tree_icons.py

import toga

class StartApp(toga.App):
    def startup(self):
        # Main window
        self.main_window = toga.MainWindow(self.name)
        self.main_window.app = self

        # Tree widget from Toga
        self.tree = toga.Tree(['Navigate'])

        # Add top level nodes on the tree
        self.tree.insert('root1')
        self.tree.insert('root2')
        r3 = self.tree.insert('root3')

        # Children nodes
        sub_r3 = self.tree.insert('sub_r3', r3)
        self.tree.insert('sub_sub_r3', sub_r3)

        # Add icons in some nodes
        r3.icon = 'icon.png'
        sub_r3.icon = 'icon.png'

        # Update the tree layout
        self.tree.apply_layout()

        # Add the main widget inside of the main application
        self.main_window.content = self.tree
        # Show the application
        self.main_window.show()

def main():
    # Start the application with the title and the name of the organization
    app = StartApp('Test Tree icons', 'org.pybee.test')
    app.main_loop()

if __name__ == '__main__':
    main()