ImportError: cannot import name A2

pack-__init__.py

from .pack_a import A1, A2
from .pack_b import B1, B2

pack-pack_a.py

from .pack_b import B1

class A1:
  def __init__(self):
    print("A1")
    B1()

class A2:
  def __init__(self):
    print("A2")

pack-pack_b.py

from .pack_a import A2

class B1:
  def __init__(self):
    print("B1")

class B2:
  def __init__(self):
    print("B2")
    A2()

test.py

from pack import A1, A2, B1, B2
A1()
B2()

Traceback (most recent call last):
  File "test.py", line 1, in <module>
    from pack import A1, A2, B1, B2
  File "/home/Blusky-Desktop/python_test/pack/__init__.py", line 1, in <module>
    from .pack_a import A1, A2
  File "/home/Blusky-Desktop/python_test/pack/pack_a.py", line 1, in <module>
    from .pack_b import B1
  File "/home/Blusky-Desktop/python_test/pack/pack_b.py", line 1, in <module>
    from .pack_a import A2
ImportError: cannot import name A2