How it works...

First, we replaced the wildcard import with the import ... as syntax to have better control over our global namespace.

Then, we defined our App class as a Tk subclass, which now is referenced via the tk namespace. To properly initialize the base class, we will call the __init__ method of the Tk class with the built-in super() function. This corresponds to the following lines:

class App(tk.Tk): 
    def __init__(self): 
        super().__init__() 
        # ... 

Now, we have a reference to the App instance with the self variable, so we will add all the Button widget as an attribute of our class.

Although it may look overkill for such a simple program, this refactoring will help us to reason about each part, the button instantiation is separated from the callback that gets executed when it is clicked, and the application bootstrapping is moved to the if __name__ == "__main__" block, which is a common practice in executable Python scripts.

We will follow this convention through all the code samples, so you can take this template as the starting point of any larger application.