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.
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 equivalent 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.)
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