3.3 Special method names

A class can implement certain operations that are invoked by special syntax (such as arithmetic operations or subscripting and slicing) by defining methods with special names. For instance, if a class defines a method named __getitem__(), and x is an instance of this class, then x[i] is equivalent to x.__getitem__(i). (The reverse is not true -- if x is a list object, x.__getitem__(i) is not equivalent to x[i].) Except where mentioned, attempts to execute an operation raise an exception when no appropriate method is defined.