Python: Not able to import module from sibling directory -

i using python 3.4

i have cloned git repo has structure this:

parent_dir/       .       .#random things       .        sub_dir/                sub_dir1/                     __init__.py                     file1.py                sub_dir2                     __init__.py                     file2.py                sub_dir3                     file3.py                __init__.py 

content of file2.py

from sub_dir.sub_dir1.file1 import attibute1 #ok sub_dir.sub_dir3.file3 import attribute1 #gives error there no module named sub_dir3 

i tried add __init__.py file sub_dir3 still gave same error. tried experiment created own project similar structure in case works fine.

is there obvious overlooking in original package working with?

my ide pycharm.

i looked through these1,2, questions, couldn't find answer.