Update the CustomJS data with Python function Bokeh

I'm trying to create a bokeh server (running with Python -m bokeh serve my_pythonfile.py) where I let the user modify some data that I want to use in a CustomJS function. I tried to make a simple example of my class here:

def __init__(self):
    self.data = 0
    self.click_me = self._get_click_button()
    self.save_data = self._get_save_data()

def _get_click_button(self) -> Button:
    click_me = Button(label='Add data')
    click_me.on_click(self.add_data)
    return click_me

def self.add_data(self):
    self.data += 1

def _get_save_data(self) -> Button:
    save = Button(label='Save data')
    callback = CustomJS(args={'data': self.return_data()}, code="""console.log(data);""")
    save.js_on_click(callback)
    return save

def return_data(self) -> int:
    return self.data

When I press on the Save data button I always get 0 in the console, I understand why but I don't know how to get the actual number of self.data, any ideas/tips?

The actual things that I want to do is to let the user download some data (a json file) and one solution that came to mind was use the html5 keyword download. In another case I wanted to use the alert keyword to let the user know that he have entered some bad data.

from Update the CustomJS data with Python function Bokeh