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.  

When implementing a class that emulates any built-in type, it is important that the emulation only be implemented to the degree that it makes sense for the object being modelled. For example, some sequences may work well with retrieval of individual elements, but extracting a slice may not make sense. (One example of this is the NodeList interface in the W3C's Document Object Model.)

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