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(python2.1-ref.info)Special method names

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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.)

Basic customization

Customizing attribute access

Emulating callable objects

Emulating sequence and mapping types

Additional methods for emulation of sequence types

Emulating numeric types