How it works...

The first lines of our application constructor instantiate StringVar and attach a callback to the write mode. The valid mode values are as follows:

• "w": Called when the variable is written
• "r": Called when the variable is read
• "u" (for unset): Called when the variable is deleted

When invoked, the callback function receives three arguments: the internal variable name, an empty string (it is used in other types of Tk variables), and the mode that triggered the operation. By declaring the method with *args, we make these arguments optional, because we are not using any of these values within the callback.

The get() method of Tk wrappers returns the current value of the variable, and the set() method updates its value. They also notify the corresponding observers, so both modifying the entry's content through the GUI or clicking on the Clear button will trigger the call to the show_message() method.