C extensions written for Python 3.10 and older which access directly to the PyFrameObject.f_back member build successfully on Python 3.11, but they can fail because f_back must not be read directly. f_back can be NULL even if the frame has an outer frame.

PyFrameObject*
PyFrame_GetBack(PyFrameObject *frame)
{
    assert(frame != NULL);
    PyFrameObject *back = frame->f_back;
    if (back == NULL && frame->f_frame->previous != NULL) {
        back = _PyFrame_GetFrameObject(frame->f_frame->previous);
    }
    Py_XINCREF(back);
    return back;
}

I suggest to remove or "hide" this member from the structure. For example, rename "f_back" to "_f_back" to advice developers to not access it directly.

See also bpo-46355: Document PyFrameObject and PyThreadState changes.

It's unclear to me if the ability to read directly the PyFrameObject.f_lineno member is a deliberate use case, or if we should enforce the usage of PyFrame_GetLineNumber().

This must be normal

f_lineno is not part of the API. No field in PyFrameObject is.

I don't know how we can stop people from using them though.

If they don't know better than pulling data out of undocumented internal struct, then I'm not sure an underscore prefix is going to help.
It won't do any harm, though.

I don't know how we can stop people from using them though.

The first option is to promote helper functions to abstract access to the PyFrameObject structure.

The second option is to make the whole structure opaque and enforce the usage of helper functions. Before being able to do that, we need to prepare Cython and a few more popular C extensions for that. Right now, Cython still has a lot of code accessing directly PyFrameObject members.

Well, we can discuss that in bpo-40421 ;-)

If they don't know better than pulling data out of undocumented internal struct, then I'm not sure an underscore prefix is going to help. It won't do any harm, though.

I propose to rename f_back to trigger a build error for existing C extensions which access it directly. So developers have to decide how to handle the change: access the renamed member, or read the doc and use a better way, call PyFrame_GetBack() :-)

I dislike that this incompatible change is "silent" and that developers may only notice the change in production, when it's too late.

Or maybe most C extensions have a a good test suite and will notice the change before it's too late.

I created bpo-46836: "[C API] Move PyFrameObject to the internal C API".

This issue has been fixed by bpo-46836:

New changeset 18b5dd6 by Victor Stinner in branch 'main':
bpo-46836: Move PyFrameObject to pycore_frame.h (GH-31530)