Wednesday, 18 November 2009

CapPython revisited

[I've had a draft of this post floating around since June, and it's about time I posted it.]

Guido van Rossum's criticisms of CapPython from earlier in the year are largely right, though I would put the emphasis differently. A lot of existing Python code will not pass CapPython's static verifier. But I was not expecting large amounts of code to pass the verifier.

CapPython was an experiment to see if Python could be tamed using only a static verifier - similar to Joe-E, a subset of Java - without resorting to automated rewriting of code. I had a hunch that it was possible. I also wanted a concrete system to show why I preferred to avoid some of the Python constructs that CapPython prohibits. Some of Joe-E's restrictions are also quite severe, such as prohibiting try...finally in order to prevent non-deterministic execution (something, incidentally, that CapPython does not try to do). It turned out that it was better to abandon the goal of being static-only when it came to loading Python modules.

My goal was that CapPython code should look like fairly idiomatic Python code (and be able to run directly under Python), not that all or most idiomatic Python code would be valid CapPython code. As a consequence, I didn't want CapPython to require special declarations, such as decorators on classes, for making objects encapsulated. Instead I wanted objects to be encapsulated by default. Maybe that was an arbitrary goal.

CapPython managed to meet that goal, but only partially, and only by depending on Python 2.x's unbound methods feature, which was removed in Python 3.0.

The reason my goal was met only partially is that in some circumstances it is necessary to wrap class objects (using a decorator such as @sealed in my earlier post) so that the class's constructor is exposed but inheritance is blocked. Whether this is necessary depends on whether the class object is authorityless. A class object is authorityless if it does not reference any authority-carrying objects (directly, or indirectly through objects captured by method function closures). If a class object is authorityless, it should be safe to grant other, untrusted modules the ability to create derived classes. Otherwise, the class object ought to be wrapped. The problem is that it is not trivial to make this determination. Furthermore, permitting inheritance is the default, and this is not a safe default.

If we have to go around putting @sealed decorators on all class definitions because the non-@sealed default is not-quite-encapsulated, we may as well consider alternatives where the default is not-at-all-encapsulated.

The main alternative is to resurrect CPython's restricted mode and add a wrapper type for making encapsulated objects (i.e. a non-broken version of Bastion along the lines of my earlier post).

If modifying CPython is considered acceptable, and one's goal is to add object-capabilities to Python in the most practical way (rather than to investigate a specific approach), resurrecting restricted mode is probably a better way to do it than CapPython.