If all occurring directories are Python packages, i.e. they all contain __init__.py, then you can use

from ..bar_dir import bar

If the directories aren’t Python packages, you can do this by messing around with sys.path, but you shouldn’t.

You can use the sys and os modules for generalized imports. In foo.py start with the lines

import sys
import os
sys.path.append(os.path.abspath('../bar_dir'))
import bar

Let’s say if you have following structure:

root
  |_ productconst.py
  |_ products
     |_ __init__.py

And if you would like to import productconst in products.__init__, then following can be used :

from ..productconst import * 

Most of the proposed answers have relied on altering sys.path. IMHO – this is not neccessary if you can retain the relative folder structure correctly. I was inspired by the response of a similar question:

Please see my worked out example below. I have followed the same folder structure as in your question

Folder structure

After expanding the folder parent_dir

bar_dir/bar.py

The file bar.py implements a skeletal class Bar

class Bar():
    def __init__(self) -> None:
        pass

    def __repr__(self) -> str:
        return "This is an instance of Bar"
    
    def some_method(self) ->None:
        pass

bar_dir/init.py

from .bar import Bar

foo_dir/foo.py

The file foo.py implements a simple class Foo which in turn references the class Bar through a constructor parameter.

from ..bar_dir import Bar

class Foo():
    def __init__(self ,bar_instance: Bar) -> None:        
        self._bar=bar_instance

    def __repr__(self) -> str:
        return f"You are inside an instance of Foo and you have just accessed bar={self._bar}"

foo_dir/init.py

from .foo import Foo

sfo_main.py

This is main runner script which glues together foo_dir and bar_dir packages

import parent_dir.bar_dir as bar_lib
import parent_dir.foo_dir as foo_lib


if __name__ == "__main__":
    f=foo_lib.Foo(bar_instance=bar_lib.Bar())
    print(f"This is an instance of foo={f}")
    print(f"{bar_lib.Bar()}")

Output from sfo_main.py

Why use a init.py ?

https://docs.python.org/3.9/tutorial/modules.html

Documentation on relative imports

https://docs.python.org/3/reference/import.html

If you’re having issues in python 3+, the following worked for me using sys.path.append("..").

sys.path.append("..")
from bar_dir import bar