I came across this situation today.  I was calling a Java function that returns an instance of a base class that I know is actually a specific descendant class.  In Java I simply recast the instance, for example:
    BaseClass theBaseClass = Factory.getInstanceByName("DerivedClassName");
    DerivedClass theDerivedClass = (DerivedClass) BaseClass;
How can I do the same thing in Jython?  If I do:
    theBaseClass = Factory.getInstanceByName("DerivedClassName")
How can I "cast" theBaseClass as DerivedClass so I can get access to the DerivedClass methods and attributes?