Tracing text changes

Tk variables allow your applications to get notified when an input changes its value. There are four variable classes in Tkinter: BooleanVar, DoubleVar, IntVar, and StringVar. Each one wraps the value of the corresponding Python type, which should match the type of the input widget attached to the variable.

This feature is particularly useful if you want to automatically update certain parts of your application based on the current state of some input widgets.