《扩展和嵌入Python解释器（译稿）》1.3 Back to the Example

1.3 Back to the Example 示例回顾

Going back to our example function, you should now be able to understand this statement:

回到我们例子中的函数，你现在应该能理解这条语句：

if (!PyArg_ParseTuple(args, "s", &command))

    return NULL;

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’).

当参数列表中检测到错误则函数返回一个NULL 指针（函数返回对象指针时发生错误的标识符），这依赖于由 PyArg_ParseTuple() 设置的异常。此外，参数的字符串的值拷贝给局部变量command。这是一个指针赋值，并且你不应当修改它所指向的字符串（所以在标准C 中，command 变量应当正确地声明为‘const char *command’）

The next statement is a call to the UNIX function system(), passing it the string we just got from PyArg_-ParseTuple():

下一条语句是对UNIX 函数 system() 的一个调用，将我们从PyArg_-ParseTuple()得到的字符串传入：

sts = system(command);

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 ofPyArg_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.

我们的spam.system()函数必须以Python对象的形式返回sts的值。这由Py_BuildValue()函数实现，它有点像PyArg_ParseTuple()的逆操作：接收一个格式转换码和任意一个 C 变量值，返回一个新的Python 对象。有关Py_BuildValue() 更多的信息会在后面给出。

return Py_BuildValue("i", sts);

In this case, it will return an integer object. (Yes, even integers are objects on the heap in Python!)

在这，它将返回一个整型数。（是的，甚至整型数在Python堆栈上也是对象！）

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):

如果有一个返回无效参数的函数（一个返回void的函数），则对应的Python函数必须返回None。你也需要这个风格（由Py_RETURN_NONE macro 实现）

Py_INCREF(Py_None);

return Py_None;

Py_Noneis 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Python特殊对象None 在C 中的名字。它是一个真正的Python对象，而不是一个NULL 指针，正如我们看到了的，在大多数上下文中 NULL 意味着错误。