이터레이터 프로토콜
*******************

특히 이터레이터를 사용하기 위한 두 함수가 있습니다.

int PyIter_Check(PyObject *o)
    * Part of the Stable ABI since version 3.8.*

   Return non-zero if the object *o* can be safely passed to
   "PyIter_Next()", and "0" otherwise.  This function always succeeds.

int PyAIter_Check(PyObject *o)
    * Part of the Stable ABI since version 3.10.*

   Return non-zero if the object *o* provides the "AsyncIterator"
   protocol, and "0" otherwise.  This function always succeeds.

   버전 3.10에 추가.

PyObject *PyIter_Next(PyObject *o)
    *Return value: New reference.** Part of the Stable ABI.*

   Return the next value from the iterator *o*.  The object must be an
   iterator according to "PyIter_Check()" (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.

이터레이터를 이터레이트하는 루프를 작성하려면, C 코드는 이런 식으로 되
어야 합니다:

   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 */
   }

type PySendResult

   The enum value used to represent different results of
   "PyIter_Send()".

   버전 3.10에 추가.

PySendResult PyIter_Send(PyObject *iter, PyObject *arg, PyObject **presult)
    * Part of the Stable ABI since version 3.10.*

   Sends the *arg* value into the iterator *iter*. Returns:

   * "PYGEN_RETURN" if iterator returns. Return value is returned via
     *presult*.

   * "PYGEN_NEXT" if iterator yields. Yielded value is returned via
     *presult*.

   * "PYGEN_ERROR" if iterator has raised and exception. *presult* is
     set to "NULL".

   버전 3.10에 추가.
