python运行时环境

python通过一个PyFrameObject结构体来描述运行时环境

typedef struct _frame {
    PyObject_VAR_HEAD
    struct _frame *f_back;	/* previous frame, or NULL */
    PyCodeObject *f_code;	/* code segment */
    PyObject *f_builtins;	/* builtin symbol table (PyDictObject) */
    PyObject *f_globals;	/* global symbol table (PyDictObject) */
    PyObject *f_locals;		/* local symbol table (any mapping) */
    PyObject **f_valuestack;	/* points after the last local */
    /* Next free slot in f_valuestack.  Frame creation sets to f_valuestack.
       Frame evaluation usually NULLs it, but a frame that yields sets it
       to the current stack top. */
    PyObject **f_stacktop;
    PyObject *f_trace;		/* Trace function */

    /* If an exception is raised in this frame, the next three are used to
     * record the exception info (if any) originally in the thread state.  See
     * comments before set_exc_info() -- it\'s not obvious.
     * Invariant:  if _type is NULL, then so are _value and _traceback.
     * Desired invariant:  all three are NULL, or all three are non-NULL.  That
     * one isn\'t currently true, but "should be".
     */
    PyObject *f_exc_type, *f_exc_value, *f_exc_traceback;

    PyThreadState *f_tstate;
    int f_lasti;		/* Last instruction if called */
    /* Call PyFrame_GetLineNumber() instead of reading this field
       directly.  As of 2.3 f_lineno is only valid when tracing is
       active (i.e. when f_trace is set).  At other times we use
       PyCode_Addr2Line to calculate the line from the current
       bytecode index. */
    int f_lineno;		/* Current line number */
    int f_iblock;		/* index in f_blockstack */
    PyTryBlock f_blockstack[CO_MAXBLOCKS]; /* for try and loop blocks */
    PyObject *f_localsplus[1];	/* locals+stack, dynamically sized */
} PyFrameObject;

　　其实一眼就能看出，PyFrameObject中静态的没有什么可以分析的。主要用来存放一些静态的信息，比如说一段CodeObject对应的字节码等等。在python程序运行过程中会不断发生变化的就是最后的f_localsplus[1]这一个值了。这里用到了一个c语言的编程技巧，f_localsplus为一个PyObject的指针数组，大小为1。这样当申请一个大小超过sizeof(PyFrameObject)的结构体对象时，超过的部分就自动分配给f_localsplus。变相的而且更方便的实现动态数组。在为整个运行时环境申请空间时所采用的代码是：

ncells = PyTuple_GET_SIZE(code->co_cellvars);
nfrees = PyTuple_GET_SIZE(code->co_freevars);
extras = code->co_stacksize + code->co_nlocals + ncells +nfrees;
f = PyObject_GC_NewVar(PyFrameObject, &PyFrame_Type, extras);
        

　　于是，整个运行时环境的空间可以被描述为：静态数据的空间+ncell的空间+nfrees的空间+运行时栈的空间+局部变量的空间。在进行逻辑管理的时候主要是下图：

Cell对象和free对象是为了实现闭包

从这一个运行时空间中我们可以看出来，实际上所有的运行时所需操作的对象都是作为局部变量来操作的