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

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

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:

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 2.2.6), `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.