Python - import function from parallel directory

Let's assume I have two parallel directories, with simple python files.
--'dir_one' with file_one.py
--'dir_two' with file_two.py

In file_one.py there is a function named function_one. How can I import it within file_two.py? I have checked literally a few dozens of sources, including answers on stackoverflow. Therefore, I am not quoting any of them here. I have tried four different soulutions, none of them works.

1. Insert 'init.py' files in all the directories - results in error 'ImportError: attempted relative import with no known parent package'
2. Make both directories a single package - not possible in this case, directories have to be able to run as independent apps.
3. Use sys module: sys.path.insert(0, 'path-to-dir_one') right before 'import dir_one' line - doesn't change anything, still get 'ModuleNotFoundError: No module named 'dir_one'.
4. try relative import: 'from .dir_one import file_one' - 'ModuleNotFoundError: No module named 'zweizen.prodalitics'.

Is it possible at all to import a function in python from parallel folder without making it part of the same package?

You can add directly load the module to sys.modules using importlib to import it. I assume you are working with a directory structure similar to this.

file1.py

a = 10

file2.py

import os

def import_from_path(module_name:str, file_path: str):
    import importlib.util
    spec = importlib.util.spec_from_file_location(module_name, file_path)
    module = importlib.util.module_from_spec(spec)
    spec.loader.exec_module(module)
    return module

file_path = os.path.abspath(os.path.join(os.path.dirname(__file__), '../dir1/file1.py'))
module_name = "file1"

file1 = import_from_path(module_name, file_path)

print(file1.a)

You can import any python file as a module using this method and you don't modify the system path too.