Note: This module is available for backward
compatibility only. If you are writing code that does not need to work with versions of Python
earlier than Python 2.2, please consider subclassing directly from the built-in list type.
This module defines a class that acts as a wrapper around list objects. It is a useful base
class for your own list-like classes, which can inherit from them and override existing
methods or add new ones. In this way one can add new behaviors to lists.
The UserList module defines the UserList
- Class that simulates a list. The instance's contents are kept in a regular list, which
is accessible via the data attribute of UserList
instances. The instance's contents are initially set to a copy of list,
defaulting to the empty list
. list can be either a regular
Python list, or an instance of UserList (or a subclass).
In addition to supporting the methods and operations of mutable sequences (see section 2.3.6), UserList instances provide
the following attribute:
- A real Python list object used to store the contents of the UserList
Subclassing requirements: Subclasses of UserList are expect
to offer a constructor which can be called with either no arguments or one argument. List
operations which return a new sequence attempt to create an instance of the actual
implementation class. To do so, it assumes that the constructor can be called with a single
parameter, which is a sequence object used as a data source.
If a derived class does not wish to comply with this requirement, all of the special
methods supported by this class will need to be overridden; please consult the sources for
information about the methods which need to be provided in that case.
Changed in version 2.0: Python versions 1.5.2 and 1.6 also
required that the constructor be callable with no parameters, and offer a mutable data attribute. Earlier versions of Python did not attempt to create
instances of the derived class.