How do I import .py files from a directory that is parallel to current script's parent directory?

Question

I have a python script that wants to import another .py file in a directory that is on the same level of its parent directory, the file system like the following

.
├── __init__.py
├── obslib
│   ├── __init__.py
│   └── jet
│       ├── __init__.py
│       ├── fakepdf.py
│       ├── fakepdf.pyc
│       ├── jet_mellin.py
│       ├── reader.py
│       ├── residuals.py
│       ├── theory.py
│       └── theory.pyc
├── readme.md
└── tools
    ├── README.md
    ├── __init__.py
    ├── __init__.pyc
    ├── bar.py
    ├── config.py
    ├── inputmod.py
    ├── multiproc.py
    ├── parallel.py
    ├── randomstr.py
    ├── reader.py
    ├── residuals.py
    ├── storage.py
    └── tools.py

File jet_mellin.py in directory jet wants to import, say, class BAR in bar.py in directory tools, it should be fine to just write

from tools.bar import BAR

However, I get error

ImportError: No module named tools.bar

Then I tried

from ..tools import bar

and get

ValueError: Attempted relative import in non-package

The only way I could make it work is following

import sys
sys.path.insert(0, '../../tools/')
from bar import BAR

But this would give rise to ambiguous situation if I want to have another script also called bar.py in some other directory. So I would like to be able to specify the path for an import like in from ..tools import bar. Could you please help me with this? Thanks in advance!