When you specify a Tkinter variable, such as textvariable, for a widget (textvariable = myvar), the widget automatically gets updated whenever the value of the variable changes. However, there might be times when, in addition to updating the widget, you need to do some extra processing at the time of reading or writing (or modifying) the variable.
Tkinter provides a method to attach a callback method that will be triggered every time the value of a variable is accessed. Thus, the callback acts as a variable observer.
The callback creation method is named trace_variable(self, mode, callback) or simply trace(self, mode, callback).
The mode argument can take a value of r, w, or u, which stand for read, write, or