Core Python Programming (2nd Edition)

13.13. Customizing Classes with Special Methods
We covered two important aspects of methods in preceding sections of this chapter: first, that methods
must be bound (to an instance of their corresponding class) before they can be invoked; and second,
that there are two special methods which provide the functionality of constructors and destructors,
namely __init__() and __del__() respectively.
In fact, __init__() and __del__() are part of a set of special methods which can be implemented. Some
have the predefined default behavior of inaction while others do not and should be implemented where
needed. These special methods allow for a powerful form of extending classes in Python. In particular,
they allow for:
● Emulating standard types
● Overloading operators
Special methods enable classes to emulate standard types by overloading standard operators such as +,
*, and even the slicing subscript and mapping operator [ ]. As with most other special reserved
identifiers, these methods begin and end with a double underscore ( __ ). Table 13.4 presents a list of
all special methods and their descriptions.
Table 13.4. Special Methods for Customizing Classes
Special Method Description
Basic Customization
C.__init__(self[, arg1, ...] ) Constructor (with any optional arguments)
[a] Constructor (with any optional argu ments); usually
C.__new__(self[, arg1, ...] )
used for setting up subclassing of immutable data types
C.__del__(self) Destructor
C.__str__(self) Printable string representation; str() built-in and print
statement
C.__repr__(self) Evaluatable string representation; repr() built-in and
'' operator
[b] Unicode string representation; unicode() built-in
C.__unicode__(self)
C.__call__(self, *args) Denote callable instances
C.__nonzero__(self) Define False value for object; bool() built-in (as of 2.2)
C.__len__(self) "Length" (appropriate for class); len() built-in