// Include/weakrefobject.h
typedef struct _PyWeakReference PyWeakReference;
struct _PyWeakReference {
PyObject_HEAD
/* The object to which this is a weak reference, or Py_None if none.
* Note that this is a stealth reference: wr_object's refcount is
* not incremented to reflect this pointer.
*/
PyObject *wr_object;
/* A callable to invoke when wr_object dies, or NULL if none. */
PyObject *wr_callback;
/* A cache for wr_object's hash code. As usual for hashes, this is -1
* if the hash code isn't known yet.
*/
Py_hash_t hash;
/* If wr_object is weakly referenced, wr_object has a doubly-linked NULL-
* terminated list of weak references to it. These are the list pointers.
* If wr_object goes away, wr_object is set to Py_None, and these pointers
* have no meaning then.
*/
PyWeakReference *wr_prev;
PyWeakReference *wr_next;
};PyWeakReference 对象很简单, 其中包含了引用的目标对象, 回调函数. 另外 PyWeakReference 对象通过 wr_prev 和 wr_next 构成一个双向链表.
弱引用对象的关键就是没有增加目标对象的引用计数.
PyTypeObject
_PyWeakref_RefType = {
PyVarObject_HEAD_INIT(&PyType_Type, 0)
"weakref",
sizeof(PyWeakReference),
0,
weakref_dealloc, /*tp_dealloc*/
0, /*tp_vectorcall_offset*/
0, /*tp_getattr*/
0, /*tp_setattr*/
0, /*tp_as_async*/
(reprfunc)weakref_repr, /*tp_repr*/
0, /*tp_as_number*/
0, /*tp_as_sequence*/
0, /*tp_as_mapping*/
(hashfunc)weakref_hash, /*tp_hash*/
(ternaryfunc)weakref_call, /*tp_call*/
0, /*tp_str*/
0, /*tp_getattro*/
0, /*tp_setattro*/
0, /*tp_as_buffer*/
Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC
| Py_TPFLAGS_BASETYPE, /*tp_flags*/
0, /*tp_doc*/
(traverseproc)gc_traverse, /*tp_traverse*/
(inquiry)gc_clear, /*tp_clear*/
(richcmpfunc)weakref_richcompare, /*tp_richcompare*/
0, /*tp_weaklistoffset*/
0, /*tp_iter*/
0, /*tp_iternext*/
weakref_methods, /*tp_methods*/
weakref_members, /*tp_members*/
0, /*tp_getset*/
0, /*tp_base*/
0, /*tp_dict*/
0, /*tp_descr_get*/
0, /*tp_descr_set*/
0, /*tp_dictoffset*/
weakref___init__, /*tp_init*/
PyType_GenericAlloc, /*tp_alloc*/
weakref___new__, /*tp_new*/
PyObject_GC_Del, /*tp_free*/
};static PyObject *
weakref___new__(PyTypeObject *type, PyObject *args, PyObject *kwargs)
{
PyWeakReference *self = NULL;
PyObject *ob, *callback = NULL;
if (parse_weakref_init_args("__new__", args, kwargs, &ob, &callback)) {
PyWeakReference *ref, *proxy;
PyWeakReference **list;
if (!PyType_SUPPORTS_WEAKREFS(Py_TYPE(ob))) {
PyErr_Format(PyExc_TypeError,
"cannot create weak reference to '%s' object",
Py_TYPE(ob)->tp_name);
return NULL;
}
if (callback == Py_None)
callback = NULL;
list = GET_WEAKREFS_LISTPTR(ob);
get_basic_refs(*list, &ref, &proxy);
if (callback == NULL && type == &_PyWeakref_RefType) {
if (ref != NULL) {
/* We can re-use an existing reference. */
Py_INCREF(ref);
return (PyObject *)ref;
}
}
/* We have to create a new reference. */
/* Note: the tp_alloc() can trigger cyclic GC, so the weakref
list on ob can be mutated. This means that the ref and
proxy pointers we got back earlier may have been collected,
so we need to compute these values again before we use
them. */
self = (PyWeakReference *) (type->tp_alloc(type, 0));
if (self != NULL) {
init_weakref(self, ob, callback);
if (callback == NULL && type == &_PyWeakref_RefType) {
insert_head(self, list);
}
else {
PyWeakReference *prev;
get_basic_refs(*list, &ref, &proxy);
prev = (proxy == NULL) ? ref : proxy;
if (prev == NULL)
insert_head(self, list);
else
insert_after(self, prev);
}
}
}
return (PyObject *)self;
}流程:
-
检查目标对象是否支持弱引用. 判断标准是:
/* Test if a type supports weak references */ #define PyType_SUPPORTS_WEAKREFS(t) ((t)->tp_weaklistoffset > 0)
-
获取目标对象的弱引用链表 list.
-
调用
get_basic_refs(*list, &ref, &proxy)获取 ref 和 proxy.ref 表示一个 callback 等于 NULL 的弱引用对象(PyWeakReference), proxy 表示一个 callback 等于 NULL 的 weakproxy 对象或者 weakcallableproxy 对象.
目标对象的弱引用链表最前面的两个节点一般是 ref 和 proxy.
-
如果 callback 等于 NULL, 而且 ref 不为 NULL, 那么可以复用 ref, 此时直接返回 ref.
-
否则新生成一个弱引用对象, 然后将其添加到目标对象的弱引用链表.
目标对象的弱引用链表的示意图:
+-----------------+
| PyObject |
+-----------------+
| PyObject_HEAD |
+-----------------+
| __dict__ |
+-----------------+
| __weakref__ | -----> +-------------------+ +-------------------+
+-----------------+ | PyWeakReference | -----> | PyWeakReference | ...
+-------------------+ +-------------------+
我们通过调用弱引用对象来获取目标对象, target_obj = wr_ojb().
调用对象的底层实现自然是 __call__, 即 tp_call.
static PyObject *
weakref_call(PyWeakReference *self, PyObject *args, PyObject *kw)
{
static char *kwlist[] = {NULL};
if (PyArg_ParseTupleAndKeywords(args, kw, ":__call__", kwlist)) {
PyObject *object = PyWeakref_GET_OBJECT(self);
Py_INCREF(object);
return (object);
}
return NULL;
}PyWeakref_GET_OBJECT 是一个宏, 作用是返回目标对象. 如果目标对象的引用计数大于 0, 那么返回目标对象, 否则返回 None.
目标对象被回收时, 首先会调用目标对象的 tp_dealloc, tp_dealloc 会调用 PyObject_ClearWeakRefs.
/* Note that there's an inlined copy-paste of handle_callback() in gcmodule.c's
* handle_weakrefs().
*/
static void
handle_callback(PyWeakReference *ref, PyObject *callback)
{
PyObject *cbresult = PyObject_CallOneArg(callback, (PyObject *)ref);
if (cbresult == NULL)
PyErr_WriteUnraisable(callback);
else
Py_DECREF(cbresult);
}
/* This function is called by the tp_dealloc handler to clear weak references.
*
* This iterates through the weak references for 'object' and calls callbacks
* for those references which have one. It returns when all callbacks have
* been attempted.
*/
void
PyObject_ClearWeakRefs(PyObject *object)
{
PyWeakReference **list;
if (object == NULL
|| !PyType_SUPPORTS_WEAKREFS(Py_TYPE(object))
|| Py_REFCNT(object) != 0)
{
PyErr_BadInternalCall();
return;
}
list = GET_WEAKREFS_LISTPTR(object);
/* Remove the callback-less basic and proxy references */
if (*list != NULL && (*list)->wr_callback == NULL) {
clear_weakref(*list);
if (*list != NULL && (*list)->wr_callback == NULL)
clear_weakref(*list);
}
if (*list != NULL) {
PyWeakReference *current = *list;
Py_ssize_t count = _PyWeakref_GetWeakrefCount(current);
PyObject *err_type, *err_value, *err_tb;
PyErr_Fetch(&err_type, &err_value, &err_tb);
if (count == 1) {
PyObject *callback = current->wr_callback;
current->wr_callback = NULL;
clear_weakref(current);
if (callback != NULL) {
if (Py_REFCNT((PyObject *)current) > 0) {
handle_callback(current, callback);
}
Py_DECREF(callback);
}
}
else {
PyObject *tuple;
Py_ssize_t i = 0;
tuple = PyTuple_New(count * 2);
if (tuple == NULL) {
_PyErr_ChainExceptions(err_type, err_value, err_tb);
return;
}
for (i = 0; i < count; ++i) {
PyWeakReference *next = current->wr_next;
if (Py_REFCNT((PyObject *)current) > 0) {
Py_INCREF(current);
PyTuple_SET_ITEM(tuple, i * 2, (PyObject *) current);
PyTuple_SET_ITEM(tuple, i * 2 + 1, current->wr_callback);
}
else {
Py_DECREF(current->wr_callback);
}
current->wr_callback = NULL;
clear_weakref(current);
current = next;
}
for (i = 0; i < count; ++i) {
PyObject *callback = PyTuple_GET_ITEM(tuple, i * 2 + 1);
/* The tuple may have slots left to NULL */
if (callback != NULL) {
PyObject *item = PyTuple_GET_ITEM(tuple, i * 2);
handle_callback((PyWeakReference *)item, callback);
}
}
Py_DECREF(tuple);
}
assert(!PyErr_Occurred());
PyErr_Restore(err_type, err_value, err_tb);
}
}PyObject_ClearWeakRefs 负责清理目标对象的弱引用链表, 同时调用弱引用的 callback.