Supporting Cyclic Garbage Collection¶
Python's support for detecting and collecting garbage which involves circular references requires support from object types which are "containers" for other objects which may also be containers. Types which do not store references to other objects, or which only store references to atomic types (such as numbers or strings), do not need to provide any explicit support for garbage collection.
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
Objects with a type with this flag set must conform with the rules documented here. For convenience these objects will be referred to as container objects.
Constructors for container types must conform to two rules:
The memory for the object must be allocated using
PyObject_GC_New()
orPyObject_GC_NewVar()
.Once all the fields which may contain references to other containers are initialized, it must call
PyObject_GC_Track()
.
-
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)¶ 为
PyObject_NewVar()
所分配对象重新调整大小。 返回调整大小后的对象或在失败时返回NULL
。 op 必须尚未被垃圾回收器追踪。
-
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.
Similarly, the deallocator for the object must conform to a similar pair of rules:
Before fields which refer to other containers are invalidated,
PyObject_GC_UnTrack()
must be called.The object's memory must be deallocated using
PyObject_GC_Del()
.
-
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.
3.8 版更變: _PyObject_GC_TRACK()
和 _PyObject_GC_UNTRACK()
宏已从公有 C API 中移除。
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)¶ 用于容器对象的遍历函数。 它的实现必须对 self 所直接包含的每个对象调用 visit 函数,visit 的形参为所包含对象和传给处理程序的 arg 值。 visit 函数调用不可附带
NULL
对象作为参数。 如果 visit 返回非零值,则该值应当被立即返回。
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)¶ 如果 o 不为
NULL
,则调用 visit 回调函数,附带参数 o 和 arg。 如果 visit 返回一个非零值,则返回该值。 使用此宏之后,tp_traverse
处理程序的形式如下:static int my_traverse(Noddy *self, visitproc visit, void *arg) { Py_VISIT(self->foo); Py_VISIT(self->bar); return 0; }
tp_clear
处理程序必须为 inquiry
类型,如果对象不可变则为 NULL
。
-
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.