a.py

import b

class A:
    def __init__(self, save_dir):
        self.save_dir = save_dir

save_dialog = A(b.prefs.get('save_dir'))

def show():
    pass

b.py

import a

class B:
    def get(self, name):
        pass

prefs = B()
a.show()

main.sh

$ python3
Python 3.6.3 (v3.6.3:2c5fed86e0, Oct  3 2017, 00:32:08) 
[GCC 4.2.1 (Apple Inc. build 5666) (dot 3)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import b
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/terapyon/tmp/effective/b.py", line 1, in <module>
    import a
  File "/Users/terapyon/tmp/effective/a.py", line 7, in <module>
    save_dialog = A(b.prefs.get('save_dir'))
AttributeError: module 'b' has no attribute 'prefs'