The 'cantSetTrue' assertion in domconfigerrorhandler1 relies on the
implementation being able to tell the difference between boolean values and
ErrorHandler types. However this is not necessarily possible in a
dynamically-typed language such as Python.

Officially, IDL boolean is still bound to Python integer, however in the
meantime integer has become a type that can be extended (eg. by the new native
boolean type in Python 2.3), so it's entirely possible to have an object that's
both a boolean and an ErrorHandler! There are arbitrary approaches one could
take to detecting non-handler objects (eg. method-sniffing), but in the absence
of static typing there's no watertight solution.