Simulating call of instance like a list or tuple

[Brett Cannon]

With Python 2.2's wonderful ability of subclassing built-ins, I began to
wonder if there was a way to call an instance like a list or tuple, but
without subclassing them.

In other words, have a class named foo with an instance called bar.  Now,
call bar like:
>>> bar
and have  it return something, just like how calling a list or  tuple like
that returned the list or  tuple value.  I can't find any special methods
that allow you to do that.  Does that exist, or is that just one of the
things that still separates built-ins and user-defined objects?

-Brett C.