how would i use if statement to add attributes to the chosen value?

I am newbie and thanks for helping me out. Am trying to create an if statement so the user would be able to enter new details and that details should be stored into the application.

I have looked online but most of the examples out there are calculations of numbers and that's not very helpful, i was hoping you guys could help me out.

Thanks

This is the code i'm working one

def add(self):
    self.carsales.append(ID ,owner,plate Number,Postcode,Price)

    if