class design - Foward declaration of classes in Python -

the following program can run successfully:

class simple(object):     def __init__(self, name):       self.name = name      def __add__(self, other):       c = composite()       c._members.append(self)       c._members.append(other)       return c      def __repr__(self):       return "simple('%s')" % self.name  class composite(object):     def __init__(self):       self._members = []      def __add__(self, obj):       if isinstance(obj, simple):         out = composite()         out._members = [k k in self._members] + [obj]       elif isinstance(obj, composite):         out = composite()         out._members = [k k in self._members + obj._members]       else:         raise typeerror       return out  if __name__ == "__main__":     s1 = simple('one')     s2 = simple('two')     s3 = simple('three')     c1 = s1 + s2     c2 = c1 + s3     print c1._members     print c2._members     # output:     # [simple('one'), simple('two')]     # [simple('one'), simple('two'), simple('three')] 

i keep definition of simple, composite, , __main__ in 3 different files, cannot because cannot import simple composite.py , cannot import composite simple.py.

how modify class definition such can keep separate files?

thank you.

ps. i've read several answers related "forward declaration" not find answer specific problem.

since none of references needed until methods called, circular imports can work here. trick use fully-qualified references.

import composite  class simple(object):    ...   def __add__(self, other):     c = composite.composite()     c._members.append(self)     c._members.append(other)     return c