I'm working on a script to migrate old C extension modules to the latest flavor of the Python C API:
https://github.com/pythoncapi/upgrade_pythoncapi

I would like to replace frame->f_code with PyFrame_GetCode(frame). The problem is that frame->f_code is a borrowed reference, whereas PyFrame_GetCode() returns a strong reference. Having a Py_Borrow() function would help to automate migration to PyFrame_GetCode():

static inline PyObject* Py_Borrow(PyObject *obj)
{
    Py_XDECREF(obj);
    return obj;
}

So frame->f_code can be replaced with Py_Borrow(PyFrame_GetCode(frame)).

Py_Borrow() is similar to Py_XDECREF() but can be used as an expression:

    PyObject *code = Py_Borrow(PyFrame_GetCode(frame));

Py_Borrow() is the opposite of Py_XNewRef(). For example, Py_Borrow(Py_XNewRef(obj)) leaves the reference count unchanged (+1 and then -1).


My pratical problem is that it's not easy not add Py_XDECREF() call when converting C code to the new C API.

Example 1:

    PyObject *code = frame->f_code;

This one is easy and can be written as:

    PyObject *code = PyFrame_GetCode(frame); Py_XDECREF(code);


Example 2:

    func(frame->f_code);

This one is more tricky. For example, the following code using a macro is wrong:

    #define Py_BORROW(obj) (Py_XDECREF(obj), obj)
    func(Py_BORROW(PyFrame_GetCode(frame->f_code)));

since it calls PyFrame_GetCode() twice when proceed by the C preprocessor and so leaks a reference:

    func(Py_XDECREF(PyFrame_GetCode(frame->f_code)), PyFrame_GetCode(frame->f_code));


Attached PR implements Py_Borrow() function as a static inline function.