PEP 795: Add deep immutability to Python

[mjp41 (Matthew Parkinson)]

@steve.dower

Was there any discussion of an immutable proxy alternative? I see it’s not mentioned in the rejected section.

We will add a section about this. We couldn’t see how to make this work in a safe way with sub-interpreters.

@ZeroIntensity

Yeah, I think this is a much more robust approach, because the wrapped object doesn’t even have to be immutable, or follow strict Py_CHECKWRITE contracts. Immutability is nice sometimes, but you still generally want some shared data between threads. Several months ago, I threw together a POC for this exact idea with subinterpreters via an immortal object proxy. There are definitely some issues with it, but I think this PEP doing something similar would play better with the ecosystem (particularly because you wouldn’t need to modify every C type out there).

We have been looking at your example on discussion on Gauging interest in arbitrary object immortalization and shared object proxies.

I am bit unsure about two things

• Are calls to SharedObjectProxys asynchronous, or synchronous? If synchronous is there a possiblity of deadlock?
• Are the parameters passed to a method on a SharedObjectProxy also proxied? If not, can that lead to sharing?

@ZeroIntensity should I post these questions on the other discussion (not sure on the etiquette of overlapping conversions on DPO)?