bpo-38787: C API for module state access from extension methods (PEP 573) (GH-19936)

Module C state is now accessible from C-defined heap type methods (PEP 573).
Patch by Marcel Plch and Petr Viktorin.

Co-authored-by: Marcel Plch <mplch@redhat.com>
Co-authored-by: Victor Stinner <vstinner@python.org>

Doc/c-api/structures.rst

@@ -147,23 +147,56 @@ Implementing functions and methods
    value of the function as exposed in Python.  The function must return a new
    reference.
 
+   The function signature is::
+
+      PyObject *PyCFunction(PyObject *self,
+                            PyObject *const *args);
 
 .. c:type:: PyCFunctionWithKeywords
 
    Type of the functions used to implement Python callables in C
    with signature :const:`METH_VARARGS | METH_KEYWORDS`.
+   The function signature is::
+
+      PyObject *PyCFunctionWithKeywords(PyObject *self,
+                                        PyObject *const *args,
+                                        PyObject *kwargs);
 
 
 .. c:type:: _PyCFunctionFast
 
    Type of the functions used to implement Python callables in C
    with signature :const:`METH_FASTCALL`.
+   The function signature is::
 
+      PyObject *_PyCFunctionFast(PyObject *self,
+                                 PyObject *const *args,
+                                 Py_ssize_t nargs);
 
 .. c:type:: _PyCFunctionFastWithKeywords
 
    Type of the functions used to implement Python callables in C
    with signature :const:`METH_FASTCALL | METH_KEYWORDS`.
+   The function signature is::
+
+      PyObject *_PyCFunctionFastWithKeywords(PyObject *self,
+                                             PyObject *const *args,
+                                             Py_ssize_t nargs,
+                                             PyObject *kwnames);
+
+.. c:type:: PyCMethod
+
+   Type of the functions used to implement Python callables in C
+   with signature :const:`METH_METHOD | METH_FASTCALL | METH_KEYWORDS`.
+   The function signature is::
+
+      PyObject *PyCMethod(PyObject *self,
+                          PyTypeObject *defining_class,
+                          PyObject *const *args,
+                          Py_ssize_t nargs,
+                          PyObject *kwnames)
+
+   .. versionadded:: 3.9
 
 
 .. c:type:: PyMethodDef
@@ -197,9 +230,7 @@ The :attr:`ml_flags` field is a bitfield which can include the following flags.
 The individual flags indicate either a calling convention or a binding
 convention.
 
-There are four basic calling conventions for positional arguments
-and two of them can be combined with :const:`METH_KEYWORDS` to support
-also keyword arguments.  So there are a total of 6 calling conventions:
+There are these calling conventions:
 
 .. data:: METH_VARARGS
 
@@ -250,6 +281,19 @@ also keyword arguments.  So there are a total of 6 calling conventions:
    .. versionadded:: 3.7
 
 
+.. data:: METH_METHOD | METH_FASTCALL | METH_KEYWORDS
+
+   Extension of :const:`METH_FASTCALL | METH_KEYWORDS` supporting the *defining
+   class*, that is, the class that contains the method in question.
+   The defining class might be a superclass of ``Py_TYPE(self)``.
+
+   The method needs to be of type :c:type:`PyCMethod`, the same as for
+   ``METH_FASTCALL | METH_KEYWORDS`` with ``defining_class`` argument added after
+   ``self``.
+
+   .. versionadded:: 3.9
+
+
 .. data:: METH_NOARGS
 
    Methods without parameters don't need to check whether arguments are given if

Doc/c-api/type.rst

@@ -109,6 +109,30 @@ Type Objects
 
    .. versionadded:: 3.4
 
+.. c:function:: PyObject* PyType_GetModule(PyTypeObject *type)
+
+   Return the module object associated with the given type when the type was
+   created using :c:func:`PyType_FromModuleAndSpec`.
+
+   If no module is associated with the given type, sets :py:class:`TypeError`
+   and returns ``NULL``.
+
+   .. versionadded:: 3.9
+
+.. c:function:: void* PyType_GetModuleState(PyTypeObject *type)
+
+   Return the state of the module object associated with the given type.
+   This is a shortcut for calling :c:func:`PyModule_GetState()` on the result
+   of :c:func:`PyType_GetModule`.
+
+   If no module is associated with the given type, sets :py:class:`TypeError`
+   and returns ``NULL``.
+
+   If the *type* has an associated module but its state is ``NULL``,
+   returns ``NULL`` without setting an exception.
+
+   .. versionadded:: 3.9
+
 
 Creating Heap-Allocated Types
 .............................
@@ -116,7 +140,7 @@ Creating Heap-Allocated Types
 The following functions and structs are used to create
 :ref:`heap types <heap-types>`.
 
-.. c:function:: PyObject* PyType_FromSpecWithBases(PyType_Spec *spec, PyObject *bases)
+.. c:function:: PyObject* PyType_FromModuleAndSpec(PyObject *module, PyType_Spec *spec, PyObject *bases)
 
    Creates and returns a heap type object from the *spec*
    (:const:`Py_TPFLAGS_HEAPTYPE`).
@@ -127,8 +151,18 @@ The following functions and structs are used to create
    If *bases* is ``NULL``, the *Py_tp_base* slot is used instead.
    If that also is ``NULL``, the new type derives from :class:`object`.
 
+   The *module* must be a module object or ``NULL``.
+   If not ``NULL``, the module is associated with the new type and can later be
+   retreived with :c:func:`PyType_GetModule`.
+
    This function calls :c:func:`PyType_Ready` on the new type.
 
+   .. versionadded:: 3.9
+
+.. c:function:: PyObject* PyType_FromSpecWithBases(PyType_Spec *spec, PyObject *bases)
+
+   Equivalent to ``PyType_FromModuleAndSpec(NULL, spec, bases)``.
+
    .. versionadded:: 3.3
 
 .. c:function:: PyObject* PyType_FromSpec(PyType_Spec *spec)

Include/cpython/methodobject.h

@@ -0,0 +1,32 @@
+#ifndef Py_CPYTHON_METHODOBJECT_H
+#  error "this header file must not be included directly"
+#endif
+
+PyAPI_DATA(PyTypeObject) PyCMethod_Type;
+
+/* Macros for direct access to these values. Type checks are *not*
+   done, so use with care. */
+#define PyCFunction_GET_FUNCTION(func) \
+        (((PyCFunctionObject *)func) -> m_ml -> ml_meth)
+#define PyCFunction_GET_SELF(func) \
+        (((PyCFunctionObject *)func) -> m_ml -> ml_flags & METH_STATIC ? \
+         NULL : ((PyCFunctionObject *)func) -> m_self)
+#define PyCFunction_GET_FLAGS(func) \
+        (((PyCFunctionObject *)func) -> m_ml -> ml_flags)
+#define PyCFunction_GET_CLASS(func) \
+    (((PyCFunctionObject *)func) -> m_ml -> ml_flags & METH_METHOD ? \
+         ((PyCMethodObject *)func) -> mm_class : NULL)
+
+typedef struct {
+    PyObject_HEAD
+    PyMethodDef *m_ml; /* Description of the C function to call */
+    PyObject    *m_self; /* Passed as 'self' arg to the C func, can be NULL */
+    PyObject    *m_module; /* The __module__ attribute, can be anything */
+    PyObject    *m_weakreflist; /* List of weak references */
+    vectorcallfunc vectorcall;
+} PyCFunctionObject;
+
+typedef struct {
+    PyCFunctionObject func;
+    PyTypeObject *mm_class; /* Class that defines this method */
+} PyCMethodObject;

Include/cpython/object.h

@@ -289,6 +289,7 @@ typedef struct _heaptypeobject {
     PyBufferProcs as_buffer;
     PyObject *ht_name, *ht_slots, *ht_qualname;
     struct _dictkeysobject *ht_cached_keys;
+    PyObject *ht_module;
     /* here are optional user slots, followed by the members. */
 } PyHeapTypeObject;
 

Include/methodobject.h

@@ -13,7 +13,7 @@ extern "C" {
 
 PyAPI_DATA(PyTypeObject) PyCFunction_Type;
 
-#define PyCFunction_Check(op) Py_IS_TYPE(op, &PyCFunction_Type)
+#define PyCFunction_Check(op) (Py_IS_TYPE(op, &PyCFunction_Type) || (PyType_IsSubtype(Py_TYPE(op), &PyCFunction_Type)))
 
 typedef PyObject *(*PyCFunction)(PyObject *, PyObject *);
 typedef PyObject *(*_PyCFunctionFast) (PyObject *, PyObject *const *, Py_ssize_t);
@@ -22,21 +22,13 @@ typedef PyObject *(*PyCFunctionWithKeywords)(PyObject *, PyObject *,
 typedef PyObject *(*_PyCFunctionFastWithKeywords) (PyObject *,
                                                    PyObject *const *, Py_ssize_t,
                                                    PyObject *);
+typedef PyObject *(*PyCMethod)(PyObject *, PyTypeObject *, PyObject *const *,
+                               size_t, PyObject *);
+
 PyAPI_FUNC(PyCFunction) PyCFunction_GetFunction(PyObject *);
 PyAPI_FUNC(PyObject *) PyCFunction_GetSelf(PyObject *);
 PyAPI_FUNC(int) PyCFunction_GetFlags(PyObject *);
 
-/* Macros for direct access to these values. Type checks are *not*
-   done, so use with care. */
-#ifndef Py_LIMITED_API
-#define PyCFunction_GET_FUNCTION(func) \
-        (((PyCFunctionObject *)func) -> m_ml -> ml_meth)
-#define PyCFunction_GET_SELF(func) \
-        (((PyCFunctionObject *)func) -> m_ml -> ml_flags & METH_STATIC ? \
-         NULL : ((PyCFunctionObject *)func) -> m_self)
-#define PyCFunction_GET_FLAGS(func) \
-        (((PyCFunctionObject *)func) -> m_ml -> ml_flags)
-#endif
 Py_DEPRECATED(3.9) PyAPI_FUNC(PyObject *) PyCFunction_Call(PyObject *, PyObject *, PyObject *);
 
 struct PyMethodDef {
@@ -52,6 +44,13 @@ typedef struct PyMethodDef PyMethodDef;
 PyAPI_FUNC(PyObject *) PyCFunction_NewEx(PyMethodDef *, PyObject *,
                                          PyObject *);
 
+#if !defined(Py_LIMITED_API) || Py_LIMITED_API+0 >= 0x03090000
+#define PyCFunction_NewEx(ML, SELF, MOD) PyCMethod_New((ML), (SELF), (MOD), NULL)
+PyAPI_FUNC(PyObject *) PyCMethod_New(PyMethodDef *, PyObject *,
+                                     PyObject *, PyTypeObject *);
+#endif
+
+
 /* Flag passed to newmethodobject */
 /* #define METH_OLDARGS  0x0000   -- unsupported now */
 #define METH_VARARGS  0x0001
@@ -84,15 +83,24 @@ PyAPI_FUNC(PyObject *) PyCFunction_NewEx(PyMethodDef *, PyObject *,
 #define METH_STACKLESS 0x0000
 #endif
 
+/* METH_METHOD means the function stores an
+ * additional reference to the class that defines it;
+ * both self and class are passed to it.
+ * It uses PyCMethodObject instead of PyCFunctionObject.
+ * May not be combined with METH_NOARGS, METH_O, METH_CLASS or METH_STATIC.
+ */
+
+#if !defined(Py_LIMITED_API) || Py_LIMITED_API+0 >= 0x03090000
+#define METH_METHOD 0x0200
+#endif
+
+
 #ifndef Py_LIMITED_API
-typedef struct {
-    PyObject_HEAD
-    PyMethodDef *m_ml; /* Description of the C function to call */
-    PyObject    *m_self; /* Passed as 'self' arg to the C func, can be NULL */
-    PyObject    *m_module; /* The __module__ attribute, can be anything */
-    PyObject    *m_weakreflist; /* List of weak references */
-    vectorcallfunc vectorcall;
-} PyCFunctionObject;
+
+#define Py_CPYTHON_METHODOBJECT_H
+#include  "cpython/methodobject.h"
+#undef Py_CPYTHON_METHODOBJECT_H
+
 #endif
 
 #ifdef __cplusplus

Include/object.h

@@ -213,6 +213,11 @@ PyAPI_FUNC(PyObject*) PyType_FromSpecWithBases(PyType_Spec*, PyObject*);
 #if !defined(Py_LIMITED_API) || Py_LIMITED_API+0 >= 0x03040000
 PyAPI_FUNC(void*) PyType_GetSlot(PyTypeObject*, int);
 #endif
+#if !defined(Py_LIMITED_API) || Py_LIMITED_API+0 >= 0x03090000
+PyAPI_FUNC(PyObject*) PyType_FromModuleAndSpec(PyObject *, PyType_Spec *, PyObject *);
+PyAPI_FUNC(PyObject *) PyType_GetModule(struct _typeobject *);
+PyAPI_FUNC(void *) PyType_GetModuleState(struct _typeobject *);
+#endif
 
 /* Generic type check */
 PyAPI_FUNC(int) PyType_IsSubtype(PyTypeObject *, PyTypeObject *);

Lib/test/test_capi.py

@@ -13,6 +13,8 @@ import threading
 import time
 import unittest
 import weakref
+import importlib.machinery
+import importlib.util
 from test import support
 from test.support import MISSING_C_DOCSTRINGS
 from test.support.script_helper import assert_python_failure, assert_python_ok
@@ -774,5 +776,76 @@ class PyMemDefaultTests(PyMemDebugTests):
     PYTHONMALLOC = ''
 
 
+class Test_ModuleStateAccess(unittest.TestCase):
+    """Test access to module start (PEP 573)"""
+
+    # The C part of the tests lives in _testmultiphase, in a module called
+    # _testmultiphase_meth_state_access.
+    # This module has multi-phase initialization, unlike _testcapi.
+
+    def setUp(self):
+        fullname = '_testmultiphase_meth_state_access'  # XXX
+        origin = importlib.util.find_spec('_testmultiphase').origin
+        loader = importlib.machinery.ExtensionFileLoader(fullname, origin)
+        spec = importlib.util.spec_from_loader(fullname, loader)
+        module = importlib.util.module_from_spec(spec)
+        loader.exec_module(module)
+        self.module = module
+
+    def test_subclass_get_module(self):
+        """PyType_GetModule for defining_class"""
+        class StateAccessType_Subclass(self.module.StateAccessType):
+            pass
+
+        instance = StateAccessType_Subclass()
+        self.assertIs(instance.get_defining_module(), self.module)
+
+    def test_subclass_get_module_with_super(self):
+        class StateAccessType_Subclass(self.module.StateAccessType):
+            def get_defining_module(self):
+                return super().get_defining_module()
+
+        instance = StateAccessType_Subclass()
+        self.assertIs(instance.get_defining_module(), self.module)
+
+    def test_state_access(self):
+        """Checks methods defined with and without argument clinic
+
+        This tests a no-arg method (get_count) and a method with
+        both a positional and keyword argument.
+        """
+
+        a = self.module.StateAccessType()
+        b = self.module.StateAccessType()
+
+        methods = {
+            'clinic': a.increment_count_clinic,
+            'noclinic': a.increment_count_noclinic,
+        }
+
+        for name, increment_count in methods.items():
+            with self.subTest(name):
+                self.assertEqual(a.get_count(), b.get_count())
+                self.assertEqual(a.get_count(), 0)
+
+                increment_count()
+                self.assertEqual(a.get_count(), b.get_count())
+                self.assertEqual(a.get_count(), 1)
+
+                increment_count(3)
+                self.assertEqual(a.get_count(), b.get_count())
+                self.assertEqual(a.get_count(), 4)
+
+                increment_count(-2, twice=True)
+                self.assertEqual(a.get_count(), b.get_count())
+                self.assertEqual(a.get_count(), 0)
+
+                with self.assertRaises(TypeError):
+                    increment_count(thrice=3)
+
+                with self.assertRaises(TypeError):
+                    increment_count(1, 2, 3)
+
+
 if __name__ == "__main__":
     unittest.main()

Lib/test/test_sys.py

@@ -1322,7 +1322,7 @@ class SizeofTest(unittest.TestCase):
                   '3P'                  # PyMappingMethods
                   '10P'                 # PySequenceMethods
                   '2P'                  # PyBufferProcs
-                  '4P')
+                  '5P')
         class newstyleclass(object): pass
         # Separate block for PyDictKeysObject with 8 keys and 5 entries
         check(newstyleclass, s + calcsize("2nP2n0P") + 8 + 5*calcsize("n2P"))

Makefile.pre.in

@@ -1104,6 +1104,7 @@ PYTHON_HEADERS= \
 		$(srcdir)/Include/cpython/initconfig.h \
 		$(srcdir)/Include/cpython/interpreteridobject.h \
 		$(srcdir)/Include/cpython/listobject.h \
+		$(srcdir)/Include/cpython/methodobject.h \
 		$(srcdir)/Include/cpython/object.h \
 		$(srcdir)/Include/cpython/objimpl.h \
 		$(srcdir)/Include/cpython/pyerrors.h \

Misc/NEWS.d/next/C API/2020-01-22-12-38-59.bpo-38787.HUH6hd.rst

@@ -0,0 +1,2 @@
+Module C state is now accessible from C-defined heap type methods (:pep:`573`).
+Patch by Marcel Plch and Petr Viktorin.

Modules/_testmultiphase.c

@@ -4,6 +4,19 @@
 
 #include "Python.h"
 
+/* State for testing module state access from methods */
+
+typedef struct {
+    int counter;
+} meth_state;
+
+/*[clinic input]
+module _testmultiphase
+
+class _testmultiphase.StateAccessType "StateAccessTypeObject *" "!StateAccessType"
+[clinic start generated code]*/
+/*[clinic end generated code: output=da39a3ee5e6b4b0d input=bab9f2fe3bd312ff]*/
+
 /* Example objects */
 typedef struct {
     PyObject_HEAD
@@ -14,6 +27,10 @@ typedef struct {
     PyObject *integer;
 } testmultiphase_state;
 
+typedef struct {
+    PyObject_HEAD
+} StateAccessTypeObject;
+
 /* Example methods */
 
 static int
@@ -42,6 +59,7 @@ Example_demo(ExampleObject *self, PyObject *args)
     Py_RETURN_NONE;
 }
 
+#include "clinic/_testmultiphase.c.h"
 
 static PyMethodDef Example_methods[] = {
     {"demo",            (PyCFunction)Example_demo,  METH_VARARGS,
@@ -102,6 +120,150 @@ static PyType_Spec Example_Type_spec = {
     Example_Type_slots
 };
 
+
+/*[clinic input]
+_testmultiphase.StateAccessType.get_defining_module
+
+    cls: defining_class
+
+Return the module of the defining class.
+[clinic start generated code]*/
+
+static PyObject *
+_testmultiphase_StateAccessType_get_defining_module_impl(StateAccessTypeObject *self,
+                                                         PyTypeObject *cls)
+/*[clinic end generated code: output=ba2a14284a5d0921 input=946149f91cf72c0d]*/
+{
+    PyObject *retval;
+    retval = PyType_GetModule(cls);
+    if (retval == NULL) {
+        return NULL;
+    }
+    Py_INCREF(retval);
+    return retval;
+}
+
+/*[clinic input]
+_testmultiphase.StateAccessType.increment_count_clinic
+
+    cls: defining_class
+    /
+    n: int = 1
+    *
+    twice: bool = False
+
+Add 'n' from the module-state counter.
+
+Pass 'twice' to double that amount.
+
+This tests Argument Clinic support for defining_class.
+[clinic start generated code]*/
+
+static PyObject *
+_testmultiphase_StateAccessType_increment_count_clinic_impl(StateAccessTypeObject *self,
+                                                            PyTypeObject *cls,
+                                                            int n, int twice)
+/*[clinic end generated code: output=3b34f86bc5473204 input=551d482e1fe0b8f5]*/
+{
+    meth_state *m_state = PyType_GetModuleState(cls);
+    if (twice) {
+        n *= 2;
+    }
+    m_state->counter += n;
+
+    Py_RETURN_NONE;
+}
+
+PyDoc_STRVAR(_StateAccessType_decrement_count__doc__,
+"decrement_count($self, /, n=1, *, twice=None)\n"
+"--\n"
+"\n"
+"Add 'n' from the module-state counter.\n"
+"Pass 'twice' to double that amount.\n"
+"(This is to test both positional and keyword arguments.");
+
+// Intentionally does not use Argument Clinic
+static PyObject *
+_StateAccessType_increment_count_noclinic(StateAccessTypeObject *self,
+                                          PyTypeObject *defining_class,
+                                          PyObject *const *args,
+                                          Py_ssize_t nargs,
+                                          PyObject *kwnames)
+{
+    if (!_PyArg_CheckPositional("StateAccessTypeObject.decrement_count", nargs, 0, 1)) {
+        return NULL;
+    }
+    long n = 1;
+    if (nargs) {
+        n = PyLong_AsLong(args[0]);
+        if (PyErr_Occurred()) {
+            return NULL;
+        }
+    }
+    if (kwnames && PyTuple_Check(kwnames)) {
+        if (PyTuple_GET_SIZE(kwnames) > 1 ||
+            PyUnicode_CompareWithASCIIString(
+                PyTuple_GET_ITEM(kwnames, 0),
+                "twice"
+            )) {
+            PyErr_SetString(
+                PyExc_TypeError,
+                "decrement_count only takes 'twice' keyword argument"
+            );
+            return NULL;
+        }
+        n *= 2;
+    }
+    meth_state *m_state = PyType_GetModuleState(defining_class);
+    m_state->counter += n;
+
+    Py_RETURN_NONE;
+}
+
+/*[clinic input]
+_testmultiphase.StateAccessType.get_count
+
+    cls: defining_class
+
+Return the value of the module-state counter.
+[clinic start generated code]*/
+
+static PyObject *
+_testmultiphase_StateAccessType_get_count_impl(StateAccessTypeObject *self,
+                                               PyTypeObject *cls)
+/*[clinic end generated code: output=64600f95b499a319 input=d5d181f12384849f]*/
+{
+    meth_state *m_state = PyType_GetModuleState(cls);
+    return PyLong_FromLong(m_state->counter);
+}
+
+static PyMethodDef StateAccessType_methods[] = {
+    _TESTMULTIPHASE_STATEACCESSTYPE_GET_DEFINING_MODULE_METHODDEF
+    _TESTMULTIPHASE_STATEACCESSTYPE_GET_COUNT_METHODDEF
+    _TESTMULTIPHASE_STATEACCESSTYPE_INCREMENT_COUNT_CLINIC_METHODDEF
+    {
+        "increment_count_noclinic",
+        (PyCFunction)(void(*)(void))_StateAccessType_increment_count_noclinic,
+        METH_METHOD|METH_FASTCALL|METH_KEYWORDS,
+        _StateAccessType_decrement_count__doc__
+    },
+    {NULL,              NULL}           /* sentinel */
+};
+
+static PyType_Slot StateAccessType_Type_slots[] = {
+    {Py_tp_doc, "Type for testing per-module state access from methods."},
+    {Py_tp_methods, StateAccessType_methods},
+    {0, NULL}
+};
+
+static PyType_Spec StateAccessType_spec = {
+    "_testimportexec.StateAccessType",
+    sizeof(StateAccessTypeObject),
+    0,
+    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_FINALIZE | Py_TPFLAGS_BASETYPE,
+    StateAccessType_Type_slots
+};
+
 /* Function of two integers returning integer */
 
 PyDoc_STRVAR(testexport_foo_doc,
@@ -193,30 +355,39 @@ static int execfunc(PyObject *m)
 
     /* Add a custom type */
     temp = PyType_FromSpec(&Example_Type_spec);
-    if (temp == NULL)
+    if (temp == NULL) {
         goto fail;
-    if (PyModule_AddObject(m, "Example", temp) != 0)
+    }
+    if (PyModule_AddObject(m, "Example", temp) != 0) {
         goto fail;
+    }
+
 
     /* Add an exception type */
     temp = PyErr_NewException("_testimportexec.error", NULL, NULL);
-    if (temp == NULL)
+    if (temp == NULL) {
         goto fail;
-    if (PyModule_AddObject(m, "error", temp) != 0)
+    }
+    if (PyModule_AddObject(m, "error", temp) != 0) {
         goto fail;
+    }
 
     /* Add Str */
     temp = PyType_FromSpec(&Str_Type_spec);
-    if (temp == NULL)
+    if (temp == NULL) {
         goto fail;
-    if (PyModule_AddObject(m, "Str", temp) != 0)
+    }
+    if (PyModule_AddObject(m, "Str", temp) != 0) {
         goto fail;
+    }
 
-    if (PyModule_AddIntConstant(m, "int_const", 1969) != 0)
+    if (PyModule_AddIntConstant(m, "int_const", 1969) != 0) {
         goto fail;
+    }
 
-    if (PyModule_AddStringConstant(m, "str_const", "something different") != 0)
+    if (PyModule_AddStringConstant(m, "str_const", "something different") != 0) {
         goto fail;
+    }
 
     return 0;
  fail:
@@ -620,6 +791,54 @@ PyInit__testmultiphase_exec_unreported_exception(PyObject *spec)
     return PyModuleDef_Init(&def_exec_unreported_exception);
 }
 
+static int
+meth_state_access_exec(PyObject *m)
+{
+    PyObject *temp;
+    meth_state *m_state;
+
+    m_state = PyModule_GetState(m);
+    if (m_state == NULL) {
+        return -1;
+    }
+
+    temp = PyType_FromModuleAndSpec(m, &StateAccessType_spec, NULL);
+    if (temp == NULL) {
+        return -1;
+    }
+    if (PyModule_AddObject(m, "StateAccessType", temp) != 0) {
+        return -1;
+    }
+
+
+    return 0;
+}
+
+static PyModuleDef_Slot meth_state_access_slots[] = {
+    {Py_mod_exec, meth_state_access_exec},
+    {0, NULL}
+};
+
+static PyModuleDef def_meth_state_access = {
+    PyModuleDef_HEAD_INIT,                      /* m_base */
+    "_testmultiphase_meth_state_access",        /* m_name */
+    PyDoc_STR("Module testing access"
+              " to state from methods."),
+    sizeof(meth_state),                         /* m_size */
+    NULL,                                       /* m_methods */
+    meth_state_access_slots,                    /* m_slots */
+    0,                                          /* m_traverse */
+    0,                                          /* m_clear */
+    0,                                          /* m_free */
+};
+
+PyMODINIT_FUNC
+PyInit__testmultiphase_meth_state_access(PyObject *spec)
+{
+    return PyModuleDef_Init(&def_meth_state_access);
+}
+
+
 /*** Helper for imp test ***/
 
 static PyModuleDef imp_dummy_def = TEST_MODULE_DEF("imp_dummy", main_slots, testexport_methods);

Modules/clinic/_testmultiphase.c.h

@@ -0,0 +1,101 @@
+/*[clinic input]
+preserve
+[clinic start generated code]*/
+
+PyDoc_STRVAR(_testmultiphase_StateAccessType_get_defining_module__doc__,
+"get_defining_module($self, /)\n"
+"--\n"
+"\n"
+"Return the module of the defining class.");
+
+#define _TESTMULTIPHASE_STATEACCESSTYPE_GET_DEFINING_MODULE_METHODDEF    \
+    {"get_defining_module", (PyCFunction)(void(*)(void))_testmultiphase_StateAccessType_get_defining_module, METH_METHOD|METH_FASTCALL|METH_KEYWORDS, _testmultiphase_StateAccessType_get_defining_module__doc__},
+
+static PyObject *
+_testmultiphase_StateAccessType_get_defining_module_impl(StateAccessTypeObject *self,
+                                                         PyTypeObject *cls);
+
+static PyObject *
+_testmultiphase_StateAccessType_get_defining_module(StateAccessTypeObject *self, PyTypeObject *cls, PyObject *const *args, Py_ssize_t nargs, PyObject *kwnames)
+{
+    PyObject *return_value = NULL;
+    static const char * const _keywords[] = { NULL};
+    static _PyArg_Parser _parser = {":get_defining_module", _keywords, 0};
+
+    if (!_PyArg_ParseStackAndKeywords(args, nargs, kwnames, &_parser
+        )) {
+        goto exit;
+    }
+    return_value = _testmultiphase_StateAccessType_get_defining_module_impl(self, cls);
+
+exit:
+    return return_value;
+}
+
+PyDoc_STRVAR(_testmultiphase_StateAccessType_increment_count_clinic__doc__,
+"increment_count_clinic($self, /, n=1, *, twice=False)\n"
+"--\n"
+"\n"
+"Add \'n\' from the module-state counter.\n"
+"\n"
+"Pass \'twice\' to double that amount.\n"
+"\n"
+"This tests Argument Clinic support for defining_class.");
+
+#define _TESTMULTIPHASE_STATEACCESSTYPE_INCREMENT_COUNT_CLINIC_METHODDEF    \
+    {"increment_count_clinic", (PyCFunction)(void(*)(void))_testmultiphase_StateAccessType_increment_count_clinic, METH_METHOD|METH_FASTCALL|METH_KEYWORDS, _testmultiphase_StateAccessType_increment_count_clinic__doc__},
+
+static PyObject *
+_testmultiphase_StateAccessType_increment_count_clinic_impl(StateAccessTypeObject *self,
+                                                            PyTypeObject *cls,
+                                                            int n, int twice);
+
+static PyObject *
+_testmultiphase_StateAccessType_increment_count_clinic(StateAccessTypeObject *self, PyTypeObject *cls, PyObject *const *args, Py_ssize_t nargs, PyObject *kwnames)
+{
+    PyObject *return_value = NULL;
+    static const char * const _keywords[] = {"n", "twice", NULL};
+    static _PyArg_Parser _parser = {"|i$p:increment_count_clinic", _keywords, 0};
+    int n = 1;
+    int twice = 0;
+
+    if (!_PyArg_ParseStackAndKeywords(args, nargs, kwnames, &_parser,
+        &n, &twice)) {
+        goto exit;
+    }
+    return_value = _testmultiphase_StateAccessType_increment_count_clinic_impl(self, cls, n, twice);
+
+exit:
+    return return_value;
+}
+
+PyDoc_STRVAR(_testmultiphase_StateAccessType_get_count__doc__,
+"get_count($self, /)\n"
+"--\n"
+"\n"
+"Return the value of the module-state counter.");
+
+#define _TESTMULTIPHASE_STATEACCESSTYPE_GET_COUNT_METHODDEF    \
+    {"get_count", (PyCFunction)(void(*)(void))_testmultiphase_StateAccessType_get_count, METH_METHOD|METH_FASTCALL|METH_KEYWORDS, _testmultiphase_StateAccessType_get_count__doc__},
+
+static PyObject *
+_testmultiphase_StateAccessType_get_count_impl(StateAccessTypeObject *self,
+                                               PyTypeObject *cls);
+
+static PyObject *
+_testmultiphase_StateAccessType_get_count(StateAccessTypeObject *self, PyTypeObject *cls, PyObject *const *args, Py_ssize_t nargs, PyObject *kwnames)
+{
+    PyObject *return_value = NULL;
+    static const char * const _keywords[] = { NULL};
+    static _PyArg_Parser _parser = {":get_count", _keywords, 0};
+
+    if (!_PyArg_ParseStackAndKeywords(args, nargs, kwnames, &_parser
+        )) {
+        goto exit;
+    }
+    return_value = _testmultiphase_StateAccessType_get_count_impl(self, cls);
+
+exit:
+    return return_value;
+}
+/*[clinic end generated code: output=39eea487e94e7f5d input=a9049054013a1b77]*/

Objects/descrobject.c

@@ -127,7 +127,11 @@ classmethod_get(PyMethodDescrObject *descr, PyObject *obj, PyObject *type)
                      ((PyTypeObject *)type)->tp_name);
         return NULL;
     }
-    return PyCFunction_NewEx(descr->d_method, type, NULL);
+    PyTypeObject *cls = NULL;
+    if (descr->d_method->ml_flags & METH_METHOD) {
+        cls = descr->d_common.d_type;
+    }
+    return PyCMethod_New(descr->d_method, type, NULL, cls);
 }
 
 static PyObject *
@@ -137,7 +141,19 @@ method_get(PyMethodDescrObject *descr, PyObject *obj, PyObject *type)
 
     if (descr_check((PyDescrObject *)descr, obj, &res))
         return res;
-    return PyCFunction_NewEx(descr->d_method, obj, NULL);
+    if (descr->d_method->ml_flags & METH_METHOD) {
+        if (PyType_Check(type)) {
+            return PyCMethod_New(descr->d_method, obj, NULL, descr->d_common.d_type);
+        } else {
+            PyErr_Format(PyExc_TypeError,
+                        "descriptor '%V' needs a type, not '%s', as arg 2",
+                        descr_name((PyDescrObject *)descr),
+                        Py_TYPE(type)->tp_name);
+            return NULL;
+        }
+    } else {
+        return PyCFunction_NewEx(descr->d_method, obj, NULL);
+    }
 }
 
 static PyObject *
@@ -335,6 +351,27 @@ exit:
     return result;
 }
 
+static PyObject *
+method_vectorcall_FASTCALL_KEYWORDS_METHOD(
+    PyObject *func, PyObject *const *args, size_t nargsf, PyObject *kwnames)
+{
+    PyThreadState *tstate = _PyThreadState_GET();
+    Py_ssize_t nargs = PyVectorcall_NARGS(nargsf);
+    if (method_check_args(func, args, nargs, NULL)) {
+        return NULL;
+    }
+     NULL;
+    PyCMethod meth = (PyCMethod) method_enter_call(tstate, func);
+    if (meth == NULL) {
+        return NULL;
+    }
+    PyObject *result = meth(args[0],
+                            ((PyMethodDescrObject *)func)->d_common.d_type,
+                            args+1, nargs-1, kwnames);
+    Py_LeaveRecursiveCall();
+    return result;
+}
+
 static PyObject *
 method_vectorcall_FASTCALL(
     PyObject *func, PyObject *const *args, size_t nargsf, PyObject *kwnames)
@@ -868,7 +905,8 @@ PyDescr_NewMethod(PyTypeObject *type, PyMethodDef *method)
 {
     /* Figure out correct vectorcall function to use */
     vectorcallfunc vectorcall;
-    switch (method->ml_flags & (METH_VARARGS | METH_FASTCALL | METH_NOARGS | METH_O | METH_KEYWORDS))
+    switch (method->ml_flags & (METH_VARARGS | METH_FASTCALL | METH_NOARGS |
+                                METH_O | METH_KEYWORDS | METH_METHOD))
     {
         case METH_VARARGS:
             vectorcall = method_vectorcall_VARARGS;
@@ -888,6 +926,9 @@ PyDescr_NewMethod(PyTypeObject *type, PyMethodDef *method)
         case METH_O:
             vectorcall = method_vectorcall_O;
             break;
+        case METH_METHOD | METH_FASTCALL | METH_KEYWORDS:
+            vectorcall = method_vectorcall_FASTCALL_KEYWORDS_METHOD;
+            break;
         default:
             PyErr_Format(PyExc_SystemError,
                          "%s() method: bad call flags", method->ml_name);

Objects/methodobject.c

@@ -10,12 +10,16 @@
 
 /* undefine macro trampoline to PyCFunction_NewEx */
 #undef PyCFunction_New
+/* undefine macro trampoline to PyCMethod_New */
+#undef PyCFunction_NewEx
 
 /* Forward declarations */
 static PyObject * cfunction_vectorcall_FASTCALL(
     PyObject *func, PyObject *const *args, size_t nargsf, PyObject *kwnames);
 static PyObject * cfunction_vectorcall_FASTCALL_KEYWORDS(
     PyObject *func, PyObject *const *args, size_t nargsf, PyObject *kwnames);
+static PyObject * cfunction_vectorcall_FASTCALL_KEYWORDS_METHOD(
+    PyObject *func, PyObject *const *args, size_t nargsf, PyObject *kwnames);
 static PyObject * cfunction_vectorcall_NOARGS(
     PyObject *func, PyObject *const *args, size_t nargsf, PyObject *kwnames);
 static PyObject * cfunction_vectorcall_O(
@@ -32,10 +36,17 @@ PyCFunction_New(PyMethodDef *ml, PyObject *self)
 
 PyObject *
 PyCFunction_NewEx(PyMethodDef *ml, PyObject *self, PyObject *module)
+{
+    return PyCMethod_New(ml, self, module, NULL);
+}
+
+PyObject *
+PyCMethod_New(PyMethodDef *ml, PyObject *self, PyObject *module, PyTypeObject *cls)
 {
     /* Figure out correct vectorcall function to use */
     vectorcallfunc vectorcall;
-    switch (ml->ml_flags & (METH_VARARGS | METH_FASTCALL | METH_NOARGS | METH_O | METH_KEYWORDS))
+    switch (ml->ml_flags & (METH_VARARGS | METH_FASTCALL | METH_NOARGS |
+                            METH_O | METH_KEYWORDS | METH_METHOD))
     {
         case METH_VARARGS:
         case METH_VARARGS | METH_KEYWORDS:
@@ -55,17 +66,44 @@ PyCFunction_NewEx(PyMethodDef *ml, PyObject *self, PyObject *module)
         case METH_O:
             vectorcall = cfunction_vectorcall_O;
             break;
+        case METH_METHOD | METH_FASTCALL | METH_KEYWORDS:
+            vectorcall = cfunction_vectorcall_FASTCALL_KEYWORDS_METHOD;
+            break;
         default:
             PyErr_Format(PyExc_SystemError,
                          "%s() method: bad call flags", ml->ml_name);
             return NULL;
     }
 
-    PyCFunctionObject *op =
-        PyObject_GC_New(PyCFunctionObject, &PyCFunction_Type);
-    if (op == NULL) {
-        return NULL;
+    PyCFunctionObject *op = NULL;
+
+    if (ml->ml_flags & METH_METHOD) {
+        if (!cls) {
+            PyErr_SetString(PyExc_SystemError,
+                            "attempting to create PyCMethod with a METH_METHOD "
+                            "flag but no class");
+            return NULL;
+        }
+        PyCMethodObject *om = PyObject_GC_New(PyCMethodObject, &PyCMethod_Type);
+        if (om == NULL) {
+            return NULL;
+        }
+        Py_INCREF(cls);
+        om->mm_class = cls;
+        op = (PyCFunctionObject *)om;
+    } else {
+        if (cls) {
+            PyErr_SetString(PyExc_SystemError,
+                            "attempting to create PyCFunction with class "
+                            "but no METH_METHOD flag");
+            return NULL;
+        }
+        op = PyObject_GC_New(PyCFunctionObject, &PyCFunction_Type);
+        if (op == NULL) {
+            return NULL;
+        }
     }
+
     op->m_weakreflist = NULL;
     op->m_ml = ml;
     Py_XINCREF(self);
@@ -107,6 +145,16 @@ PyCFunction_GetFlags(PyObject *op)
     return PyCFunction_GET_FLAGS(op);
 }
 
+PyTypeObject *
+PyCMethod_GetClass(PyObject *op)
+{
+    if (!PyCFunction_Check(op)) {
+        PyErr_BadInternalCall();
+        return NULL;
+    }
+    return PyCFunction_GET_CLASS(op);
+}
+
 /* Methods (the standard built-in methods, that is) */
 
 static void
@@ -118,6 +166,7 @@ meth_dealloc(PyCFunctionObject *m)
     }
     Py_XDECREF(m->m_self);
     Py_XDECREF(m->m_module);
+    Py_XDECREF(PyCFunction_GET_CLASS(m));
     PyObject_GC_Del(m);
 }
 
@@ -196,6 +245,7 @@ meth_traverse(PyCFunctionObject *m, visitproc visit, void *arg)
 {
     Py_VISIT(m->m_self);
     Py_VISIT(m->m_module);
+    Py_VISIT(PyCFunction_GET_CLASS(m));
     return 0;
 }
 
@@ -314,6 +364,13 @@ PyTypeObject PyCFunction_Type = {
     0,                                          /* tp_dict */
 };
 
+PyTypeObject PyCMethod_Type = {
+    PyVarObject_HEAD_INIT(&PyType_Type, 0)
+    .tp_name = "builtin_method",
+    .tp_basicsize = sizeof(PyCMethodObject),
+    .tp_base = &PyCFunction_Type,
+};
+
 /* Vectorcall functions for each of the PyCFunction calling conventions,
  * except for METH_VARARGS (possibly combined with METH_KEYWORDS) which
  * doesn't use vectorcall.
@@ -385,6 +442,22 @@ cfunction_vectorcall_FASTCALL_KEYWORDS(
     return result;
 }
 
+static PyObject *
+cfunction_vectorcall_FASTCALL_KEYWORDS_METHOD(
+    PyObject *func, PyObject *const *args, size_t nargsf, PyObject *kwnames)
+{
+    PyThreadState *tstate = _PyThreadState_GET();
+    PyTypeObject *cls = PyCFunction_GET_CLASS(func);
+    Py_ssize_t nargs = PyVectorcall_NARGS(nargsf);
+    PyCMethod meth = (PyCMethod)cfunction_enter_call(tstate, func);
+    if (meth == NULL) {
+        return NULL;
+    }
+    PyObject *result = meth(PyCFunction_GET_SELF(func), cls, args, nargs, kwnames);
+    _Py_LeaveRecursiveCall(tstate);
+    return result;
+}
+
 static PyObject *
 cfunction_vectorcall_NOARGS(
     PyObject *func, PyObject *const *args, size_t nargsf, PyObject *kwnames)

Objects/object.c

@@ -1789,6 +1789,7 @@ _PyTypes_Init(void)
     INIT_TYPE(&PyCode_Type, "code");
     INIT_TYPE(&PyFrame_Type, "frame");
     INIT_TYPE(&PyCFunction_Type, "builtin function");
+    INIT_TYPE(&PyCMethod_Type, "builtin method");
     INIT_TYPE(&PyMethod_Type, "method");
     INIT_TYPE(&PyFunction_Type, "function");
     INIT_TYPE(&PyDictProxy_Type, "dict proxy");

Objects/typeobject.c

@@ -2708,6 +2708,9 @@ type_new(PyTypeObject *metatype, PyObject *args, PyObject *kwds)
     if (qualname != NULL && _PyDict_DelItemId(dict, &PyId___qualname__) < 0)
         goto error;
 
+    /* Set ht_module */
+    et->ht_module = NULL;
+
     /* Set tp_doc to a copy of dict['__doc__'], if the latter is there
        and is a string.  The __doc__ accessor will first look for tp_doc;
        if that fails, it will still look into __dict__.
@@ -2939,6 +2942,12 @@ PyType_FromSpec_tp_traverse(PyObject *self, visitproc visit, void *arg)
 
 PyObject *
 PyType_FromSpecWithBases(PyType_Spec *spec, PyObject *bases)
+{
+    return PyType_FromModuleAndSpec(NULL, spec, bases);
+}
+
+PyObject *
+PyType_FromModuleAndSpec(PyObject *module, PyType_Spec *spec, PyObject *bases)
 {
     PyHeapTypeObject *res;
     PyObject *modname;
@@ -2998,6 +3007,9 @@ PyType_FromSpecWithBases(PyType_Spec *spec, PyObject *bases)
     Py_INCREF(res->ht_qualname);
     type->tp_name = spec->name;
 
+    Py_XINCREF(module);
+    res->ht_module = module;
+
     /* Adjust for empty tuple bases */
     if (!bases) {
         base = &PyBaseObject_Type;
@@ -3176,6 +3188,40 @@ PyType_GetSlot(PyTypeObject *type, int slot)
     return  *(void**)(((char*)type) + slotoffsets[slot]);
 }
 
+PyObject *
+PyType_GetModule(PyTypeObject *type)
+{
+    assert(PyType_Check(type));
+    if (!_PyType_HasFeature(type, Py_TPFLAGS_HEAPTYPE)) {
+        PyErr_Format(
+            PyExc_TypeError,
+            "PyType_GetModule: Type '%s' is not a heap type",
+            type->tp_name);
+        return NULL;
+    }
+
+    PyHeapTypeObject* et = (PyHeapTypeObject*)type;
+    if (!et->ht_module) {
+        PyErr_Format(
+            PyExc_TypeError,
+            "PyType_GetModule: Type '%s' has no associated module",
+            type->tp_name);
+        return NULL;
+    }
+    return et->ht_module;
+
+}
+
+void *
+PyType_GetModuleState(PyTypeObject *type)
+{
+    PyObject *m = PyType_GetModule(type);
+    if (m == NULL) {
+        return NULL;
+    }
+    return PyModule_GetState(m);
+}
+
 /* Internal API to look for a name through the MRO, bypassing the method cache.
    This returns a borrowed reference, and might set an exception.
    'error' is set to: -1: error with exception; 1: error without exception; 0: ok */
@@ -3503,8 +3549,10 @@ type_dealloc(PyTypeObject *type)
     Py_XDECREF(et->ht_name);
     Py_XDECREF(et->ht_qualname);
     Py_XDECREF(et->ht_slots);
-    if (et->ht_cached_keys)
+    if (et->ht_cached_keys) {
         _PyDictKeys_DecRef(et->ht_cached_keys);
+    }
+    Py_XDECREF(et->ht_module);
     Py_TYPE(type)->tp_free((PyObject *)type);
 }
 
@@ -3694,6 +3742,7 @@ type_traverse(PyTypeObject *type, visitproc visit, void *arg)
     Py_VISIT(type->tp_mro);
     Py_VISIT(type->tp_bases);
     Py_VISIT(type->tp_base);
+    Py_VISIT(((PyHeapTypeObject *)type)->ht_module);
 
     /* There's no need to visit type->tp_subclasses or
        ((PyHeapTypeObject *)type)->ht_slots, because they can't be involved
@@ -3715,10 +3764,13 @@ type_clear(PyTypeObject *type)
        the dict, so that other objects caught in a reference cycle
        don't start calling destroyed methods.
 
-       Otherwise, the only field we need to clear is tp_mro, which is
+       Otherwise, the we need to clear tp_mro, which is
        part of a hard cycle (its first element is the class itself) that
        won't be broken otherwise (it's a tuple and tuples don't have a
-       tp_clear handler).  None of the other fields need to be
+       tp_clear handler).
+       We also need to clear ht_module, if present: the module usually holds a
+       reference to its class. None of the other fields need to be
+
        cleared, and here's why:
 
        tp_cache:
@@ -3743,8 +3795,11 @@ type_clear(PyTypeObject *type)
         ((PyHeapTypeObject *)type)->ht_cached_keys = NULL;
         _PyDictKeys_DecRef(cached_keys);
     }
-    if (type->tp_dict)
+    if (type->tp_dict) {
         PyDict_Clear(type->tp_dict);
+    }
+    Py_CLEAR(((PyHeapTypeObject *)type)->ht_module);
+
     Py_CLEAR(type->tp_mro);
 
     return 0;

PCbuild/pythoncore.vcxproj

@@ -138,6 +138,7 @@
     <ClInclude Include="..\Include\cpython\import.h" />
     <ClInclude Include="..\Include\cpython\initconfig.h" />
     <ClInclude Include="..\Include\cpython\listobject.h" />
+    <ClInclude Include="..\Include\cpython\methodobject.h" />
     <ClInclude Include="..\Include\cpython\object.h" />
     <ClInclude Include="..\Include\cpython\objimpl.h" />
     <ClInclude Include="..\Include\cpython\pyerrors.h" />

PCbuild/pythoncore.vcxproj.filters

@@ -111,6 +111,9 @@
     <ClInclude Include="..\Include\cpython\listobject.h">
       <Filter>Include</Filter>
     </ClInclude>
+    <ClInclude Include="..\Include\cpython\methodobject.h">
+      <Filter>Include</Filter>
+    </ClInclude>
     <ClInclude Include="..\Include\cpython\object.h">
       <Filter>Include</Filter>
     </ClInclude>

Tools/clinic/clinic.py

@@ -657,9 +657,14 @@ class CLanguage(Language):
                 if not p.is_optional():
                     min_pos = i
 
+        requires_defining_class = any(
+            isinstance(p.converter, defining_class_converter)
+            for p in parameters)
+
         meth_o = (len(parameters) == 1 and
               parameters[0].is_positional_only() and
               not converters[0].is_optional() and
+              not requires_defining_class and
               not new_or_init)
 
         # we have to set these things before we're done:
@@ -717,6 +722,11 @@ class CLanguage(Language):
             {c_basename}({self_type}{self_name}, PyObject *const *args, Py_ssize_t nargs, PyObject *kwnames)
             """)
 
+        parser_prototype_def_class = normalize_snippet("""
+            static PyObject *
+            {c_basename}({self_type}{self_name}, PyTypeObject *cls, PyObject *const *args, Py_ssize_t nargs, PyObject *kwnames)
+        """)
+
         # parser_body_fields remembers the fields passed in to the
         # previous call to parser_body. this is used for an awful hack.
         parser_body_fields = ()
@@ -824,7 +834,7 @@ class CLanguage(Language):
 
             parser_definition = parser_body(parser_prototype, '    {option_group_parsing}')
 
-        elif pos_only == len(parameters):
+        elif not requires_defining_class and pos_only == len(parameters):
             if not new_or_init:
                 # positional-only, but no option groups
                 # we only need one call to _PyArg_ParseStack
@@ -891,7 +901,7 @@ class CLanguage(Language):
                 parser_prototype = parser_prototype_fastcall_keywords
                 argname_fmt = 'args[%d]'
                 declarations = normalize_snippet("""
-                    static const char * const _keywords[] = {{{keywords}, NULL}};
+                    static const char * const _keywords[] = {{{keywords} NULL}};
                     static _PyArg_Parser _parser = {{NULL, _keywords, "{name}", 0}};
                     PyObject *argsbuf[%s];
                     """ % len(converters))
@@ -909,7 +919,7 @@ class CLanguage(Language):
                 parser_prototype = parser_prototype_keyword
                 argname_fmt = 'fastargs[%d]'
                 declarations = normalize_snippet("""
-                    static const char * const _keywords[] = {{{keywords}, NULL}};
+                    static const char * const _keywords[] = {{{keywords} NULL}};
                     static _PyArg_Parser _parser = {{NULL, _keywords, "{name}", 0}};
                     PyObject *argsbuf[%s];
                     PyObject * const *fastargs;
@@ -923,6 +933,9 @@ class CLanguage(Language):
                         goto exit;
                     }}
                     """ % (min_pos, max_pos, min_kw_only), indent=4)]
+            if requires_defining_class:
+                flags = 'METH_METHOD|' + flags
+                parser_prototype = parser_prototype_def_class
 
             add_label = None
             for i, p in enumerate(parameters):
@@ -983,11 +996,11 @@ class CLanguage(Language):
                     parser_code.append("%s:" % add_label)
             else:
                 declarations = (
-                    'static const char * const _keywords[] = {{{keywords}, NULL}};\n'
+                    'static const char * const _keywords[] = {{{keywords} NULL}};\n'
                     'static _PyArg_Parser _parser = {{"{format_units}:{name}", _keywords, 0}};')
                 if not new_or_init:
                     parser_code = [normalize_snippet("""
-                        if (!_PyArg_ParseStackAndKeywords(args, nargs, kwnames, &_parser,
+                        if (!_PyArg_ParseStackAndKeywords(args, nargs, kwnames, &_parser{parse_arguments_comma}
                             {parse_arguments})) {{
                             goto exit;
                         }}
@@ -1021,6 +1034,9 @@ class CLanguage(Language):
             if parses_keywords:
                 assert parses_positional
 
+            if requires_defining_class:
+                raise ValueError("Slot methods cannot access their defining class.")
+
             if not parses_keywords:
                 fields.insert(0, normalize_snippet("""
                     if ({self_type_check}!_PyArg_NoKeywords("{name}", kwargs)) {{
@@ -1297,9 +1313,13 @@ class CLanguage(Language):
         template_dict['declarations'] = format_escape("\n".join(data.declarations))
         template_dict['initializers'] = "\n\n".join(data.initializers)
         template_dict['modifications'] = '\n\n'.join(data.modifications)
-        template_dict['keywords'] = '"' + '", "'.join(data.keywords) + '"'
+        template_dict['keywords'] = ' '.join('"' + k + '",' for k in data.keywords)
         template_dict['format_units'] = ''.join(data.format_units)
         template_dict['parse_arguments'] = ', '.join(data.parse_arguments)
+        if data.parse_arguments:
+            template_dict['parse_arguments_comma'] = ',';
+        else:
+            template_dict['parse_arguments_comma'] = '';
         template_dict['impl_parameters'] = ", ".join(data.impl_parameters)
         template_dict['impl_arguments'] = ", ".join(data.impl_arguments)
         template_dict['return_conversion'] = format_escape("".join(data.return_conversion).rstrip())
@@ -2730,6 +2750,25 @@ class bool_converter(CConverter):
                 """.format(argname=argname, paramname=self.name)
         return super().parse_arg(argname, displayname)
 
+class defining_class_converter(CConverter):
+    """
+    A special-case converter:
+    this is the default converter used for the defining class.
+    """
+    type = 'PyTypeObject *'
+    format_unit = ''
+    show_in_signature = False
+
+    def converter_init(self, *, type=None):
+        self.specified_type = type
+
+    def render(self, parameter, data):
+        self._render_self(parameter, data)
+
+    def set_template_dict(self, template_dict):
+        template_dict['defining_class_name'] = self.name
+
+
 class char_converter(CConverter):
     type = 'char'
     default_type = (bytes, bytearray)
@@ -4508,6 +4547,19 @@ class DSLParser:
             else:
                 fail("A 'self' parameter, if specified, must be the very first thing in the parameter block.")
 
+        if isinstance(converter, defining_class_converter):
+            _lp = len(self.function.parameters)
+            if _lp == 1:
+                if (self.parameter_state != self.ps_required):
+                    fail("A 'defining_class' parameter cannot be marked optional.")
+                if value is not unspecified:
+                    fail("A 'defining_class' parameter cannot have a default value.")
+                if self.group:
+                    fail("A 'defining_class' parameter cannot be in an optional group.")
+            else:
+                fail("A 'defining_class' parameter, if specified, must either be the first thing in the parameter block, or come just after 'self'.")
+
+
         p = Parameter(parameter_name, kind, function=self.function, converter=converter, default=value, group=self.group)
 
         if parameter_name in self.function.parameters: