我们可以看到这两标识符被使用在操作码 SETUP_WITH 中：
| /** From: Python/ceval.c **/ TARGET(SETUP_WITH) { _Py_IDENTIFIER(__exit__); _Py_IDENTIFIER(__enter__); PyObject *mgr = TOP(); PyObject *exit = special_lookup(mgr, &PyId___exit__), *enter; PyObject *res; |
现在，看一眼宏 _Py_IDENTIFIER 定义
| /** From: Include/object.h **/ /********************* String Literals ****************************************/ /* This structure helps managing static strings. The basic usage goes like this: Instead of doing r = PyObject_CallMethod(o, "foo", "args", ...); do _Py_IDENTIFIER(foo); ... r = _PyObject_CallMethodId(o, &PyId_foo, "args", ...); PyId_foo is a static variable, either on block level or file level. On first usage, the string "foo" is interned, and the structures are linked. On interpreter shutdown, all strings are released (through _PyUnicode_ClearStaticStrings). Alternatively, _Py_static_string allows to choose the variable name. _PyUnicode_FromId returns a borrowed reference to the interned string. _PyObject_{Get,Set,Has}AttrId are __getattr__ versions using _Py_Identifier*. */ typedef struct _Py_Identifier { struct _Py_Identifier *next; const char* string; PyObject *object; } _Py_Identifier; #define _Py_static_string_init(value) { 0, value, 0 } #define _Py_static_string(varname, value) static _Py_Identifier varname = _Py_static_string_init(value) #define _Py_IDENTIFIER(varname) _Py_static_string(PyId_##varname, #varname) |
嗯，注释部分已经说明得很清楚了。通过一番查找，我们发现了可以用来从字典找固定字符串的函数 _PyDict_GetItemId，所以我们操作码的查找部分的代码就是长这样滴。
| /** Our callback function will be named op_target **/ PyObject *target = NULL; _Py_IDENTIFIER(op_target); target = _PyDict_GetItemId(f->f_globals, &PyId_op_target); if (target == NULL && _PyErr_OCCURRED()) { if (!PyErr_ExceptionMatches(PyExc_KeyError)) goto error; PyErr_Clear(); DISPATCH(); } |
为了方便理解，对这一段代码做一些说明：
f 是当前的帧，f->f_globals 是它的全局区域 如果我们没有找到 op_target，我们将会检查这个异常是不是 KeyError goto error; 是一种在 main loop 中抛出异常的方法 PyErr_Clear() 抑制了当前异常的抛出，而 DISPATCH() 触发了下一个操作码的执行下一步就是收集我们想要的堆栈信息。
| /** This code create a list with all the values on the current stack **/ PyObject *value = PyList_New(0); for (i = 1 ; i <= STACK_LEVEL(); i++) { tmp = PEEK(i); if (tmp == NULL) { tmp = Py_None; } PyList_Append(value, tmp); } |
最后一步就是调用我们的回调函数！我们用 call_function 来搞定这件事，我们通过研究操作码 CALL_FUNCTION 的实现来学习怎么使用 call_function 。
