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.

To create a container type, the tp_flags field of the type object must include the Py_TPFLAGS_HAVE_GC and provide an implementation of the tp_traverse handler. If instances of the type are mutable, a tp_clear implementation must also be provided.

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. The memory for the object must be allocated using PyObject_GC_New or 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.

PyObject_GC_New(TYPE, typeobj)

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

Do not call this directly to allocate memory for an object; call the type’s tp_alloc slot instead.

Ao preencher o slot tp_alloc de um tipo, PyType_GenericAlloc() é preferível a uma função personalizada que simplesmente chama esta macro.

Memory allocated by this macro must be freed with PyObject_GC_Del() (usually called via the object’s tp_free slot).

Ver também

PyObject_GC_NewVar(TYPE, typeobj, size)

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

Do not call this directly to allocate memory for an object; call the type’s tp_alloc slot instead.

Ao preencher o slot tp_alloc de um tipo, PyType_GenericAlloc() é preferível a uma função personalizada que simplesmente chama esta macro.

Memory allocated by this macro must be freed with PyObject_GC_Del() (usually called via the object’s tp_free slot).

Ver também

PyObject *PyUnstable_Object_GC_NewWithExtraData(PyTypeObject *type, size_t extra_size)
Esta é uma API Instável. Isso pode se alterado sem aviso em lançamentos menores.

Analogous to PyObject_GC_New but allocates extra_size bytes at the end of the object (at offset tp_basicsize). The allocated memory is initialized to zeros, except for the Python object header.

The extra data will be deallocated with the object, but otherwise it is not managed by Python.

Memory allocated by this function must be freed with PyObject_GC_Del() (usually called via the object’s tp_free slot).

Aviso

The function is marked as unstable because the final mechanism for reserving extra data after an instance is not yet decided. For allocating a variable number of fields, prefer using PyVarObject and tp_itemsize instead.

Adicionado na versão 3.12.

PyObject_GC_Resize(TYPE, op, newsize)

Resize an object allocated by PyObject_NewVar. Returns the resized object of type TYPE* (refers to any C type) or NULL on failure.

op must be of type PyVarObject* and must not be tracked by the collector yet. newsize must be of type Py_ssize_t.

void PyObject_GC_Track(PyObject *op)
Parte da ABI Estável.

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)
Parte da ABI Estável desde a versão 3.9.

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().

Adicionado na versão 3.9.

int PyObject_GC_IsFinalized(PyObject *op)
Parte da ABI Estável desde a versão 3.9.

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().

Adicionado na versão 3.9.

void PyObject_GC_Del(void *op)
Parte da ABI Estável.

Releases memory allocated to an object using PyObject_GC_New or PyObject_GC_NewVar.

Do not call this directly to free an object’s memory; call the type’s tp_free slot instead.

Do not use this for memory allocated by PyObject_New, PyObject_NewVar, or related allocation functions; use PyObject_Free() instead.

Ver também

void PyObject_GC_UnTrack(void *op)
Parte da ABI Estável.

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:

typedef int (*visitproc)(PyObject *object, void *arg)
Parte da ABI Estável.

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_clear handler must be of the inquiry type, or NULL if the object is immutable.

typedef int (*inquiry)(PyObject *self)
Parte da ABI Estável.

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.

Traversal

The tp_traverse handler must have the following type:

typedef int (*traverseproc)(PyObject *self, visitproc visit, void *arg)
Parte da ABI Estável.

Traversal function for a garbage-collected object, used by the garbage collector to detect reference cycles. 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.

A typical tp_traverse function calls the Py_VISIT() convenience macro on each of the instance’s members that are Python objects that the instance owns. For example, this is a (slightly outdated) traversal function for the threading.local class:

static int
local_traverse(PyObject *op, visitproc visit, void *arg)
{
    localobject *self = (localobject *) op;
    Py_VISIT(Py_TYPE(self));
    Py_VISIT(self->args);
    Py_VISIT(self->kw);
    Py_VISIT(self->dict);
    return 0;
}

Nota

Py_VISIT() requires the visit and arg parameters to local_traverse() to have these specific names; don’t name them just anything.

Instances of heap-allocated types hold a reference to their type. Their traversal function must therefore visit the type:

Py_VISIT(Py_TYPE(self));

Alternately, the type may delegate this responsibility by calling tp_traverse of a heap-allocated superclass (or another heap-allocated type, if applicable). If they do not, the type object may not be garbage-collected.

If the Py_TPFLAGS_MANAGED_DICT bit is set in the tp_flags field, the traverse function must call PyObject_VisitManagedDict() like this:

int err = PyObject_VisitManagedDict((PyObject*)self, visit, arg);
if (err) {
    return err;
}

Only the members that the instance owns (by having strong references to them) must be visited. For instance, if an object supports weak references via the tp_weaklist slot, the pointer supporting the linked list (what tp_weaklist points to) must not be visited as the instance does not directly own the weak references to itself.

The traversal function has a limitation:

Aviso

The traversal function must not have any side effects. Implementations may not modify the reference counts of any Python objects nor create or destroy any Python objects, directly or indirectly.

This means that most Python C API functions may not be used, since they can raise a new exception, return a new reference to a result object, have internal logic that uses side effects. Also, unless documented otherwise, functions that happen to not have side effects may start having them in future versions, without warning.

For a list of safe functions, see a separate section below.

Nota

The Py_VISIT() call may be skipped for those members that provably cannot participate in reference cycles. In the local_traverse example above, there is also a self->key member, but it can only be NULL or a Python string and therefore cannot be part of a reference cycle.

On the other hand, even if you know a member can never be part of a cycle, as a debugging aid you may want to visit it anyway just so the gc module’s get_referents() function will include it.

Nota

The tp_traverse function can be called from any thread.

Detalhes da implementação do CPython: Garbage collection is a “stop-the-world” operation: even in free threading builds, only one thread state is attached when tp_traverse handlers run.

Alterado na versão 3.9: Heap-allocated types are expected to visit Py_TYPE(self) in tp_traverse. In earlier versions of Python, due to bug 40217, doing this may lead to crashes in subclasses.

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:

Py_VISIT(o)

If the PyObject* o is not NULL, call the visit callback, with arguments o and arg. If visit returns a non-zero value, then return it.

This corresponds roughly to:

#define Py_VISIT(o)                             \
   if (op) {                                    \
      int visit_result = visit(o, arg);         \
      if (visit_result != 0) {                  \
         return visit_result;                   \
      }                                         \
   }

Traversal-safe functions

The following functions and macros are safe to use in a tp_traverse handler:

“DuringGC” functions

The following functions should only be used in a tp_traverse handler; calling them in other contexts may have unintended consequences.

These functions act like their counterparts without the _DuringGC suffix, but they are guaranteed to not have side effects, they do not set an exception on failure, and they return/set borrowed references as detailed in the individual documentation.

Note that these functions may fail (return NULL or -1), but as they do not set an exception, no error information is available. In some cases, failure is not distinguishable from a successful NULL result.

void *PyObject_GetTypeData_DuringGC(PyObject *o, PyTypeObject *cls)
void *PyObject_GetItemData_DuringGC(PyObject *o)
void *PyType_GetModuleState_DuringGC(PyTypeObject *type)
void *PyModule_GetState_DuringGC(PyObject *module)
int PyModule_GetToken_DuringGC(PyObject *module, void **result)
Parte da ABI Estável desde a versão 3.15.

See “DuringGC” functions for common information.

Adicionado na versão 3.15.0a8 (unreleased).

Ver também

PyObject_GetTypeData(), PyObject_GetItemData(), PyType_GetModuleState(), PyModule_GetState(), PyModule_GetToken(), PyType_GetBaseByToken()

int PyType_GetBaseByToken_DuringGC(PyTypeObject *type, void *tp_token, PyTypeObject **result)
Parte da ABI Estável desde a versão 3.15.

See “DuringGC” functions for common information.

Sets *result to a borrowed reference rather than a strong one. The reference is valid for the duration of the tp_traverse handler call.

Adicionado na versão 3.15.0a8 (unreleased).

Ver também

PyType_GetBaseByToken()

PyObject *PyType_GetModule_DuringGC(PyTypeObject *type)
PyObject *PyType_GetModuleByToken_DuringGC(PyTypeObject *type, const void *mod_token)
Retorna valor: Referência emprestada. Parte da ABI Estável desde a versão 3.15.

See “DuringGC” functions for common information.

These functions return a borrowed reference, which is valid for the duration of the tp_traverse handler call.

Adicionado na versão 3.15.0a8 (unreleased).

Ver também

PyType_GetModule(), PyType_GetModuleByToken()

Controlando o estado do coletor de lixo

The C-API provides the following functions for controlling garbage collection runs.

Py_ssize_t PyGC_Collect(void)
Parte da ABI Estável.

Perform a full garbage collection, if the garbage collector is enabled. (Note that gc.collect() runs it unconditionally.)

Returns the number of collected + unreachable objects which cannot be collected. If the garbage collector is disabled or already collecting, returns 0 immediately. Errors during garbage collection are passed to sys.unraisablehook. This function does not raise exceptions.

int PyGC_Enable(void)
Parte da ABI Estável desde a versão 3.10.

Enable the garbage collector: similar to gc.enable(). Returns the previous state, 0 for disabled and 1 for enabled.

Adicionado na versão 3.10.

int PyGC_Disable(void)
Parte da ABI Estável desde a versão 3.10.

Disable the garbage collector: similar to gc.disable(). Returns the previous state, 0 for disabled and 1 for enabled.

Adicionado na versão 3.10.

int PyGC_IsEnabled(void)
Parte da ABI Estável desde a versão 3.10.

Query the state of the garbage collector: similar to gc.isenabled(). Returns the current state, 0 for disabled and 1 for enabled.

Adicionado na versão 3.10.

Querying Garbage Collector State

The C-API provides the following interface for querying information about the garbage collector.

void PyUnstable_GC_VisitObjects(gcvisitobjects_t callback, void *arg)
Esta é uma API Instável. Isso pode se alterado sem aviso em lançamentos menores.

Run supplied callback on all live GC-capable objects. arg is passed through to all invocations of callback.

Aviso

If new objects are (de)allocated by the callback it is undefined if they will be visited.

Garbage collection is disabled during operation. Explicitly running a collection in the callback may lead to undefined behaviour e.g. visiting the same objects multiple times or not at all.

Adicionado na versão 3.12.

typedef int (*gcvisitobjects_t)(PyObject *object, void *arg)

Type of the visitor function to be passed to PyUnstable_GC_VisitObjects(). arg is the same as the arg passed to PyUnstable_GC_VisitObjects. Return 1 to continue iteration, return 0 to stop iteration. Other return values are reserved for now so behavior on returning anything else is undefined.

Adicionado na versão 3.12.