Zope 2.4 includes an Interface module that can be used outside Zope.
It provides lots of nice introspection methods.

quoting the ZopeDevGuide:

from Interface import Base

class Hello(Base):
    """ The Hello interface provides greetings. """
    def hello(self, name):
        """ Say hello to the name """

class HelloComponent:
     __implements__ = Hello

     def hello(self, name):
         return "hello %s!" % name
#############

Hello looks like a class, but it's not:
> >> Hello
<Interface Hello at 109189c> 
> >> type(Hello)
<type 'instance'> 
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