\member{data} attribute of \class{UserDict} instances. If
\var{initialdata} is provided, \member{data} is initialized with its
contents; note that a reference to \var{initialdata} will not be kept,
-allowing it be used for other purposes.
+allowing it be used for other purposes. \note{For backward compatibility,
+instances of \class{UserDict} are not iterable.}
+\end{classdesc}
+
+\begin{classdesc}{IterableUserDict}{\optional{initialdata}}
+Subclass of \class{UserDict} that supports direct iteration (e.g.
+\code{for key in myDict}).
\end{classdesc}
In addition to supporting the methods and operations of mappings (see
-section \ref{typesmapping}), \class{UserDict} instances provide the
-following attribute:
+section \ref{typesmapping}), \class{UserDict} and
+\class{IterableUserDict} instances provide the following attribute:
\begin{memberdesc}{data}
A real dictionary used to store the contents of the \class{UserDict}