3.6 UserList -- Class wrapper for list objects
Keywords: lib

# 3.6 `UserList` -- Class wrapper for list objects

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:

`UserList` ([list])
  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 ), `UserList` instances provide the following attribute:

`data`
  A real Python list object used to store the contents of the `UserList` class.

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.