Pythonの動的代入のトラップ解析

def test():
    globals()['a2'] = 4
test()
print a2   # 输出 4

def aaaa():
    print locals()
    for i in ['a', 'b', 'c']:
        locals()[i] = 1
    print locals()
    print a
aaaa()

{}
{'i': 'c', 'a': 1, 'c': 1, 'b': 1}
Traceback (most recent call last):
  File "5.py", line 17, in <module>
    aaaa()
  File "5.py", line 16, in aaaa
    print a
NameError: global name &#39;a&#39; is not defined

def aaaa():
    print locals()
    s = 'test'                    # 加入显示赋值 s       
    for i in ['a', 'b', 'c']:
        locals()[i] = 1
    print locals()
    print s                       # 打印局部变量 s 
    print a
aaaa()

{}
{'i': 'c', 'a': 1, 's': 'test', 'b': 1, 'c': 1}
test
Traceback (most recent call last):
  File "5.py", line 19, in <module>
    aaaa()
  File "5.py", line 18, in aaaa
    print a
NameError: global name &#39;a&#39; is not defined

13           0 LOAD_GLOBAL              0 (locals)
              3 CALL_FUNCTION            0
              6 PRINT_ITEM
              7 PRINT_NEWLINE
 14           8 LOAD_CONST               1 ('test')
             11 STORE_FAST               0 (s)
 15          14 SETUP_LOOP              36 (to 53)
             17 LOAD_CONST               2 ('a')
             20 LOAD_CONST               3 ('b')
             23 LOAD_CONST               4 ('c')
             26 BUILD_LIST               3
             29 GET_ITER
        >>   30 FOR_ITER                19 (to 52)
             33 STORE_FAST               1 (i)
 16          36 LOAD_CONST               5 (1)
             39 LOAD_GLOBAL              0 (locals)
             42 CALL_FUNCTION            0
             45 LOAD_FAST                1 (i)
             48 STORE_SUBSCR
             49 JUMP_ABSOLUTE           30
        >>   52 POP_BLOCK
 17     >>   53 LOAD_GLOBAL              0 (locals)
             56 CALL_FUNCTION            0
             59 PRINT_ITEM
             60 PRINT_NEWLINE
 18          61 LOAD_FAST                0 (s)
             64 PRINT_ITEM
             65 PRINT_NEWLINE
 19          66 LOAD_GLOBAL              1 (a)
             69 PRINT_ITEM
             70 PRINT_NEWLINE
             71 LOAD_CONST               0 (None)
             74 RETURN_VALUE
None

// ceval.c  从上往下, 依次是相应函数或者变量的定义
// 指令源码
TARGET(STORE_FAST)
{
    v = POP();
    SETLOCAL(oparg, v);
    FAST_DISPATCH();
}
--------------------
// SETLOCAL 宏定义      
#define SETLOCAL(i, value)      do { PyObject *tmp = GETLOCAL(i); \
                                     GETLOCAL(i) = value; \
                                     Py_XDECREF(tmp); } while (0)
-------------------- 
// GETLOCAL 宏定义                                    
#define GETLOCAL(i)     (fastlocals[i])     
-------------------- 
// fastlocals 真面目
PyObject * PyEval_EvalFrameEx(PyFrameObject *f, int throwflag){
    // 省略其他无关代码
   fastlocals = f->f_localsplus;
....
}

// bltinmodule.c
static PyMethodDef builtin_methods[] = {
    ...
    // 找到 locals 函数对应的内置函数是 builtin_locals 
    {"locals",          (PyCFunction)builtin_locals,     METH_NOARGS, locals_doc},
    ...
}
-----------------------------
// builtin_locals 的定义
static PyObject *
builtin_locals(PyObject *self)
{
    PyObject *d;
    d = PyEval_GetLocals();
    Py_XINCREF(d);
    return d;
}
-----------------------------
PyObject *
PyEval_GetLocals(void)
{
    PyFrameObject *current_frame = PyEval_GetFrame();  // 获取当前堆栈对象
    if (current_frame == NULL)
        return NULL;
    PyFrame_FastToLocals(current_frame); // 初始化和填充 f_locals
    return current_frame->f_locals;
}
-----------------------------
// 初始化和填充 f_locals 的具体实现
void
PyFrame_FastToLocals(PyFrameObject *f)
{
    /* Merge fast locals into f->f_locals */
    PyObject *locals, *map;
    PyObject **fast;
    PyObject *error_type, *error_value, *error_traceback;
    PyCodeObject *co;
    Py_ssize_t j;
    int ncells, nfreevars;
    if (f == NULL)
        return;
    locals = f->f_locals;
    // 如果locals为空, 就新建一个字典对象
    if (locals == NULL) {
        locals = f->f_locals = PyDict_New();  
        if (locals == NULL) {
            PyErr_Clear(); /* Can't report it :-( */
            return;
        }
    }
    co = f->f_code;
    map = co->co_varnames;
    if (!PyTuple_Check(map))
        return;
    PyErr_Fetch(&error_type, &error_value, &error_traceback);
    fast = f->f_localsplus;
    j = PyTuple_GET_SIZE(map);
    if (j > co->co_nlocals)
        j = co->co_nlocals;
    // 将 f_localsplus 写入 locals
    if (co->co_nlocals)
        map_to_dict(map, j, locals, fast, 0);
    ncells = PyTuple_GET_SIZE(co->co_cellvars);
    nfreevars = PyTuple_GET_SIZE(co->co_freevars);
    if (ncells || nfreevars) {
        // 将 co_cellvars 写入 locals
        map_to_dict(co->co_cellvars, ncells,
                    locals, fast + co->co_nlocals, 1);
        if (co->co_flags & CO_OPTIMIZED) {
            // 将 co_freevars 写入 locals
            map_to_dict(co->co_freevars, nfreevars,
                        locals, fast + co->co_nlocals + ncells, 1);
        }
    }
    PyErr_Restore(error_type, error_value, error_traceback);
}