Getting the selected value from combobox in Tkinter

I've figured out what's wrong in the code.

First, as James said the brackets should be removed when binding justamethod to the combobox.

Second, regarding the type error, this is because justamethod is an event handler, so it should take two parameters, self and event, like this,

def justamethod (self, event): 

After making these changes the code is working well.