# Estruturas comuns de objetos

Existe um grande número de estruturas usadas para a definição de tipos objeto para o Python. Esta seção descreve essas estruturas e como são usadas.

## Macros e tipos de objetos base

Todos os objetos Python por fim compartilham um pequeno número de campos no começo da representação o objeto na memória. Esses são representados pelos tipos `PyObject` e `PyVarObject`, que são definidos, por sua vez, pelas expansões de alguns macros também, utilizados, direta ou indiretamente, na definição de todos outros objetos Python. Macros adicionais podem ser encontrados em contagem de referências.

type PyObject
  Parte da API Limitada. (Somente alguns membros são parte da ABI estável.)

  Todos os tipos de objeto são extensões deste tipo. Este é um tipo que contém as informações necessárias para que o Python trate um ponteiro para um objeto como tal. Em uma versão de “lançamento” normal, ele contém apenas a contagem de referências do objeto e um ponteiro para o objeto do tipo correspondente. Nada é declarado como um `PyObject`, mas todo ponteiro para um objeto Python pode ser convertido para um PyObject*.

  Os membros não devem ser acessados diretamente; em vez disso, use macros como `Py_REFCNT` e `Py_TYPE`.

  Py_ssize_t ob_refcnt
    Parte da ABI Estável.

    A contagem de referências do objeto, conforme retornada por `Py_REFCNT`. Não use este campo diretamente; em vez disso, use funções e macros como `Py_REFCNT`, `Py_INCREF()` e `Py_DecRef()`.

    O tipo de campo pode ser diferente de `Py_ssize_t`, dependendo da configuração e da plataforma de construção.

  PyTypeObject *ob_type
    Parte da ABI Estável.

    O tipo do objeto. Não use este campo diretamente; use `Py_TYPE` e `Py_SET_TYPE()` em vez disso.

type PyVarObject
  Parte da API Limitada. (Somente alguns membros são parte da ABI estável.)

  Uma extensão de `PyObject` que adiciona o campo `ob_size`. Isso se destina a objetos que possuem alguma noção de comprimento.

  Assim como com `PyObject`, os membros não devem ser acessados diretamente; em vez disso, use macros como `Py_SIZE`, `Py_REFCNT` e `Py_TYPE`.

  Py_ssize_t ob_size
    Parte da ABI Estável.

    Um campo de tamanho, cujo conteúdo deve ser considerado um detalhe de implementação interna do objeto.

    Não utilize este campo diretamente; utilize `Py_SIZE` em vez disso.

    Funções de criação de objetos, como `PyObject_NewVar()`, geralmente definem este campo com o tamanho solicitado (número de itens). Após a criação, valores arbitrários podem ser armazenados em `ob_size` usando `Py_SET_SIZE`.

    Para obter o comprimento publicamente exposto de um objeto, conforme retornado pela função Python `len()`, use `PyObject_Length()` em vez disso.

PyObject_HEAD

  Este é um macro usado ao declarar novos tipos que representam objetos sem comprimento variável. O macro PyObject_HEAD se expande para:

      PyObject ob_base;

  Veja documentação de `PyObject` acima.

PyObject_VAR_HEAD

  Esta é uma macro usada ao declarar novos tipos que representam objetos com um comprimento que varia de instância para instância. A macro PyObject_VAR_HEAD se expande para:

      PyVarObject ob_base;

  Veja documentação de `PyVarObject` acima.

PyTypeObject PyBaseObject_Type
  Parte da ABI Estável.

  A classe base de todos os outros objetos, equivalente a `object` em Python.

int Py_Is(PyObject *x, PyObject *y)
  Parte da ABI Estável desde a versão 3.10.

  Testa se o objeto x é o objeto y, o mesmo que `x is y` em Python.

  Adicionado na versão 3.10.

int Py_IsNone(PyObject *x)
  Parte da ABI Estável desde a versão 3.10.

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

  Adicionado na versão 3.10.

int Py_IsTrue(PyObject *x)
  Parte da ABI Estável desde a versão 3.10.

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

  Adicionado na versão 3.10.

int Py_IsFalse(PyObject *x)
  Parte da ABI Estável desde a versão 3.10.

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

  Adicionado na versão 3.10.

PyTypeObject *Py_TYPE(PyObject *o)
  Retorna valor: Referência emprestada. Parte da ABI Estável desde a versão 3.14.

  Get the type of the Python object o.

  The returned reference is borrowed from o. Do not release it with `Py_DECREF()` or similar.

  Alterado na versão 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`.

  Adicionado na versão 3.9.

void Py_SET_TYPE(PyObject *o, PyTypeObject *type)

  Set the type of object o to type, without any checking or reference counting.

  This is a very low-level operation. Consider instead setting the Python attribute `__class__` using `PyObject_SetAttrString()` or similar.

  Note that assigning an incompatible type can lead to undefined behavior.

  If type is a heap type, the caller must create a new reference to it. Similarly, if the old type of o is a heap type, the caller must release a reference to that type.

  Adicionado na versão 3.9.

Py_ssize_t Py_SIZE(PyVarObject *o)

  Get the `ob_size` field of o.

  Alterado na versão 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 `ob_size` field of o to size.

  Adicionado na versão 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
  Parte da ABI Estável.

  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.

  A assinatura da função é:

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

type PyCFunctionWithKeywords
  Parte da ABI Estável.

  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
  Parte da ABI Estável desde a versão 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
  Parte da ABI Estável desde a versão 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)

  Adicionado na versão 3.9.

type PyMethodDef
  Parte da ABI Estável (incluindo todos os membros).

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

  const char *ml_name

    Nome do método.

  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
  Parte da ABI Estável.

  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
  Parte da ABI Estável desde a versão 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).

  Adicionado na versão 3.7.

  Alterado na versão 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.

  Adicionado na versão 3.7.

METH_METHOD
  Parte da ABI Estável desde a versão 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`.

  Adicionado na versão 3.9.

METH_NOARGS
  Parte da ABI Estável.

  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
  Parte da ABI Estável.

  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 use 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
  Parte da ABI Estável.

  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
  Parte da ABI Estável.

  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
  Parte da ABI Estável.

  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)
  Retorna valor: Nova referência. Parte da ABI Estável desde a versão 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`.

  Ver também

  `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`.

  Adicionado na versão 3.9.

PyTypeObject PyCFunction_Type
  Parte da ABI Estável.

  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)
  Retorna valor: Nova referência. Parte da ABI Estável.

  Equivalente a `PyCMethod_New(ml, self, module, NULL)`.

PyObject *PyCFunction_New(PyMethodDef *ml, PyObject *self)
  Retorna valor: Nova referência. Parte da ABI Estável desde a versão 3.4.

  Equivalente a `PyCMethod_New(ml, self, NULL, NULL)`.

int PyCFunction_GetFlags(PyObject *func)
  Parte da ABI Estável.

  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)
  Parte da ABI Estável.

  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)
  Parte da ABI Estável.

  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
  Parte da ABI Estável (incluindo todos os membros).

  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. This member must be defined with `Py_T_PYSSIZET`, and either `Py_READONLY` or `Py_READONLY | Py_RELATIVE_OFFSET`. 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.

  Alterado na versão 3.12: `PyMemberDef` is always available. Previously, it required including `"structmember.h"`.

  Alterado na versão 3.14: `Py_RELATIVE_OFFSET` is now allowed for `"__vectorcalloffset__"`, `"__dictoffset__"` and `"__weaklistoffset__"`.

PyObject *PyMember_GetOne(const char *obj_addr, struct PyMemberDef *m)
  Parte da ABI Estável.

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

  Alterado na versão 3.12: `PyMember_GetOne` is always available. Previously, it required including `"structmember.h"`.

int PyMember_SetOne(char *obj_addr, struct PyMemberDef *m, PyObject *o)
  Parte da ABI Estável.

  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.

  Alterado na versão 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
  Parte da ABI Estável desde a versão 3.12.

  Not writable.

Py_AUDIT_READ
  Parte da ABI Estável desde a versão 3.12.

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

Py_RELATIVE_OFFSET
  Parte da ABI Estável desde a versão 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 the `Py_tp_members` `slot` when creating a class using negative `basicsize`. It is mandatory in that case. When setting `tp_members` from the slot during class creation, Python clears the flag and sets `PyMemberDef.offset` to the offset from the `PyObject` struct.

Alterado na versão 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.

Alterado na versão 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 | Tipo em C | Tipo em Python
--- | --- | ---
Py_T_BYTE Parte da ABI Estável desde a versão 3.12. | char | `int`
Py_T_SHORT Parte da ABI Estável desde a versão 3.12. | short | `int`
Py_T_INT Parte da ABI Estável desde a versão 3.12. | int | `int`
Py_T_LONG Parte da ABI Estável desde a versão 3.12. | long | `int`
Py_T_LONGLONG Parte da ABI Estável desde a versão 3.12. | long long | `int`
Py_T_UBYTE Parte da ABI Estável desde a versão 3.12. | unsigned char | `int`
Py_T_UINT Parte da ABI Estável desde a versão 3.12. | unsigned int | `int`
Py_T_USHORT Parte da ABI Estável desde a versão 3.12. | unsigned short | `int`
Py_T_ULONG Parte da ABI Estável desde a versão 3.12. | unsigned long | `int`
Py_T_ULONGLONG Parte da ABI Estável desde a versão 3.12. | unsigned long long | `int`
Py_T_PYSSIZET Parte da ABI Estável desde a versão 3.12. | Py_ssize_t | `int`
Py_T_FLOAT Parte da ABI Estável desde a versão 3.12. | float | `float`
Py_T_DOUBLE Parte da ABI Estável desde a versão 3.12. | double | `float`
Py_T_BOOL Parte da ABI Estável desde a versão 3.12. | char (escrito como 0 ou 1) | `bool`
Py_T_STRING Parte da ABI Estável desde a versão 3.12. | const char* (*) | `str` (RO)
Py_T_STRING_INPLACE Parte da ABI Estável desde a versão 3.12. | const char[] (*) | `str` (RO)
Py_T_CHAR Parte da ABI Estável desde a versão 3.12. | char (0-127) | `str` (**)
Py_T_OBJECT_EX Parte da ABI Estável desde a versão 3.12. | PyObject* | `object` (D)

> (*): 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): implica `Py_READONLY`.

> (D): pode ser deletado, neste caso o ponteiro é definido para `NULL`. Ler um ponteiro `NULL` levanta uma exceção `AttributeError`.

Adicionado na versão 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

  Sempre `None`. Deve ser usado com `Py_READONLY`.

### Defining Getters and Setters

type PyGetSetDef
  Parte da ABI Estável (incluindo todos os membros).

  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*)
  Parte da ABI Estável.

  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*)
  Parte da ABI Estável.

  `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.