Type InterfaceClass

    object --+        
             |        
          type --+    
                 |    
DefinitionsTracker --+
                     |
                    InterfaceClass

Known Subclasses:

HaroldClass

A class with contractual interface.

 class X (object):
     __metaclass__ = InterfaceClass
     p_ro = property (g)
     p_rw = property (g,s)
     def m (self, x, y=1, z=1)

 class O1 (X):
     p_ro = property (None,s) 
     ## illegal: p_ro should be readable

 class O2 (X):
     p_ro = property (g,s)
     ## legal: p_ro is readable

 class O3 (X):
     p_rw = property (g)
     ## illegal: p_rw should be settable

 class O4 (X):
     def m (self)
     ## illegal: m should accept 2,3 or 4 parameters

 class O5 (X):
     def m (self, a, b, c)
     ## illegal: m should accept 2 or 3 parameters

 class O6 (X):
     def m (self, a)
     ## illegal: m should accept 3 or 4 parameters

 class O7 (X):
     def m (self, *args, **kw)
     ## legal: m does accept 2, 3 or 4 parameters