How to handle cyclic imports in python?

Question

I'm new to python and I have directory like this

dir
--a.py
--b.py
--__init__.py

a.py has

from b import B
class A:
    #class body

b.py has

from a import A
class B:
    #class body

But when I try to run a.py I get error like:

File "/b.py", line 1, in <module
ImportError: cannot import name B

how should I resolve this?

__init__.py is empty.