I don't have any example at hand, but you need...
- Include the brython.js and stdlib in your HTML
- In your python script section of the HTML, import the function you want to call from hello_world.py
- again, in your python script section of the HTML, bind the function with the 'click' event of the button.
Use the developer functions of your browser (usually, F12 key) to view any network request/error while brython is importing the hello_world.py file. For beginers, the HTML and the PY file must be in the same folder. it's possible to put the HTML and the PY files in different folders, but for now, maintain it in the same folder.
as you can see, the trick is to think in the python script of the HTML as the 'main' function of your app.