Move handler execution into a separate class method

[LeafyLappa]

The change allows consumers of the library to modify exactly how handler resource methods are called. My case is dependency injection (with lagom):

class ResourceWithDependencyInjection(Resource):
    dependency_container: Container

    @classmethod
    async def execute_handler(cls, request, request_context, handler_name, *args, **kwargs):
        resource = cls(request, request_context, *args, **kwargs)
        handler = getattr(resource, handler_name, None)
        handler_with_deps = cls.dependency_container.partial(handler)
        response = await handler_with_deps(*args, **kwargs)
        return response

    @classmethod
    def register_routes(cls, app, base_path: str = '', container: Container = Container()):
        cls.dependency_container = container
        super().register_routes(app, base_path)

Regardless, it's a nice refactor in the spirit of SRP.

[vladmunteanu]

This is a nice proposal @LeafyLappa, thanks for the PR!
I'm going to merge this, but I would really appreciate if you had some time to write a brief section in the documentation about dependency injection and how you use it.
Although I'm personally not a heavy user of dependency injection, I can definitely understand why people accustomed with FastAPI or other frameworks would be happy to have this functionality here.

[LeafyLappa]

This PR does not add dependency injection per se, it only allowed me to write a simple (likely not at all scalable, extensible or even thread safe) integration for lagom and a rather quirky one at how it does its thing... storing the dependency container inside a class field, freely accessible from inside the handler. I could publish it, I feel its place would be at https://lagom-di.readthedocs.io/en/latest/framework_integrations though, and then it could be mentioned in the documentation of this project!

[vladmunteanu]

@LeafyLappa

This PR does not add dependency injection per se

Yes, that was clear.

Well, no rush with this, but I think it would be cool to see an example nonetheless.

[vladmunteanu]

@LeafyLappa The test suite found some extra white spaces in there apparently.

[LeafyLappa]

Please run it again, the last commit should fix the issue.