The author says:
That’s exactly the thing that makes C/C++ so different from most other languages. It distinguishes between reference and value calls. Python does not know about these things. People have tried to resolve this problem by trying to guess which calling method is correct in a place. But as in the Zen of Python is already stated 'Explicit is better than implicit.' So we don’t want the system to guess as things can go terribly wrong. Therefore, so called 'calling policies' (mentioned on slide 32 in the flash animation here) have to be used to disambiguate in a way that the programmer wants.