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.