def relpath(file):
    """ Always locate to the correct relative path. """
    from sys import _getframe
    from pathlib import Path
    frame = _getframe(1)
    curr_file = Path(frame.f_code.co_filename)
    return str(curr_file.parent.joinpath(file).resolve())

someprj
|- relpath.py
|- A
    |- a.py
    |- AA
        |- aa.py
        |- AAA
            |- aaa.py
|- B
    |- b.txt

# A/a.py
def show_path():
    from relpath import relpath
    print(relpath('../B/b.txt'))

if __name__ == '__main__':
    from A.AA import aa
    from A.AA.AAA import aaa
    show_path()  # -> '/someprj/B/b.txt'
    aa.show_path()  # -> '/someprj/B/b.txt'
    aaa.show_path()  # -> '/someprj/B/b.txt'


# A/AA/aa.py
def show_path():
    from relpath import relpath
    print(relpath('../../B/b.txt'))

if __name__ == '__main__':
    from A import a
    from A.AA.AAA import aaa
    show_path()  # -> '/someprj/B/b.txt'
    a.show_path()  # -> '/someprj/B/b.txt'
    aaa.show_path()  # -> '/someprj/B/b.txt'


# A/AA/AAA/aaa.py
def show_path():
    from relpath import relpath
    print(relpath('../../../B/b.txt'))

if __name__ == '__main__':
    from A import a
    from A.AA import aa
    show_path()  # -> '/someprj/B/b.txt'
    a.show_path()  # -> '/someprj/B/b.txt'
    aa.show_path()  # -> '/someprj/B/b.txt'