If the problem is accidental use of the result of PyFloat_AS_DOUBLE() as an lvalue, why not use the comma operator to ensure that the result is an rvalue?

The C99 standard says "A comma operator does not yield an lvalue" in §6.5.17; I imagine there is similar text in other versions of the standard.

The idea would be to define a helper macro like this:

    /* As expr, but can only be used as an rvalue. */
    #define Py_RVALUE(expr) ((void)0, (expr))

and then use the helper where needed, for example:

    #define PyFloat_AS_DOUBLE(op) Py_RVALUE(((PyFloatObject *)(op))->ob_fval)