Object proxies

[Korijn]

Closes #103

🚧🚧🚧 Work in progress

I'm just starting to get to grips with the complexity of this topic. It's been a long time since I looked at object proxying.

The boilerplate is done.

The main complexity will be in object_proxy.py. In particular the challenge is that there's no watertight method of trapping attribute access only, without also trapping method calls. So for example

class Foo:
    def __init__(self):
        self.bar = 5

    def baz(self):
        return 5

    def quux(self):
        return self.bar

f = Foo()

• It's obvious that f.bar should be reactive.
• However I think f.quux should not be reactive (since f.bar is already reactive).
• f.baz is not stateful, so it doesn't need to be reactive.

The list of attributes to trap is dynamic, like the keys of a dict, but we cannot replace the internal f.__dict__. All we have to work with is a trap on f.__getattribute__ but that also fires for the f.bar and f.quux.

Another challenge is that on objects, you can define something like __iter__ to make a type iterable, while by default classes don't have an __iter__ implementation.

🤔🤔🤔💭💭💭 Needs a lot more thought...

[Korijn]

I've spent another two hours on this and am abandoning this feature (again). It just isn't possible in a generic sense. Good luck to the next guy. Problems in a nutshell:

• proxying all attribute gets and sets generically is very complicated
  • proxying __getattribute__ and __setattr__ gets you a long way, however instance methods, static methods, class methods, instance attributes and class attributes all flow through these functions so you need a mechanism to distinguish state from logic and I couldn't find one that's foolproof
  • if you proxy __getattribute__ on Proxy, that means Proxy.__init__ will also trigger the traps but the traps rely on Proxy.__init__ to have already completed
  • magic methods must be proxied since they bypass __getattribute__
  • most magic methods (e.g. __len__) are optional on classes
• unlike dict, list and set, there is no generic .copy() method you can rely on so detecting if something has actually changed is difficult
• how do you traverse a class? which attributes should you traverse? how do you exclude the attributes defined on Proxy (such as target)?

... and a lot more problems

The next best alternative that does work in a generic sense is to declare a reactive dict on your class and work with that.