Design Crisis - Global State

[0Hughman0]

The use of a global state is very useful to Cassini.

env.project and env.o are needed for the jl_gui and magics to work.

It is also very useful for extensions to do things.

However! It does create a lot of problems.

It's quite a pain that, for example, essentially, Tier objects are invalid unless a Project instance has been created, but we rely on constantly checking this within Tier methods, which has a major code smell.

Furthermore, this makes testing a challenge, because we have to initialise a new project every time we want to test methods of a Tier instance.

[0Hughman0]

def requires_project(wrapped):
    @functools.wraps(wrapped)
    def wrapper(*args, **kwargs):        
        if not env.project:
            raise RuntimeError()
        return wrapped(*args, **kwargs, project=env.project)

class C:

    @requires_project
    def my_method(self, arg1, arg2, *, project: Project):
        return project

[0Hughman0]

Maybe a custom exception e.g.

class CassiniEnvError(Exception):
     pass

[0Hughman0]

I think we should try and make it possible to initialise all objects without globals like env.project or env.shareable_project existing?

[0Hughman0]

Thing is initialising a Project object is not expensive.

setting up the files for it however is expensive...

[0Hughman0]

I could just pass the Project as an argument in the __init__ and keep a reference to that project as an attribute?

The object needs a project to work, so in some sense it makes sense.

Then we just make Project implicitly pass itself through get_tier.

[0Hughman0]

The project could also explicitly be passed in the constructor? Rather than sneaking it via the back door.

Only thing then is instances then look after their own reference to the project.

If the project is subsequently changed (... which it shouldn't be!) then that reference is stale and harder to keep track of.

[0Hughman0]

Ok, I think here's what we do.

This global state is used in the context of what is the current notebook. Where we don't need to know the current notebook, we don't need it. Tiers don't really need to know the current notebook. So I think passing in from project is the right thing to do.

[0Hughman0]

Ok, there is a problem, which is that the class attributes do not have an instance of project they can access...

I need a way for the class to accept its project...

[0Hughman0]

To look after caches we can do this:

class _Env:

    def __init__(...):
        self.caches = []

    def create_cache(self):
        cache = dict()
        self.caches.append(cache)
        return cache

Should make life a lot easier to keep track of all these caches.

[0Hughman0]

#141 makes this change... I like it!