I just realized that doing
Is it possible to inline such a function, as I would do in C using macro or using inline keyword?
No. Before reaching this specific instruction, Python interpreters don't even know if there's such a function, much less what it does.
As noted in comments, PyPy will inline automatically (the above still holds - it "simply" generates an optimized version at runtime, benefits from it, but breaks out of it when it's invalidated), although in this specific case that doesn't help as implementing NumPy on PyPy started only shortly ago and isn't even beta level to this day. But the bottom line is: Don't worry about optimizations on this level in Python. Either the implementations optimize it themselves or they don't, it's not your responsibility.