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 “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 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:
A memória para o objeto deve ser alocada usando
PyObject_GC_New()
ouPyObject_GC_NewVar()
.Uma vez que todos os campos que podem conter referências a outros containers foram inicializados, deve-se chamar
PyObject_GC_Track()
.
Da mesma forma, o desalocador para o objeto deve estar em conformidade com regras semelhantes:
Antes que os campos que fazer referência a outros containers sejam invalidados,
PyObject_GC_UnTrack()
deve ser chamado.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 likePyType_FromSpecWithBases()
orPyType_FromSpec()
the interpreter will automatically populate thetp_flags
,tp_traverse
andtp_clear
fields if the type inherits from a class that implements the garbage collector protocol and the child class does not include thePy_TPFLAGS_HAVE_GC
flag.
-
TYPE*
PyObject_GC_New
(TYPE, PyTypeObject *type)¶ Analogous to
PyObject_New()
but for container objects with thePy_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 thePy_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 orNULL
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()
orPyObject_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 thetp_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 thetp_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.