Resolve circular import error in python

Possible Duplicate:
Circular (or cyclic) imports in Python

I have class B that imports and creates instances of class A. Class A needs reference to B in its contructor and so includes B.

from a import A
class B:
  def __init__(self):
    self.a = A()

from b import B
class A:
  def __init__(self, ref):
    assert isinstance(ref, B)
    self.ref = ref

This doesn't work. The main file imports B and uses it... not. Something with the imports is wrong.

Error from file a ImportError: cannot import name B