\strong{Subclassing requirements:}

Subclasses of \class{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.

\versionchanged[Python versions 1.5.2 and 1.6 also required that the

constructor be callable with no parameters, and offer

a mutable \member{data} attribute. Earlier versions

of Python did not attempt to create instances of the

derived class]{2.0}