9. State

Global State
Node State

Global State

Lona defines a global state object in server.state, that is thread-safe and can be used to share application specific data between views, middlewares and all other parts of a Lona app that have access to the server reference.

server.state behaves like a Python dictionary, and implements the same locking interface as HTML nodes.

from datetime import datetime

from lona.html import HTML, H1, P
from lona import View, App

app = App(__file__)

# set initial state
app.settings.INITIAL_STATE = {
    'start_time': datetime.now(),
}


@app.route('/')
class StateViewView(View):
    def handle_request(self, request):

        # get start_time from global state
        start_time = this.server.state['start_time']

        return HTML(
            H1('Start Time'),
            P(f'This application runs since {start_time}')
        )


if __name__ == '__main__':
    app.run()

Node State

Every Lona node has its own state object that is only available on the server, and gets not synchronized with the browser. This can be used to attach sensitive data to a node, like a database id, to retrieve it later.

Node.state has the same API as the global state object, but its lock is shared with the nodes lock. That means you can change the server-side and the browser-side data in one transaction.

from datetime import datetime

from lona.html import HTML, H1, P, Button
from lona import View, App

app = App(__file__)

# this is sensitive data
database = {
    312: 'Bob':
    320: 'Alice'
    222: 'Carl',
}


@app.route('/')
class StateViewView(View):
    def delete_from_database(self, input_event):

        # retrieve state from the clicked button
        database_id = input_event.node.state['id']

        # remove key from the database
        database.remove(database_id)

    def handle_request(self, request):
        html = HTML(
            H1('Delete Database Rows'),
        )

        for key, value in database.items():
            button = Button(
                f'Delete {value}',
                handle_click=delete_from_database,

                # set state that should not be visible in the browser
                state={
                    'id': key,
                },
            )

            html.append(button)

        return html


if __name__ == '__main__':
    app.run()