迭代器协议
**********

2.2 新版功能.

迭代器有两个函数。

int PyIter_Check(PyObject *o)

   返回 true ， 如果对象 *o* 支持迭代器协议的话。

   This function can return a false positive in the case of old-style
   classes because those classes always define a "tp_iternext" slot
   with logic that either invokes a "next()" method or raises a
   "TypeError".

PyObject* PyIter_Next(PyObject *o)
    *Return value: New reference.*

   Return the next value from the iteration *o*.  The object must be
   an iterator (it is up to the caller to check this).  If there are
   no remaining values, returns *NULL* with no exception set.  If an
   error occurs while retrieving the item, returns *NULL* and passes
   along the exception.

要为迭代器编写一个一个循环，C代码应该看起来像这样

   PyObject *iterator = PyObject_GetIter(obj);
   PyObject *item;

   if (iterator == NULL) {
       /* propagate error */
   }

   while ((item = PyIter_Next(iterator))) {
       /* do something with item */
       ...
       /* release reference when done */
       Py_DECREF(item);
   }

   Py_DECREF(iterator);

   if (PyErr_Occurred()) {
       /* propagate error */
   }
   else {
       /* continue doing useful work */
   }
