參照計數
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本节介绍的宏被用于管理 Python 对象的引用计数。

void Py_INCREF(PyObject *o)

   Increment the reference count for object *o*.  The object must not
   be *NULL*; if you aren't sure that it isn't *NULL*, use
   "Py_XINCREF()".

void Py_XINCREF(PyObject *o)

   Increment the reference count for object *o*.  The object may be
   *NULL*, in which case the macro has no effect.

void Py_DECREF(PyObject *o)

   Decrement the reference count for object *o*.  The object must not
   be *NULL*; if you aren't sure that it isn't *NULL*, use
   "Py_XDECREF()".  If the reference count reaches zero, the object's
   type's deallocation function (which must not be *NULL*) is invoked.

   警告:

     释放函数可导致任意 Python 代码被发起调用（例如当一个带有
     "__del__()" 方法的类实例被释放时就是如此）。 虽然此类代码中的异常
     不会被传播，但被执行的代码能够自由访问所有 Python 全局变量。 这意
     味着任何可通过全局变量获取的对象在  "Py_DECREF()" 被发起调用之前
     都应当处于完好状态。 例如，从一个列表中删除对象的代码应当将被删除
     对象的引用拷贝到一个临时变量中，更新列表数据结构，然后再为临时变
     量调用 "Py_DECREF()"。

void Py_XDECREF(PyObject *o)

   Decrement the reference count for object *o*.  The object may be
   *NULL*, in which case the macro has no effect; otherwise the effect
   is the same as for "Py_DECREF()", and the same warning applies.

void Py_CLEAR(PyObject *o)

   Decrement the reference count for object *o*.  The object may be
   *NULL*, in which case the macro has no effect; otherwise the effect
   is the same as for "Py_DECREF()", except that the argument is also
   set to *NULL*.  The warning for "Py_DECREF()" does not apply with
   respect to the object passed because the macro carefully uses a
   temporary variable and sets the argument to *NULL* before
   decrementing its reference count.

   当要减少在垃圾回收期间可能会被遍历的变量的值时，使用该宏是一个好主
   意。

以下函数适用于 Python 的运行时动态嵌入: "Py_IncRef(PyObject *o)",
"Py_DecRef(PyObject *o)"。 它们分别只是 "Py_XINCREF()" 和
"Py_XDECREF()" 的简单导出函数版本。

以下函数或宏仅可在解释器核心内部使用: "_Py_Dealloc()",
"_Py_ForgetReference()", "_Py_NewReference()" 以及全局变量
"_Py_RefTotal"。
