It sounds like Python's abstract base classes (http://www.python.org/dev/peps/pep-3119/) & mix-ins (which is, INMHO a good idea for Python, I haven't used Arc enough to judge).
"Abstract Base Class" also happens to be the term preferred in C++ for Haskell's "type class" and Java's "interface". So it probably is-a python abstract base class ^^.
That said the problem with using a "base" class with an inheritance model is that it's deucedly difficult to think in terms of multiple inheritance (multiple inheritance is so hard most languages don't use it). Mix-ins help ameliorate this by being a has-a relationship.
