Methods and the self parameter; constructors (__init__) with default arguments

A method is a function defined inside a class. Its first parameter receives the object the method was called on, and by strong convention it is named self. When you write obj.method(5), Python translates it into ClassName.method(obj, 5), passing the object automatically as the first argument. You never pass self yourself in a normal call, but you must always list it in the method definition.