Newbie - circular import problem

[sw]

Can you give me some pointers on resolving this circular inclusion? I've got 
two classes, in two files:

--a.py--
import b
class ClassA:
    def __init__(self):
        print 'ClassA'

--b.py--
import a
class ClassB(ClassA):
    def __init__(self):
        print 'ClassB'
 
If I try to access ClassB (for example, by firing up python and importing b), I
get the error:

  File "b.py", line 2, in ?
    class ClassB(ClassA):
    NameError: name 'ClassA' is not defined

Could someone explain what is going wrong here? I'm still getting up to speed
with python, but, to me, this looks like the import of b is occuring before
ClassA has been defined. Is that right? How can I correct this, apart from
amalgamating the two into a single file?

Many thanks for any guidance.