Accessing the contents of a project's root directory in Python
I never heard of __root__.py and don't think that is a good idea.

Instead create a files.py in a utils module:

MAIN_DIRECTORY = dirname(dirname(__file__))
def get_full_path(*path):
    return join(MAIN_DIRECTORY, *path)


You can then import this function from your main.py:

from utils.files import get_full_path

path_to_map = get_full_path('res', 'map.png')


So my project directory tree looks like this:

project_main_folder/
    utils/
        __init__.py (empty)
        files.py
    main.py


When you import a module the Python interpreter searches if there's a built-in module with that name (that's why your module name should be different or you should use relative imports). If it hasn't found a module with that name it will (among others in sys.path) search in the directory containing your script. You can find further information in the documentation.