The Python C API provides "AS" functions to convert an object to another type, like PyFloat_AS_DOUBLE(). These macros can be abused to be used as l-value: "PyFloat_AS_DOUBLE(obj) = new_value;". It prevents to change the PyFloat implementation and makes life harder for Python implementations other than CPython.
I propose to convert these macros to static inline functions to disallow using them as l-value.
I made a similar change for Py_REFCNT(), Py_TYPE() and Py_SIZE(). For these functions, I added "SET" variants: Py_SET_REFCNT(), Py_SET_TYPE(), Py_SET_SIZE(). Here, I don't think that the l-value case is legit, and so I don't see the need to add a way to *set* a value.
For example, I don't think that PyFloat_SET_DOUBLE(obj, value) would make sense. A Python float object is supposed to be immutable.