I had more time to think about this issue.

First, I decided to always include "pythoncapi_compat.h" and so there is no strict requirement to add new C API functions to Python.

Second, the _Py_Borrow() function looked nice, but there is a practical issue: its return type is always PyObject* which makes it not convenient to "convert" a function returning a different type.

For example, I replaced "frame->f_code" with "((PyCodeObject *)_Py_Borrow(PyFrame_GetCode(frame)))", but this expression requires 3 levels of parenthesis, it's long and it's hard to read :-( The outter parenthesis is needed if the modified expresion is used to get a code member, like: "frame->f_code->co_name".

I chose another approach: add a "Borrow" variant of the functions that I need.

For example, add _PyFrameGetCodeBorrow() variant of PyFrameGetCode(). This function is only added to pythoncapi_compat.h, and so Python doesn't have to be modified.

I reject my issue :-)