Common Object Structures
************************

There are a large number of structures which are used in the
definition of object types for Python.  This section describes these
structures and how they are used.


Base object types and macros
============================

All Python objects ultimately share a small number of fields at the
beginning of the object’s representation in memory.  These are
represented by the "PyObject" and "PyVarObject" types, which are
defined, in turn, by the expansions of some macros also used, whether
directly or indirectly, in the definition of all other Python objects.
Additional macros can be found under reference counting.

type PyObject
    * Part of the Limited API. (Only some members are part of the
   stable ABI.)*

   All object types are extensions of this type.  This is a type which
   contains the information Python needs to treat a pointer to an
   object as an object.  In a normal “release” build, it contains only
   the object’s reference count and a pointer to the corresponding
   type object. Nothing is actually declared to be a "PyObject", but
   every pointer to a Python object can be cast to a PyObject*.
   Access to the members must be done by using the macros "Py_REFCNT"
   and "Py_TYPE".

type PyVarObject
    * Part of the Limited API. (Only some members are part of the
   stable ABI.)*

   This is an extension of "PyObject" that adds the "ob_size" field.
   This is only used for objects that have some notion of *length*.
   This type does not often appear in the Python/C API. Access to the
   members must be done by using the macros "Py_REFCNT", "Py_TYPE",
   and "Py_SIZE".

PyObject_HEAD

   This is a macro used when declaring new types which represent
   objects without a varying length.  The PyObject_HEAD macro expands
   to:

      PyObject ob_base;

   See documentation of "PyObject" above.

PyObject_VAR_HEAD

   This is a macro used when declaring new types which represent
   objects with a length that varies from instance to instance. The
   PyObject_VAR_HEAD macro expands to:

      PyVarObject ob_base;

   See documentation of "PyVarObject" above.

PyTypeObject PyBaseObject_Type
    * Part of the Stable ABI.*

   The base class of all other objects, the same as "object" in
   Python.

int Py_Is(PyObject *x, PyObject *y)
    * Part of the Stable ABI since version 3.10.*

   Test if the *x* object is the *y* object, the same as "x is y" in
   Python.

   Added in version 3.10.

int Py_IsNone(PyObject *x)
    * Part of the Stable ABI since version 3.10.*

   Test if an object is the "None" singleton, the same as "x is None"
   in Python.

   Added in version 3.10.

int Py_IsTrue(PyObject *x)
    * Part of the Stable ABI since version 3.10.*

   Test if an object is the "True" singleton, the same as "x is True"
   in Python.

   Added in version 3.10.

int Py_IsFalse(PyObject *x)
    * Part of the Stable ABI since version 3.10.*

   Test if an object is the "False" singleton, the same as "x is
   False" in Python.

   Added in version 3.10.

PyTypeObject *Py_TYPE(PyObject *o)
    *Return value: Borrowed reference.*

   Get the type of the Python object *o*.

   Return a *borrowed reference*.

   Use the "Py_SET_TYPE()" function to set an object type.

   Changed in version 3.11: "Py_TYPE()" is changed to an inline static
   function. The parameter type is no longer const PyObject*.

int Py_IS_TYPE(PyObject *o, PyTypeObject *type)

   Return non-zero if the object *o* type is *type*. Return zero
   otherwise. Equivalent to: "Py_TYPE(o) == type".

   Added in version 3.9.

void Py_SET_TYPE(PyObject *o, PyTypeObject *type)

   Set the object *o* type to *type*.

   Added in version 3.9.

Py_ssize_t Py_SIZE(PyVarObject *o)

   Get the size of the Python object *o*.

   Use the "Py_SET_SIZE()" function to set an object size.

   Changed in version 3.11: "Py_SIZE()" is changed to an inline static
   function. The parameter type is no longer const PyVarObject*.

void Py_SET_SIZE(PyVarObject *o, Py_ssize_t size)

   Set the object *o* size to *size*.

   Added in version 3.9.

PyObject_HEAD_INIT(type)

   This is a macro which expands to initialization values for a new
   "PyObject" type.  This macro expands to:

      _PyObject_EXTRA_INIT
      1, type,

PyVarObject_HEAD_INIT(type, size)

   This is a macro which expands to initialization values for a new
   "PyVarObject" type, including the "ob_size" field. This macro
   expands to:

      _PyObject_EXTRA_INIT
      1, type, size,


Implementing functions and methods
==================================

type PyCFunction
    * Part of the Stable ABI.*

   Type of the functions used to implement most Python callables in C.
   Functions of this type take two PyObject* parameters and return one
   such value.  If the return value is "NULL", an exception shall have
   been set.  If not "NULL", the return value is interpreted as the
   return value of the function as exposed in Python.  The function
   must return a new reference.

   The function signature is:

      PyObject *PyCFunction(PyObject *self,
                            PyObject *args);

type PyCFunctionWithKeywords
    * Part of the Stable ABI.*

   Type of the functions used to implement Python callables in C with
   signature METH_VARARGS | METH_KEYWORDS. The function signature is:

      PyObject *PyCFunctionWithKeywords(PyObject *self,
                                        PyObject *args,
                                        PyObject *kwargs);

type PyCFunctionFast
    * Part of the Stable ABI since version 3.13.*

   Type of the functions used to implement Python callables in C with
   signature "METH_FASTCALL". The function signature is:

      PyObject *PyCFunctionFast(PyObject *self,
                                PyObject *const *args,
                                Py_ssize_t nargs);

type PyCFunctionFastWithKeywords
    * Part of the Stable ABI since version 3.13.*

   Type of the functions used to implement Python callables in C with
   signature METH_FASTCALL | METH_KEYWORDS. The function signature is:

      PyObject *PyCFunctionFastWithKeywords(PyObject *self,
                                            PyObject *const *args,
                                            Py_ssize_t nargs,
                                            PyObject *kwnames);

type PyCMethod

   Type of the functions used to implement Python callables in C with
   signature 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)

   Added in version 3.9.

type PyMethodDef
    * Part of the Stable ABI (including all members).*

   Structure used to describe a method of an extension type.  This
   structure has four fields:

   const char *ml_name

      Name of the method.

   PyCFunction ml_meth

      Pointer to the C implementation.

   int ml_flags

      Flags bits indicating how the call should be constructed.

   const char *ml_doc

      Points to the contents of the docstring.

The "ml_meth" is a C function pointer. The functions may be of
different types, but they always return PyObject*.  If the function is
not of the "PyCFunction", the compiler will require a cast in the
method table. Even though "PyCFunction" defines the first parameter as
PyObject*, it is common that the method implementation uses the
specific C type of the *self* object.

The "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 these calling conventions:

METH_VARARGS
    * Part of the Stable ABI.*

   This is the typical calling convention, where the methods have the
   type "PyCFunction". The function expects two PyObject* values. The
   first one is the *self* object for methods; for module functions,
   it is the module object.  The second parameter (often called
   *args*) is a tuple object representing all arguments. This
   parameter is typically processed using "PyArg_ParseTuple()" or
   "PyArg_UnpackTuple()".

METH_KEYWORDS

   Can only be used in certain combinations with other flags:
   METH_VARARGS | METH_KEYWORDS, METH_FASTCALL | METH_KEYWORDS and
   METH_METHOD | METH_FASTCALL | METH_KEYWORDS.

METH_VARARGS | METH_KEYWORDS
   Methods with these flags must be of type "PyCFunctionWithKeywords".
   The function expects three parameters: *self*, *args*, *kwargs*
   where *kwargs* is a dictionary of all the keyword arguments or
   possibly "NULL" if there are no keyword arguments.  The parameters
   are typically processed using "PyArg_ParseTupleAndKeywords()".

METH_FASTCALL
    * Part of the Stable ABI since version 3.7.*

   Fast calling convention supporting only positional arguments. The
   methods have the type "PyCFunctionFast". The first parameter is
   *self*, the second parameter is a C array of PyObject* values
   indicating the arguments and the third parameter is the number of
   arguments (the length of the array).

   Added in version 3.7.

   Changed in version 3.10: "METH_FASTCALL" is now part of the stable
   ABI.

METH_FASTCALL | METH_KEYWORDS
   Extension of "METH_FASTCALL" supporting also keyword arguments,
   with methods of type "PyCFunctionFastWithKeywords". Keyword
   arguments are passed the same way as in the vectorcall protocol:
   there is an additional fourth PyObject* parameter which is a tuple
   representing the names of the keyword arguments (which are
   guaranteed to be strings) or possibly "NULL" if there are no
   keywords.  The values of the keyword arguments are stored in the
   *args* array, after the positional arguments.

   Added in version 3.7.

METH_METHOD
    * Part of the Stable ABI since version 3.7.*

   Can only be used in the combination with other flags: METH_METHOD |
   METH_FASTCALL | METH_KEYWORDS.

METH_METHOD | METH_FASTCALL | METH_KEYWORDS
   Extension of 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 "PyCMethod", the same as for
   "METH_FASTCALL | METH_KEYWORDS" with "defining_class" argument
   added after "self".

   Added in version 3.9.

METH_NOARGS
    * Part of the Stable ABI.*

   Methods without parameters don’t need to check whether arguments
   are given if they are listed with the "METH_NOARGS" flag.  They
   need to be of type "PyCFunction".  The first parameter is typically
   named *self* and will hold a reference to the module or object
   instance.  In all cases the second parameter will be "NULL".

   The function must have 2 parameters. Since the second parameter is
   unused, "Py_UNUSED" can be used to prevent a compiler warning.

METH_O
    * Part of the Stable ABI.*

   Methods with a single object argument can be listed with the
   "METH_O" flag, instead of invoking "PyArg_ParseTuple()" with a
   ""O"" argument. They have the type "PyCFunction", with the *self*
   parameter, and a PyObject* parameter representing the single
   argument.

These two constants are not used to indicate the calling convention
but the binding when used with methods of classes.  These may not be
used for functions defined for modules.  At most one of these flags
may be set for any given method.

METH_CLASS
    * Part of the Stable ABI.*

   The method will be passed the type object as the first parameter
   rather than an instance of the type.  This is used to create *class
   methods*, similar to what is created when using the "classmethod()"
   built-in function.

METH_STATIC
    * Part of the Stable ABI.*

   The method will be passed "NULL" as the first parameter rather than
   an instance of the type.  This is used to create *static methods*,
   similar to what is created when using the "staticmethod()" built-in
   function.

One other constant controls whether a method is loaded in place of
another definition with the same method name.

METH_COEXIST
    * Part of the Stable ABI.*

   The method will be loaded in place of existing definitions.
   Without *METH_COEXIST*, the default is to skip repeated
   definitions.  Since slot wrappers are loaded before the method
   table, the existence of a *sq_contains* slot, for example, would
   generate a wrapped method named "__contains__()" and preclude the
   loading of a corresponding PyCFunction with the same name.  With
   the flag defined, the PyCFunction will be loaded in place of the
   wrapper object and will co-exist with the slot.  This is helpful
   because calls to PyCFunctions are optimized more than wrapper
   object calls.

PyTypeObject PyCMethod_Type

   The type object corresponding to Python C method objects. This is
   available as "types.BuiltinMethodType" in the Python layer.

int PyCMethod_Check(PyObject *op)

   Return true if *op* is an instance of the "PyCMethod_Type" type or
   a subtype of it. This function always succeeds.

int PyCMethod_CheckExact(PyObject *op)

   This is the same as "PyCMethod_Check()", but does not account for
   subtypes.

PyObject *PyCMethod_New(PyMethodDef *ml, PyObject *self, PyObject *module, PyTypeObject *cls)
    *Return value: New reference.** Part of the Stable ABI since
   version 3.9.*

   Turn *ml* into a Python *callable* object. The caller must ensure
   that *ml* outlives the *callable*. Typically, *ml* is defined as a
   static variable.

   The *self* parameter will be passed as the *self* argument to the C
   function in "ml->ml_meth" when invoked. *self* can be "NULL".

   The *callable* object’s "__module__" attribute can be set from the
   given *module* argument. *module* should be a Python string, which
   will be used as name of the module the function is defined in. If
   unavailable, it can be set to "None" or "NULL".

   See also: "function.__module__"

   The *cls* parameter will be passed as the *defining_class* argument
   to the C function. Must be set if "METH_METHOD" is set on
   "ml->ml_flags".

   Added in version 3.9.

PyTypeObject PyCFunction_Type
    * Part of the Stable ABI.*

   The type object corresponding to Python C function objects. This is
   available as "types.BuiltinFunctionType" in the Python layer.

int PyCFunction_Check(PyObject *op)

   Return true if *op* is an instance of the "PyCFunction_Type" type
   or a subtype of it. This function always succeeds.

int PyCFunction_CheckExact(PyObject *op)

   This is the same as "PyCFunction_Check()", but does not account for
   subtypes.

PyObject *PyCFunction_NewEx(PyMethodDef *ml, PyObject *self, PyObject *module)
    *Return value: New reference.** Part of the Stable ABI.*

   Equivalent to "PyCMethod_New(ml, self, module, NULL)".

PyObject *PyCFunction_New(PyMethodDef *ml, PyObject *self)
    *Return value: New reference.** Part of the Stable ABI since
   version 3.4.*

   Equivalent to "PyCMethod_New(ml, self, NULL, NULL)".

int PyCFunction_GetFlags(PyObject *func)
    * Part of the Stable ABI.*

   Get the function’s flags on *func* as they were passed to
   "ml_flags".

   If *func* is not a C function object, this fails with an exception.
   *func* must not be "NULL".

   This function returns the function’s flags on success, and "-1"
   with an exception set on failure.

int PyCFunction_GET_FLAGS(PyObject *func)

   This is the same as "PyCFunction_GetFlags()", but without error or
   type checking.

PyCFunction PyCFunction_GetFunction(PyObject *func)
    * Part of the Stable ABI.*

   Get the function pointer on *func* as it was passed to "ml_meth".

   If *func* is not a C function object, this fails with an exception.
   *func* must not be "NULL".

   This function returns the function pointer on success, and "NULL"
   with an exception set on failure.

int PyCFunction_GET_FUNCTION(PyObject *func)

   This is the same as "PyCFunction_GetFunction()", but without error
   or type checking.

PyObject *PyCFunction_GetSelf(PyObject *func)
    * Part of the Stable ABI.*

   Get the “self” object on *func*. This is the object that would be
   passed to the first argument of a "PyCFunction". For C function
   objects created through a "PyMethodDef" on a "PyModuleDef", this is
   the resulting module object.

   If *func* is not a C function object, this fails with an exception.
   *func* must not be "NULL".

   This function returns a *borrowed reference* to the “self” object
   on success, and "NULL" with an exception set on failure.

PyObject *PyCFunction_GET_SELF(PyObject *func)

   This is the same as "PyCFunction_GetSelf()", but without error or
   type checking.


Accessing attributes of extension types
=======================================

type PyMemberDef
    * Part of the Stable ABI (including all members).*

   Structure which describes an attribute of a type which corresponds
   to a C struct member. When defining a class, put a NULL-terminated
   array of these structures in the "tp_members" slot.

   Its fields are, in order:

   const char *name

      Name of the member. A NULL value marks the end of a
      "PyMemberDef[]" array.

      The string should be static, no copy is made of it.

   int type

      The type of the member in the C struct. See Member types for the
      possible values.

   Py_ssize_t offset

      The offset in bytes that the member is located on the type’s
      object struct.

   int flags

      Zero or more of the Member flags, combined using bitwise OR.

   const char *doc

      The docstring, or NULL. The string should be static, no copy is
      made of it. Typically, it is defined using "PyDoc_STR".

   By default (when "flags" is "0"), members allow both read and write
   access. Use the "Py_READONLY" flag for read-only access. Certain
   types, like "Py_T_STRING", imply "Py_READONLY". Only
   "Py_T_OBJECT_EX" (and legacy "T_OBJECT") members can be deleted.

   For heap-allocated types (created using "PyType_FromSpec()" or
   similar), "PyMemberDef" may contain a definition for the special
   member ""__vectorcalloffset__"", corresponding to
   "tp_vectorcall_offset" in type objects. These must be defined with
   "Py_T_PYSSIZET" and "Py_READONLY", for example:

      static PyMemberDef spam_type_members[] = {
          {"__vectorcalloffset__", Py_T_PYSSIZET,
           offsetof(Spam_object, vectorcall), Py_READONLY},
          {NULL}  /* Sentinel */
      };

   (You may need to "#include <stddef.h>" for "offsetof()".)

   The legacy offsets "tp_dictoffset" and "tp_weaklistoffset" can be
   defined similarly using ""__dictoffset__"" and
   ""__weaklistoffset__"" members, but extensions are strongly
   encouraged to use "Py_TPFLAGS_MANAGED_DICT" and
   "Py_TPFLAGS_MANAGED_WEAKREF" instead.

   Changed in version 3.12: "PyMemberDef" is always available.
   Previously, it required including ""structmember.h"".

PyObject *PyMember_GetOne(const char *obj_addr, struct PyMemberDef *m)
    * Part of the Stable ABI.*

   Get an attribute belonging to the object at address *obj_addr*.
   The attribute is described by "PyMemberDef" *m*.  Returns "NULL" on
   error.

   Changed in version 3.12: "PyMember_GetOne" is always available.
   Previously, it required including ""structmember.h"".

int PyMember_SetOne(char *obj_addr, struct PyMemberDef *m, PyObject *o)
    * Part of the Stable ABI.*

   Set an attribute belonging to the object at address *obj_addr* to
   object *o*. The attribute to set is described by "PyMemberDef" *m*.
   Returns "0" if successful and a negative value on failure.

   Changed in version 3.12: "PyMember_SetOne" is always available.
   Previously, it required including ""structmember.h"".


Member flags
------------

The following flags can be used with "PyMemberDef.flags":

Py_READONLY
    * Part of the Stable ABI since version 3.12.*

   Not writable.

Py_AUDIT_READ
    * Part of the Stable ABI since version 3.12.*

   Emit an "object.__getattr__" audit event before reading.

Py_RELATIVE_OFFSET
    * Part of the Stable ABI since version 3.12.*

   Indicates that the "offset" of this "PyMemberDef" entry indicates
   an offset from the subclass-specific data, rather than from
   "PyObject".

   Can only be used as part of "Py_tp_members" "slot" when creating a
   class using negative "basicsize". It is mandatory in that case.

   This flag is only used in "PyType_Slot". When setting "tp_members"
   during class creation, Python clears it and sets
   "PyMemberDef.offset" to the offset from the "PyObject" struct.

Changed in version 3.10: The "RESTRICTED", "READ_RESTRICTED" and
"WRITE_RESTRICTED" macros available with "#include "structmember.h""
are deprecated. "READ_RESTRICTED" and "RESTRICTED" are equivalent to
"Py_AUDIT_READ"; "WRITE_RESTRICTED" does nothing.

Changed in version 3.12: The "READONLY" macro was renamed to
"Py_READONLY". The "PY_AUDIT_READ" macro was renamed with the "Py_"
prefix. The new names are now always available. Previously, these
required "#include "structmember.h"". The header is still available
and it provides the old names.


Member types
------------

"PyMemberDef.type" can be one of the following macros corresponding to
various C types. When the member is accessed in Python, it will be
converted to the equivalent Python type. When it is set from Python,
it will be converted back to the C type. If that is not possible, an
exception such as "TypeError" or "ValueError" is raised.

Unless marked (D), attributes defined this way cannot be deleted using
e.g. "del" or "delattr()".

+----------------------------------+-------------------------------+------------------------+
| Macro name                       | C type                        | Python type            |
|==================================|===============================|========================|
| Py_T_BYTE  * Part of the Stable  | char                          | "int"                  |
| ABI since version 3.12.*         |                               |                        |
+----------------------------------+-------------------------------+------------------------+
| Py_T_SHORT  * Part of the Stable | short                         | "int"                  |
| ABI since version 3.12.*         |                               |                        |
+----------------------------------+-------------------------------+------------------------+
| Py_T_INT  * Part of the Stable   | int                           | "int"                  |
| ABI since version 3.12.*         |                               |                        |
+----------------------------------+-------------------------------+------------------------+
| Py_T_LONG  * Part of the Stable  | long                          | "int"                  |
| ABI since version 3.12.*         |                               |                        |
+----------------------------------+-------------------------------+------------------------+
| Py_T_LONGLONG  * Part of the     | long long                     | "int"                  |
| Stable ABI since version 3.12.*  |                               |                        |
+----------------------------------+-------------------------------+------------------------+
| Py_T_UBYTE  * Part of the Stable | unsigned char                 | "int"                  |
| ABI since version 3.12.*         |                               |                        |
+----------------------------------+-------------------------------+------------------------+
| Py_T_UINT  * Part of the Stable  | unsigned int                  | "int"                  |
| ABI since version 3.12.*         |                               |                        |
+----------------------------------+-------------------------------+------------------------+
| Py_T_USHORT  * Part of the       | unsigned short                | "int"                  |
| Stable ABI since version 3.12.*  |                               |                        |
+----------------------------------+-------------------------------+------------------------+
| Py_T_ULONG  * Part of the Stable | unsigned long                 | "int"                  |
| ABI since version 3.12.*         |                               |                        |
+----------------------------------+-------------------------------+------------------------+
| Py_T_ULONGLONG  * Part of the    | unsigned long long            | "int"                  |
| Stable ABI since version 3.12.*  |                               |                        |
+----------------------------------+-------------------------------+------------------------+
| Py_T_PYSSIZET  * Part of the     | Py_ssize_t                    | "int"                  |
| Stable ABI since version 3.12.*  |                               |                        |
+----------------------------------+-------------------------------+------------------------+
| Py_T_FLOAT  * Part of the Stable | float                         | "float"                |
| ABI since version 3.12.*         |                               |                        |
+----------------------------------+-------------------------------+------------------------+
| Py_T_DOUBLE  * Part of the       | double                        | "float"                |
| Stable ABI since version 3.12.*  |                               |                        |
+----------------------------------+-------------------------------+------------------------+
| Py_T_BOOL  * Part of the Stable  | char (written as 0 or 1)      | "bool"                 |
| ABI since version 3.12.*         |                               |                        |
+----------------------------------+-------------------------------+------------------------+
| Py_T_STRING  * Part of the       | const char* (*)               | "str" (RO)             |
| Stable ABI since version 3.12.*  |                               |                        |
+----------------------------------+-------------------------------+------------------------+
| Py_T_STRING_INPLACE  * Part of   | const char[] (*)              | "str" (RO)             |
| the Stable ABI since version     |                               |                        |
| 3.12.*                           |                               |                        |
+----------------------------------+-------------------------------+------------------------+
| Py_T_CHAR  * Part of the Stable  | char (0-127)                  | "str" (**)             |
| ABI since version 3.12.*         |                               |                        |
+----------------------------------+-------------------------------+------------------------+
| Py_T_OBJECT_EX  * Part of the    | PyObject*                     | "object" (D)           |
| Stable ABI since version 3.12.*  |                               |                        |
+----------------------------------+-------------------------------+------------------------+

   (*): Zero-terminated, UTF8-encoded C string. With "Py_T_STRING" the
   C representation is a pointer; with "Py_T_STRING_INPLACE" the
   string is stored directly in the structure.

   (**): String of length 1. Only ASCII is accepted.

   (RO): Implies "Py_READONLY".

   (D): Can be deleted, in which case the pointer is set to "NULL".
   Reading a "NULL" pointer raises "AttributeError".

Added in version 3.12: In previous versions, the macros were only
available with "#include "structmember.h"" and were named without the
"Py_" prefix (e.g. as "T_INT"). The header is still available and
contains the old names, along with the following deprecated types:

T_OBJECT

   Like "Py_T_OBJECT_EX", but "NULL" is converted to "None". This
   results in surprising behavior in Python: deleting the attribute
   effectively sets it to "None".

T_NONE

   Always "None". Must be used with "Py_READONLY".


Defining Getters and Setters
----------------------------

type PyGetSetDef
    * Part of the Stable ABI (including all members).*

   Structure to define property-like access for a type. See also
   description of the "PyTypeObject.tp_getset" slot.

   const char *name

      attribute name

   getter get

      C function to get the attribute.

   setter set

      Optional C function to set or delete the attribute. If "NULL",
      the attribute is read-only.

   const char *doc

      optional docstring

   void *closure

      Optional user data pointer, providing additional data for getter
      and setter.

typedef PyObject *(*getter)(PyObject*, void*)
    * Part of the Stable ABI.*

   The "get" function takes one PyObject* parameter (the instance) and
   a user data pointer (the associated "closure"):

   It should return a new reference on success or "NULL" with a set
   exception on failure.

typedef int (*setter)(PyObject*, PyObject*, void*)
    * Part of the Stable ABI.*

   "set" functions take two PyObject* parameters (the instance and the
   value to be set) and a user data pointer (the associated
   "closure"):

   In case the attribute should be deleted the second parameter is
   "NULL". Should return "0" on success or "-1" with a set exception
   on failure.
