AttributeError: ‘module’ object has no attribute

I have two python modules:

a.py

import b

def hello():
  print "hello"

print "a.py"
print hello()
print b.hi()

b.py

import a

def hi():
  print "hi"

When I run a.py, I get:

AttributeError: 'module' object has no attribute 'hi'

What does the error mean? How do I fix it?