是否可以从解释器中运行的python代码写入sys._getframe()返回的python框架对象？

Question

这个问题的答案,在解释器中有一些脚手架来检查框架对象,可以通过它来检索sys._getframe().框架对象看起来是只读的,但我在文档中找不到任何明显说明这一点的东西.有人可以确认这些对象是可写的(以某种方式)还是只读的？

import sys

def foobar():
    xx='foo'
    ff = sys._getframe()
    ff.f_locals['xx'] = 'bar'
    print xx

if __name__ == '__main__':
    foobar()

这foo会在运行时输出' ',但下面的帖子演示了从交互式shell中的当前帧运行时变量是否可写.

Answer 1

从CPython源代码Objects/frameobject.c:

static PyMemberDef frame_memberlist[] = {
    {"f_back",      T_OBJECT,       OFF(f_back),    RO},
    {"f_code",      T_OBJECT,       OFF(f_code),    RO},
    {"f_builtins",  T_OBJECT,       OFF(f_builtins),RO},
    {"f_globals",   T_OBJECT,       OFF(f_globals), RO},
    {"f_lasti",     T_INT,          OFF(f_lasti),   RO},
    {"f_exc_type",  T_OBJECT,       OFF(f_exc_type)},
    {"f_exc_value", T_OBJECT,       OFF(f_exc_value)},
    {"f_exc_traceback", T_OBJECT,   OFF(f_exc_traceback)},
    {NULL}    /* Sentinel */
};
...
static PyGetSetDef frame_getsetlist[] = {
    {"f_locals",    (getter)frame_getlocals, NULL, NULL},
    {"f_lineno",    (getter)frame_getlineno,
                    (setter)frame_setlineno, NULL},
    {"f_trace",     (getter)frame_gettrace, (setter)frame_settrace, NULL},
    {"f_restricted",(getter)frame_getrestricted,NULL, NULL},
    {0}
};

对于PyMemberDef,标志RO或READONLY表示它的属性是只读的.对于PyGetSetDef,如果它只有一个吸气剂,它是只读的.这就意味着所有的属性,但是f_exc_type,f_exc_value,f_exc_traceback和f_trace创建后是只读的.在数据模型下的文档中也提到了这一点.

属性引用的对象不一定是只读的.你可以这样做:

>>> f = sys._getframe()
>>> f.f_locals['foo'] = 3
>>> foo
3
>>>

虽然这在解释器中起作用,但它在内部函数中失败了.执行引擎为局部变量(f_fastlocals)使用一个单独的数组,它f_locals在访问时合并,但反之则不然.

>>> def foo():
...   x = 3
...   f = sys._getframe()
...   print f.f_locals['x']
...   x = 4
...   print f.f_locals['x']
...   d = f.f_locals
...   x = 5
...   print d['x']
...   f.f_locals
...   print d['x']
...
>>> foo()
3
4
4
5
>>>

在全局框架中,f_local指的是f_globals,这使得这个技巧在解释器中起作用.修改f_globals工作,但影响整个模块.