2017-07-02 14:13 GMT+02:00 Steven D'Aprano <steve@pearwood.info>:
That only solves the problem of mysum being modified, not whether the arguments are ints. You still need to know whether it is safe to call some low-level (fast) integer addition routine, or whether you have to go through the (slow) high-level Python code.
In any case, guards are a kind of runtime check. It might not be an explicit isinstance() check, but it is logically implies one. If x was an int, and nothing has changed, then x is still an int.
If Victor is around, he might like to comment on how his FAT Python handles this.
FAT Python design is generic, you are free to implement any kind of check. A check is an object which provides a C callback. FAT Python provides the following guards: GuardArgType GuardBuiltins GuardDict GuardFunc GuardGlobals About "mysym being modified", it's handled by this guard: http://fatoptimizer.readthedocs.io/en/latest/fat.html#GuardFunc Right now, only the func.__code__ is watched. It's not enought, but it's a compromise to keep my implementation simple :-) Tomorrow, if FAT Python becomes a real thing, the builtin function type can be modified to add a version as we have for dictionaries, and the version will be increased for any function modification: argument defaults, arguments, name, etc. We would only have to modify GuardFunc implementation, not users of this guard. To really respect the Python semantics, guards became more complex than expected. GuardBuiltins doesn't only check that len() is still the same function in builtins. It only has to the globals name of the current frame, globals()[name] and builtins of the current frame. Python allows crazy stuff like running a single function with custom builtin functions: see exec() function. Victor