#sub/__init__.py


#sub/sub.py
#autogenerated
import root_sub

#sub/root_sub
#autogenerated
print('root_sub')

#main.py
import sub.sub
print ('main')

#############################
py -3 main.py

Traceback (most recent call last):
  File "main.py", line 1, in <module>
    import sub.sub
  File "e:\test\sub\sub.py", line 1, in <module>
    import root_sub
ImportError: No module named 'root_sub'