Suporte a Coleta de Lixo Cíclica

O suporte do Python para detectar e coletar o lixo, que envolve referencias circulares, requer suporte dos tipos de objetos que são “contêiners” para outros objetos que também podem ser contêiners. 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 container, o tp_flags campo do tipo do objeto deve incluir o Py_TPFLAGS_HAVE_GC`e fornecer uma implementação do :c:member:`~PyTypeObject.tp_traverse manipulador. Se as instâncias do tipo forem mutáveis, uma:c:member:~PyTypeObject.tp_clear implementação também deverá ser fornecida.

Py_TPFLAGS_HAVE_GC

Objetos com esse tipo de sinalizador definido devem seguir as 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 containers foram inicializados, deve-se chamar PyObject_GC_Track().

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.

void _PyObject_GC_TRACK(PyObject *op)

A macro version of PyObject_GC_Track(). It should not be used for extension modules.

Obsoleto desde a versão 3.6: This macro is removed from Python 3.8.

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 containers sejam invalidados, PyObject_GC_UnTrack() deve ser chamado.

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

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.

void _PyObject_GC_UNTRACK(PyObject *op)

A macro version of PyObject_GC_UnTrack(). It should not be used for extension modules.

Obsoleto desde a versão 3.6: This macro is removed from Python 3.8.

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.