I need to have check box and select multiple items in python

I have 'raja', 'rani' ,'anny' and so on upto 15 names. Each name has a history of all the sales done for the past 5 years. I want to choose one person, or n persons and find their cumulative sales made. For this I need have a drop box / drop down menu or a check box to pick their names and get the user's input and aggregate. I am able to write a UI for single input in tkinter. But how could I get multiple inputs from the user?