Python动态赋值的陷阱分析

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

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 'a' 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 'a' 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);
}