceptions(PyObject *module);
 int _PyTestCapi_Init_Code(PyObject *module);
+int _PyTestCapi_Init_Buffer(PyObject *module);
 int _PyTestCapi_Init_PyOS(PyObject *module);
 int _PyTestCapi_Init_Immortal(PyObject *module);
 

Modules/_testcapimodule.c

@@ -3959,7 +3959,6 @@ static PyTypeObject MyList_Type = {
     MyList_new,                                 /* tp_new */
 };
 
-
 /* Test PEP 560 */
 
 typedef struct {
@@ -4310,6 +4309,9 @@ PyInit__testcapi(void)
     if (_PyTestCapi_Init_Code(m) < 0) {
         return NULL;
     }
+    if (_PyTestCapi_Init_Buffer(m) < 0) {
+        return NULL;
+    }
     if (_PyTestCapi_Init_PyOS(m) < 0) {
         return NULL;
     }

Objects/clinic/memoryobject.c.h

@@ -62,6 +62,66 @@ exit:
     return return_value;
 }
 
+PyDoc_STRVAR(memoryview__from_flags__doc__,
+"_from_flags($type, /, object, flags)\n"
+"--\n"
+"\n"
+"Create a new memoryview object which references the given object.");
+
+#define MEMORYVIEW__FROM_FLAGS_METHODDEF    \
+    {"_from_flags", _PyCFunction_CAST(memoryview__from_flags), METH_FASTCALL|METH_KEYWORDS|METH_CLASS, memoryview__from_flags__doc__},
+
+static PyObject *
+memoryview__from_flags_impl(PyTypeObject *type, PyObject *object, int flags);
+
+static PyObject *
+memoryview__from_flags(PyTypeObject *type, PyObject *const *args, Py_ssize_t nargs, PyObject *kwnames)
+{
+    PyObject *return_value = NULL;
+    #if defined(Py_BUILD_CORE) && !defined(Py_BUILD_CORE_MODULE)
+
+    #define NUM_KEYWORDS 2
+    static struct {
+        PyGC_Head _this_is_not_used;
+        PyObject_VAR_HEAD
+        PyObject *ob_item[NUM_KEYWORDS];
+    } _kwtuple = {
+        .ob_base = PyVarObject_HEAD_INIT(&PyTuple_Type, NUM_KEYWORDS)
+        .ob_item = { &_Py_ID(object), &_Py_ID(flags), },
+    };
+    #undef NUM_KEYWORDS
+    #define KWTUPLE (&_kwtuple.ob_base.ob_base)
+
+    #else  // !Py_BUILD_CORE
+    #  define KWTUPLE NULL
+    #endif  // !Py_BUILD_CORE
+
+    static const char * const _keywords[] = {"object", "flags", NULL};
+    static _PyArg_Parser _parser = {
+        .keywords = _keywords,
+        .fname = "_from_flags",
+        .kwtuple = KWTUPLE,
+    };
+    #undef KWTUPLE
+    PyObject *argsbuf[2];
+    PyObject *object;
+    int flags;
+
+    args = _PyArg_UnpackKeywords(args, nargs, NULL, kwnames, &_parser, 2, 2, 0, argsbuf);
+    if (!args) {
+        goto exit;
+    }
+    object = args[0];
+    flags = _PyLong_AsInt(args[1]);
+    if (flags == -1 && PyErr_Occurred()) {
+        goto exit;
+    }
+    return_value = memoryview__from_flags_impl(type, object, flags);
+
+exit:
+    return return_value;
+}
+
 PyDoc_STRVAR(memoryview_release__doc__,
 "release($self, /)\n"
 "--\n"
@@ -356,4 +416,4 @@ skip_optional_pos:
 exit:
     return return_value;
 }
-/*[clinic end generated code: output=a832f2fc44e4794c input=a9049054013a1b77]*/
+/*[clinic end generated code: output=01613814112cedd7 input=a9049054013a1b77]*/

Objects/memoryobject.c

@@ -85,7 +85,7 @@ mbuf_alloc(void)
 }
 
 static PyObject *
-_PyManagedBuffer_FromObject(PyObject *base)
+_PyManagedBuffer_FromObject(PyObject *base, int flags)
 {
     _PyManagedBufferObject *mbuf;
 
@@ -93,7 +93,7 @@ _PyManagedBuffer_FromObject(PyObject *base)
     if (mbuf == NULL)
         return NULL;
 
-    if (PyObject_GetBuffer(base, &mbuf->master, PyBUF_FULL_RO) < 0) {
+    if (PyObject_GetBuffer(base, &mbuf->master, flags) < 0) {
         mbuf->master.obj = NULL;
         Py_DECREF(mbuf);
         return NULL;
@@ -777,11 +777,12 @@ PyMemoryView_FromBuffer(const Py_buffer *info)
     return mv;
 }
 
-/* Create a memoryview from an object that implements the buffer protocol.
+/* Create a memoryview from an object that implements the buffer protocol,
+   using the given flags.
    If the object is a memoryview, the new memoryview must be registered
    with the same managed buffer. Otherwise, a new managed buffer is created. */
 PyObject *
-PyMemoryView_FromObject(PyObject *v)
+PyMemoryView_FromObjectAndFlags(PyObject *v, int flags)
 {
     _PyManagedBufferObject *mbuf;
 
@@ -792,7 +793,7 @@ PyMemoryView_FromObject(PyObject *v)
     }
     else if (PyObject_CheckBuffer(v)) {
         PyObject *ret;
-        mbuf = (_PyManagedBufferObject *)_PyManagedBuffer_FromObject(v);
+        mbuf = (_PyManagedBufferObject *)_PyManagedBuffer_FromObject(v, flags);
         if (mbuf == NULL)
             return NULL;
         ret = mbuf_add_view(mbuf, NULL);
@@ -805,6 +806,14 @@ PyMemoryView_FromObject(PyObject *v)
         Py_TYPE(v)->tp_name);
     return NULL;
 }
+/* Create a memoryview from an object that implements the buffer protocol.
+   If the object is a memoryview, the new memoryview must be registered
+   with the same managed buffer. Otherwise, a new managed buffer is created. */
+PyObject *
+PyMemoryView_FromObject(PyObject *v)
+{
+    return PyMemoryView_FromObjectAndFlags(v, PyBUF_FULL_RO);
+}
 
 /* Copy the format string from a base object that might vanish. */
 static int
@@ -851,7 +860,7 @@ memory_from_contiguous_copy(const Py_buffer *src, char order)
     if (bytes == NULL)
         return NULL;
 
-    mbuf = (_PyManagedBufferObject *)_PyManagedBuffer_FromObject(bytes);
+    mbuf = (_PyManagedBufferObject *)_PyManagedBuffer_FromObject(bytes, PyBUF_FULL_RO);
     Py_DECREF(bytes);
     if (mbuf == NULL)
         return NULL;
@@ -968,6 +977,24 @@ memoryview_impl(PyTypeObject *type, PyObject *object)
 }
 
 
+/*[clinic input]
+@classmethod
+memoryview._from_flags
+
+    object: object
+    flags: int
+
+Create a new memoryview object which references the given object.
+[clinic start generated code]*/
+
+static PyObject *
+memoryview__from_flags_impl(PyTypeObject *type, PyObject *object, int flags)
+/*[clinic end generated code: output=bf71f9906c266ee2 input=f5f82fd0e744356b]*/
+{
+    return PyMemoryView_FromObjectAndFlags(object, flags);
+}
+
+
 /****************************************************************************/
 /*                         Previously in abstract.c                         */
 /****************************************************************************/
@@ -3184,6 +3211,7 @@ static PyMethodDef memory_methods[] = {
     MEMORYVIEW_TOLIST_METHODDEF
     MEMORYVIEW_CAST_METHODDEF
     MEMORYVIEW_TOREADONLY_METHODDEF
+    MEMORYVIEW__FROM_FLAGS_METHODDEF
     {"__enter__",   memory_enter, METH_NOARGS, NULL},
     {"__exit__",    memory_exit, METH_VARARGS, NULL},
     {NULL,          NULL}

Objects/object.c

@@ -14,6 +14,7 @@
 #include "pycore_pymem.h"         // _PyMem_IsPtrFreed()
 #include "pycore_pystate.h"       // _PyThreadState_GET()
 #include "pycore_symtable.h"      // PySTEntry_Type
+#include "pycore_typeobject.h"    // _PyBufferWrapper_Type
 #include "pycore_unionobject.h"   // _PyUnion_Type
 #include "pycore_interpreteridobject.h"  // _PyInterpreterID_Type
 
@@ -2084,6 +2085,7 @@ static PyTypeObject* static_types[] = {
     &_PyAsyncGenASend_Type,
     &_PyAsyncGenAThrow_Type,
     &_PyAsyncGenWrappedValue_Type,
+    &_PyBufferWrapper_Type,
     &_PyContextTokenMissing_Type,
     &_PyCoroWrapper_Type,
     &_Py_GenericAliasIterType,

Objects/typeobject.c

@@ -6,6 +6,7 @@
 #include "pycore_symtable.h"      // _Py_Mangle()
 #include "pycore_dict.h"          // _PyDict_KeysSize()
 #include "pycore_initconfig.h"    // _PyStatus_OK()
+#include "pycore_memoryobject.h"  // PyMemoryView_FromObjectAndFlags()
 #include "pycore_moduleobject.h"  // _PyModule_GetDef()
 #include "pycore_object.h"        // _PyType_HasFeature()
 #include "pycore_long.h"          // _PyLong_IsNegative()
@@ -8059,6 +8060,58 @@ wrap_descr_delete(PyObject *self, PyObject *args, void *wrapped)
     Py_RETURN_NONE;
 }
 
+static PyObject *
+wrap_buffer(PyObject *self, PyObject *args, void *wrapped)
+{
+    PyObject *arg = NULL;
+
+    if (!PyArg_UnpackTuple(args, "", 1, 1, &arg)) {
+        return NULL;
+    }
+    Py_ssize_t flags = PyNumber_AsSsize_t(arg, PyExc_OverflowError);
+    if (flags == -1 && PyErr_Occurred()) {
+        return NULL;
+    }
+    if (flags > INT_MAX) {
+        PyErr_SetString(PyExc_OverflowError,
+                        "buffer flags too large");
+        return NULL;
+    }
+
+    return PyMemoryView_FromObjectAndFlags(self, Py_SAFE_DOWNCAST(flags, Py_ssize_t, int));
+}
+
+static PyObject *
+wrap_releasebuffer(PyObject *self, PyObject *args, void *wrapped)
+{
+    PyObject *arg = NULL;
+    if (!PyArg_UnpackTuple(args, "", 1, 1, &arg)) {
+        return NULL;
+    }
+    if (!PyMemoryView_Check(arg)) {
+        PyErr_SetString(PyExc_TypeError,
+                        "expected a memoryview object");
+        return NULL;
+    }
+    PyMemoryViewObject *mview = (PyMemoryViewObject *)arg;
+    if (mview->view.obj != self) {
+        PyErr_SetString(PyExc_ValueError,
+                        "memoryview's buffer is not this object");
+        return NULL;
+    }
+    if (mview->flags & _Py_MEMORYVIEW_RELEASED) {
+        PyErr_SetString(PyExc_ValueError,
+                        "memoryview's buffer has already been released");
+        return NULL;
+    }
+    PyObject *res = PyObject_CallMethodNoArgs((PyObject *)mview, &_Py_ID(release));
+    if (res == NULL) {
+        return NULL;
+    }
+    Py_DECREF(res);
+    Py_RETURN_NONE;
+}
+
 static PyObject *
 wrap_init(PyObject *self, PyObject *args, void *wrapped, PyObject *kwds)
 {
@@ -8895,6 +8948,132 @@ slot_tp_finalize(PyObject *self)
     PyErr_SetRaisedException(exc);
 }
 
+typedef struct _PyBufferWrapper {
+    PyObject_HEAD
+    PyObject *mv;
+    PyObject *obj;
+} PyBufferWrapper;
+
+static int
+bufferwrapper_traverse(PyBufferWrapper *self, visitproc visit, void *arg)
+{
+    Py_VISIT(self->mv);
+    Py_VISIT(self->obj);
+    return 0;
+}
+
+static void
+bufferwrapper_dealloc(PyObject *self)
+{
+    PyBufferWrapper *bw = (PyBufferWrapper *)self;
+
+    _PyObject_GC_UNTRACK(self);
+    Py_XDECREF(bw->mv);
+    Py_XDECREF(bw->obj);
+    Py_TYPE(self)->tp_free(self);
+}
+
+static void
+bufferwrapper_releasebuf(PyObject *self, Py_buffer *view)
+{
+    PyBufferWrapper *bw = (PyBufferWrapper *)self;
+
+    assert(PyMemoryView_Check(bw->mv));
+    Py_TYPE(bw->mv)->tp_as_buffer->bf_releasebuffer(bw->mv, view);
+    if (Py_TYPE(bw->obj)->tp_as_buffer != NULL
+        && Py_TYPE(bw->obj)->tp_as_buffer->bf_releasebuffer != NULL) {
+        Py_TYPE(bw->obj)->tp_as_buffer->bf_releasebuffer(bw->obj, view);
+    }
+}
+
+static PyBufferProcs bufferwrapper_as_buffer = {
+    .bf_releasebuffer = bufferwrapper_releasebuf,
+};
+
+
+PyTypeObject _PyBufferWrapper_Type = {
+    PyVarObject_HEAD_INIT(&PyType_Type, 0)
+    .tp_name = "_buffer_wrapper",
+    .tp_basicsize = sizeof(PyBufferWrapper),
+    .tp_alloc = PyType_GenericAlloc,
+    .tp_free = PyObject_GC_Del,
+    .tp_traverse = (traverseproc)bufferwrapper_traverse,
+    .tp_dealloc = bufferwrapper_dealloc,
+    .tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC,
+    .tp_as_buffer = &bufferwrapper_as_buffer,
+};
+
+static int
+slot_bf_getbuffer(PyObject *self, Py_buffer *buffer, int flags)
+{
+    PyObject *flags_obj = PyLong_FromLong(flags);
+    if (flags_obj == NULL) {
+        return -1;
+    }
+    PyBufferWrapper *wrapper = NULL;
+    PyObject *stack[2] = {self, flags_obj};
+    PyObject *ret = vectorcall_method(&_Py_ID(__buffer__), stack, 2);
+    if (ret == NULL) {
+        goto fail;
+    }
+    if (!PyMemoryView_Check(ret)) {
+        PyErr_Format(PyExc_TypeError,
+                     "__buffer__ returned non-memoryview object");
+        goto fail;
+    }
+
+    if (PyObject_GetBuffer(ret, buffer, flags) < 0) {
+        goto fail;
+    }
+    assert(buffer->obj == ret);
+
+    wrapper = PyObject_GC_New(PyBufferWrapper, &_PyBufferWrapper_Type);
+    if (wrapper == NULL) {
+        goto fail;
+    }
+    wrapper->mv = ret;
+    wrapper->obj = Py_NewRef(self);
+    _PyObject_GC_TRACK(wrapper);
+
+    buffer->obj = (PyObject *)wrapper;
+    Py_DECREF(ret);
+    Py_DECREF(flags_obj);
+    return 0;
+
+fail:
+    Py_XDECREF(wrapper);
+    Py_XDECREF(ret);
+    Py_DECREF(flags_obj);
+    return -1;
+}
+
+static void
+slot_bf_releasebuffer(PyObject *self, Py_buffer *buffer)
+{
+    PyObject *mv;
+    if (Py_TYPE(buffer->obj) == &_PyBufferWrapper_Type) {
+        // Make sure we pass the same memoryview to
+        // __release_buffer__() that __buffer__() returned.
+        mv = Py_NewRef(((PyBufferWrapper *)buffer->obj)->mv);
+    }
+    else {
+        mv = PyMemoryView_FromBuffer(buffer);
+        if (mv == NULL) {
+            PyErr_WriteUnraisable(self);
+            return;
+        }
+    }
+    PyObject *stack[2] = {self, mv};
+    PyObject *ret = vectorcall_method(&_Py_ID(__release_buffer__), stack, 2);
+    Py_DECREF(mv);
+    if (ret == NULL) {
+        PyErr_WriteUnraisable(self);
+    }
+    else {
+        Py_DECREF(ret);
+    }
+}
+
 static PyObject *
 slot_am_await(PyObject *self)
 {
@@ -8962,6 +9141,7 @@ an all-zero entry.
 
 #undef TPSLOT
 #undef FLSLOT
+#undef BUFSLOT
 #undef AMSLOT
 #undef ETSLOT
 #undef SQSLOT
@@ -8981,6 +9161,8 @@ an all-zero entry.
 #define ETSLOT(NAME, SLOT, FUNCTION, WRAPPER, DOC) \
     {#NAME, offsetof(PyHeapTypeObject, SLOT), (void *)(FUNCTION), WRAPPER, \
      PyDoc_STR(DOC), .name_strobj = &_Py_ID(NAME) }
+#define BUFSLOT(NAME, SLOT, FUNCTION, WRAPPER, DOC) \
+    ETSLOT(NAME, as_buffer.SLOT, FUNCTION, WRAPPER, DOC)
 #define AMSLOT(NAME, SLOT, FUNCTION, WRAPPER, DOC) \
     ETSLOT(NAME, as_async.SLOT, FUNCTION, WRAPPER, DOC)
 #define SQSLOT(NAME, SLOT, FUNCTION, WRAPPER, DOC) \
@@ -9062,6 +9244,13 @@ static pytype_slotdef slotdefs[] = {
            "Create and return new object.  See help(type) for accurate signature."),
     TPSLOT(__del__, tp_finalize, slot_tp_finalize, (wrapperfunc)wrap_del, ""),
 
+    BUFSLOT(__buffer__, bf_getbuffer, slot_bf_getbuffer, wrap_buffer,
+            "__buffer__($self, flags, /)\n--\n\n"
+            "Return a buffer object that exposes the underlying memory of the object."),
+    BUFSLOT(__release_buffer__, bf_releasebuffer, slot_bf_releasebuffer, wrap_releasebuffer,
+            "__release_buffer__($self, /)\n--\n\n"
+            "Release the buffer object that exposes the underlying memory of the object."),
+
     AMSLOT(__await__, am_await, slot_am_await, wrap_unaryfunc,
            "__await__($self, /)\n--\n\nReturn an iterator to be used in await expression."),
     AMSLOT(__aiter__, am_aiter, slot_am_aiter, wrap_unaryfunc,
@@ -9208,8 +9397,12 @@ slotptr(PyTypeObject *type, int ioffset)
 
     /* Note: this depends on the order of the members of PyHeapTypeObject! */
     assert(offset >= 0);
-    assert((size_t)offset < offsetof(PyHeapTypeObject, as_buffer));
-    if ((size_t)offset >= offsetof(PyHeapTypeObject, as_sequence)) {
+    assert((size_t)offset < offsetof(PyHeapTypeObject, ht_name));
+    if ((size_t)offset >= offsetof(PyHeapTypeObject, as_buffer)) {
+        ptr = (char *)type->tp_as_buffer;
+        offset -= offsetof(PyHeapTypeObject, as_buffer);
+    }
+    else if ((size_t)offset >= offsetof(PyHeapTypeObject, as_sequence)) {
         ptr = (char *)type->tp_as_sequence;
         offset -= offsetof(PyHeapTypeObject, as_sequence);
     }

PCbuild/_testcapi.vcxproj

@@ -110,6 +110,7 @@
     <ClCompile Include="..\Modules\_testcapi\structmember.c" />
     <ClCompile Include="..\Modules\_testcapi\exceptions.c" />
     <ClCompile Include="..\Modules\_testcapi\code.c" />
+    <ClCompile Include="..\Modules\_testcapi\buffer.c" />
     <ClCompile Include="..\Modules\_testcapi\pyos.c" />
     <ClCompile Include="..\Modules\_testcapi\immortal.c" />
   </ItemGroup>

PCbuild/_testcapi.vcxproj.filters

@@ -60,6 +60,9 @@
     <ClCompile Include="..\Modules\_testcapi\code.c">
       <Filter>Source Files</Filter>
     </ClCompile>
+    <ClCompile Include="..\Modules\_testcapi\buffer.c">
+      <Filter>Source Files</Filter>
+    </ClCompile>
     <ClCompile Include="..\Modules\_testcapi\pyos.c">
       <Filter>Source Files</Filter>
     </ClCompile>

Tools/build/generate_global_objects.py

@@ -121,6 +121,8 @@ IDENTIFIERS = [
     '__xor__',
     '__divmod__',
     '__rdivmod__',
+    '__buffer__',
+    '__release_buffer__',
 ]
 
 NON_GENERATED_IMMORTAL_OBJECTS = [

Tools/c-analyzer/cpython/globals-to-fix.tsv

@@ -86,6 +86,7 @@ Objects/sliceobject.c	-	PyEllipsis_Type	-
 Objects/sliceobject.c	-	PySlice_Type	-
 Objects/tupleobject.c	-	PyTupleIter_Type	-
 Objects/tupleobject.c	-	PyTuple_Type	-
+Objects/typeobject.c	-	_PyBufferWrapper_Type	-
 Objects/typeobject.c	-	PyBaseObject_Type	-
 Objects/typeobject.c	-	PySuper_Type	-
 Objects/typeobject.c	-	PyType_Type	-

Tools/c-analyzer/cpython/ignored.tsv

@@ -404,6 +404,7 @@ Modules/_testbuffer.c	ndarray_memoryview_from_buffer	strides	-
 Modules/_testbuffer.c	ndarray_memoryview_from_buffer	suboffsets	-
 Modules/_testbuffer.c	ndarray_push	kwlist	-
 Modules/_testbuffer.c	staticarray_init	kwlist	-
+Modules/_testcapi/buffer.c	-	testBufType	-
 Modules/_testcapi/code.c	get_code_extra_index	key	-
 Modules/_testcapi/datetime.c	-	test_run_counter	-
 Modules/_testcapi/exceptions.c	-	PyRecursingInfinitelyError_Type	-