參照計數
********

The macros in this section are used for managing reference counts of
Python objects.

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: The deallocation function can cause arbitrary Python code
     to be invoked (e.g. when a class instance with a "__del__()"
     method is deallocated).  While exceptions in such code are not
     propagated, the executed code has free access to all Python
     global variables. This means that any object that is reachable
     from a global variable should be in a consistent state before
     "Py_DECREF()" is invoked.  For example, code to delete an object
     from a list should copy a reference to the deleted object in a
     temporary variable, update the list data structure, and then call
     "Py_DECREF()" for the temporary variable.

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.

   It is a good idea to use this macro whenever decrementing the
   reference count of an object that might be traversed during garbage
   collection.

The following functions are for runtime dynamic embedding of Python:
"Py_IncRef(PyObject *o)", "Py_DecRef(PyObject *o)". They are simply
exported function versions of "Py_XINCREF()" and "Py_XDECREF()",
respectively.

The following functions or macros are only for use within the
interpreter core: "_Py_Dealloc()", "_Py_ForgetReference()",
"_Py_NewReference()", as well as the global variable "_Py_RefTotal".
