The Event Object 

General event callbacks must accept one argument event that is a Tkinter event object. 

Such an event object has several attributes describing the event: 

char 
A single-character string that is the key's code (only for keyboard events) 

keysym 
A string that is the key's symbolic name (only for keyboard events) 

num 
Button number (only for mouse-button events); 1 and up 

x, y 
Mouse position, in pixels, relative to the upper left corner of the widget 

x_root , y_root 
Mouse position, in pixels, relative to the upper left corner of the screen 

widget 
The widget in which the event has occurred