Going back to our example function, you should now be able to understand this statement:
It returns NULL (the error indicator for functions returning object pointers) if an error is detected in the argument list, relying on the exception set by PyArg_ParseTuple(). Otherwise the string value of the argument has been copied to the local variable command. This is a pointer assignment and you are not supposed to modify the string to which it points (so in Standard C, the variable command should properly be declared as "const char *command").
回到我們前面的例子，如果PyArg_ParseTuple()函數在執行過程中發生異常，那麼這段代碼需要返回一個NULL值表示錯誤。否則的話，表示參數的字符串會被複制到本地變量command中。command是一個指向字符串的指針常量，在這裏並不能去修改這個指針所指向的字符串。
The next statement is a call to the Unix function system(), passing it the string we just got from PyArg_ParseTuple():
Our spam.system() function must return the value of sts as a Python object. This is done using the function Py_BuildValue(), which is something like the inverse of PyArg_ParseTuple(): it takes a format string and an arbitrary number of C values, and returns a new Python object. More info on Py_BuildValue() is given later.
接下去我們要做的就是把剛纔得到的代表參數的字符串傳入函數system中。當我們在python中調用spam.system()時，我們必須把sts表示成一個python對象。這需要通過Py_BuildValue()函數來實現，這在某種程度上似乎與PyArg_ParseTuple()函數的作用相反：它使用一個format字符串和任意個C代碼中的值，把它們轉換爲一個python對象。
return Py_BuildValue("i", sts);
In this case, it will return an integer object. (Yes, even integers are objects on the heap in Python!)
If you have a C function that returns no useful argument (a function returning void), the corresponding Python function must return None. You need this idiom to do so (which is implemented by the Py_RETURN_NONE macro):
如果用C代碼編寫的函數沒有返回值，那麼相應的python函數需要返回None。需要使用下面這種用法
Py_None is the C name for the special Python object None. It is a genuine Python object rather than a NULL pointer, which means ``error'' in most contexts, as we have seen.
Py_None是C語言中用來代表python 對象None的，它不同於NULL指針，它是真正的python對象，而後者通常用來表示錯誤，就像我們見到的那樣。