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.
This is Python's approach to operator overloading, allowing
classes to define their own behavior with respect to language
operators. For instance, if a class defines
a method named __getitem__(), and
x is an instance of
this class, then
x[i] is equivalent3.2 to
x.__getitem__(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.)