Esta documentação é de uma versão antiga do Python que não é mais atualizada. Você deveria atualizar e ler a Documentação do Python para versão estável atual.

# Suporte a Coleta Cíclica de Lixo

O suporte do Python para detectar e coletar o lixo, que envolve referencias circulares, requer suporte dos tipos de objetos que são “contêineres” para outros objetos que também podem ser contêineres. Tipos que não armazenam referências a outros tipos de objetos, ou que apenas armazenam referências a tipos atômicos (como números ou strings), não precisam fornecer nenhum suporte explicito para coleta de lixo.

Para criar um tipo contêiner, o `tp_flags` campo do tipo do objeto deve incluir o `Py_TPFLAGS_HAVE_GC` e fornecer uma implementação do `tp_traverse` manipulador. Se as instâncias do tipo forem mutáveis, uma `tp_clear` implementação também deverá ser fornecida.

`Py_TPFLAGS_HAVE_GC`

  Objetos com esse tipo de sinalizador definido devem estar em conformidade com regras documentadas aqui. Por conveniência esses objetos serão referenciados como objetos de contêiner.

Construtores para tipos de contêiner devem obedecer a duas regras:

1. A memória para o objeto deve ser alocada usando `PyObject_GC_New()` ou `PyObject_GC_NewVar()`.

2. Uma vez que todos os campos que podem conter referências a outros contêineres foram inicializados, deve-se chamar `PyObject_GC_Track()`.

Da mesma forma, o desalocador para o objeto deve estar em conformidade com regras semelhantes:

1. Antes que os campos que fazer referência a outros contêineres sejam invalidados, `PyObject_GC_UnTrack()` deve ser chamado.

2. A memória destinada ao objeto deve ser desalocada usando `PyObject_GC_Del()`.

   Aviso

   If a type adds the Py_TPFLAGS_HAVE_GC, then it must implement at least a `tp_traverse` handler or explicitly use one from its subclass or subclasses.

   When calling `PyType_Ready()` or some of the APIs that indirectly call it like `PyType_FromSpecWithBases()` or `PyType_FromSpec()` the interpreter will automatically populate the `tp_flags`, `tp_traverse` and `tp_clear` fields if the type inherits from a class that implements the garbage collector protocol and the child class does not include the `Py_TPFLAGS_HAVE_GC` flag.

TYPE* `PyObject_GC_New`(TYPE, PyTypeObject *type)

  Analogous to `PyObject_New()` but for container objects with the `Py_TPFLAGS_HAVE_GC` flag set.

TYPE* `PyObject_GC_NewVar`(TYPE, PyTypeObject *type, Py_ssize_t size)

  Analogous to `PyObject_NewVar()` but for container objects with the `Py_TPFLAGS_HAVE_GC` flag set.

TYPE* `PyObject_GC_Resize`(TYPE, PyVarObject *op, Py_ssize_t newsize)

  Resize an object allocated by `PyObject_NewVar()`. Returns the resized object or `NULL` on failure. op must not be tracked by the collector yet.

void `PyObject_GC_Track`(PyObject *op)

  Adds the object op to the set of container objects tracked by the collector. The collector can run at unexpected times so objects must be valid while being tracked. This should be called once all the fields followed by the `tp_traverse` handler become valid, usually near the end of the constructor.

int `PyObject_IS_GC`(PyObject *obj)

  Returns non-zero if the object implements the garbage collector protocol, otherwise returns 0.

  The object cannot be tracked by the garbage collector if this function returns 0.

int `PyObject_GC_IsTracked`(PyObject *op)

  Returns 1 if the object type of op implements the GC protocol and op is being currently tracked by the garbage collector and 0 otherwise.

  This is analogous to the Python function `gc.is_tracked()`.

  Novo na versão 3.9.

int `PyObject_GC_IsFinalized`(PyObject *op)

  Returns 1 if the object type of op implements the GC protocol and op has been already finalized by the garbage collector and 0 otherwise.

  This is analogous to the Python function `gc.is_finalized()`.

  Novo na versão 3.9.

void `PyObject_GC_Del`(void *op)

  Releases memory allocated to an object using `PyObject_GC_New()` or `PyObject_GC_NewVar()`.

void `PyObject_GC_UnTrack`(void *op)

  Remove the object op from the set of container objects tracked by the collector. Note that `PyObject_GC_Track()` can be called again on this object to add it back to the set of tracked objects. The deallocator (`tp_dealloc` handler) should call this for the object before any of the fields used by the `tp_traverse` handler become invalid.

Alterado na versão 3.8: The `_PyObject_GC_TRACK()` and `_PyObject_GC_UNTRACK()` macros have been removed from the public C API.

The `tp_traverse` handler accepts a function parameter of this type:

int `(*visitproc)`(PyObject *object, void *arg)

  Type of the visitor function passed to the `tp_traverse` handler. The function should be called with an object to traverse as object and the third parameter to the `tp_traverse` handler as arg. The Python core uses several visitor functions to implement cyclic garbage detection; it’s not expected that users will need to write their own visitor functions.

The `tp_traverse` handler must have the following type:

int `(*traverseproc)`(PyObject *self, visitproc visit, void *arg)

  Traversal function for a container object. Implementations must call the visit function for each object directly contained by self, with the parameters to visit being the contained object and the arg value passed to the handler. The visit function must not be called with a `NULL` object argument. If visit returns a non-zero value that value should be returned immediately.

To simplify writing `tp_traverse` handlers, a `Py_VISIT()` macro is provided. In order to use this macro, the `tp_traverse` implementation must name its arguments exactly visit and arg:

void `Py_VISIT`(PyObject *o)

  If o is not `NULL`, call the visit callback, with arguments o and arg. If visit returns a non-zero value, then return it. Using this macro, `tp_traverse` handlers look like:

      static int
      my_traverse(Noddy *self, visitproc visit, void *arg)
      {
          Py_VISIT(self->foo);
          Py_VISIT(self->bar);
          return 0;
      }

The `tp_clear` handler must be of the `inquiry` type, or `NULL` if the object is immutable.

int `(*inquiry)`(PyObject *self)

  Drop references that may have created reference cycles. Immutable objects do not have to define this method since they can never directly create reference cycles. Note that the object must still be valid after calling this method (don’t just call `Py_DECREF()` on a reference). The collector will call this method if it detects that this object is involved in a reference cycle.