How to change the values of the push buttons in QT designer

I am using the standard dialog box with ok and cancel options.i Have also added one more button exit.

self.dlg.show()                    
result=self.dlg.exec_()
print 'The result is',result

it is giving the result of 1 for ok. but it is giving the result of 0 for both cancel and the exit buttons. I want to assign different functionalities to both the buttons. So how to chnage the result values for both the buttons? like i want to print the result value of exit to be 2.