allow backend variables starting with _

[Lendemor]

Description

Allow the use of variables starting with _ within a State.
Those variable will be considered as backend variables, and will never be sent to the frontend.
You can read/write a backend variable from within an event handler.

Backend variables can also be used within a computed var to generate a value that will be sent to the frontend.

(This is a non-breaking change because pydantic already ignored any variable starting with _)

Closes #307

Note: I added test for the utils function is_backend_variable() but I'm not too sure how to test the backend variable, I'm open to suggestions if I need to add them.

Checklist

All Submissions:

• Have you followed the guidelines stated in CONTRIBUTING.md file?
• Have you checked to ensure there aren't any other open Pull Requests for the desired changed?

Type of change

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

New Feature Submission:

• Does your submission pass the tests?
• Have you linted your code locally prior to submission?

Changes To Core Features:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your core changes, as applicable?
• Have you successfully ran tests with your changes locally?

[picklelo]

Sweet - just one question otherwise looks good :)

[picklelo]

Don't we need to add this as a Pydantic field to the pc.State class?

class State(Base, ABC):
    """The state of the app."""
    ...
    # Backend vars that are never sent to the client
    backend_vars: ClassVar[Dict[str, Var]] = {}

[Lendemor]

I didn't notice any problem without the declaration, but I agree we should add it.

However, Dict[str, Var] => Dict[str, Any] (because backend variables are never transformed into Var, they are simply pure Python variables.
I'm not sure what the ClassVar does, but if it's needed I can add it.

[picklelo]

ClassVar just makes it so it's not tied to the instance but the class instead. So it won't appear in the .dict and .json methods I believe.

[picklelo]

ClassVar just makes it so it's not tied to the instance but the class instead. So it won't appear in the .dict and .json methods I believe.

[picklelo]

Nit: don't need the type in the docstring since we use type annotations