helloworld.py

'''
Toga uses a  style algorithm called Pack, which is a bit like CSS lite. https://toga.beeware.org/en/stable/reference/style/pack/
'''
import toga

def button_handler(widget): # handler is just a function that takes an activated widget as a first argument, and possibly other arguments      
    print("hello")

def build(app): # tells Toga what content to display in app window 
    box = toga.Box() # holds one or more widgets and for margins around widgets

    button = toga.Button("Hello world", on_press=button_handler) # button definition
    button.style.margin = 50 # https://toga.beeware.org/en/stable/reference/style/pack/#margin
    button.style.flex = 1 # https://toga.beeware.org/en/stable/reference/style/pack/#flex
    box.add(button) # add button to box

    return box # return the outer box that holds all the UI content. This box will be the content of the app's main window

def main():
    return toga.App("First App", "org.beeware.toga.examples.tutorial", startup=build) # instantiate the App class, creates window with menu; identifier is used when registering any app-specific system resource. By convention, the identifier is a "reversd domain name". 

if __name__ == '__main__':
    main().main_loop() # Call to main_loop() is blocking and will not return until the main app is closed.