Is it possible to write to a python frame object as returned by sys._getframe() from python code running within the interpreter?

Question

Apropos of This question, there is a bit of scaffolding within the interpreter to inspect frame objects, which can be retrieved by sys._getframe(). The frame objects appear to be read only, but I can't find anything obvious in the docs that explicitly states this. Can someone confirm whether these objects are writeable (in some way) or read only?

import sys

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

if __name__ == '__main__':
    foobar()

This prints out 'foo' when run but the post below demonstrates the variable being writable when run from the current frame in an interactive shell.

Answer 1

From CPython source, 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}
};

For the PyMemberDef, the flags RO or READONLY means it's attributes are read-only. For the PyGetSetDef, if it only has a getter, it's read only. This means all attributes but f_exc_type, f_exc_value, f_exc_traceback and f_trace are read-only after creation. This is also mentioned in the docs, under Data model.

The objects referred to by the attributes is not necessarily read-only. You could do this:

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

Though this works in the interpreter, it fails inside functions. The execution engine uses a separate array for local variables (f_fastlocals), which is merged into f_locals on access, but the converse is not true.

>>> 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
>>>

On the global frame, f_local refers to f_globals, which makes this trick work in the interpreter. Modifying f_globals works, but affects the whole module.

Answer 2

The f_locals['foo'] example by NXC works because the code is in module scope. In that case, f_locals is f_globals, and f_globals is both modifiable and modifications are reflected in the module.

Inside of function scope, locals() and f_locals are writable, but "[changes may not affect the values of local variables used by the interpreter]." 1 It's an implementation choice. In CPython there's a optimized bytecode for local variables, LOAD_FAST. In Python, local variables are (almost always) known once the function is defined, and CPython uses an index lookup to get the variable value, rather than a dictionary lookup.

In theory the dictionary lookup could proxy that table, but that's a lot of work for little gain.

The exceptions to "local variables are known" are if the function uses an exec statement, and the deprecated case of "from module import *". The generated byte code is different, and slower, for these cases.