CheatSheet: Create a relative path from an absolute or another relative path.

create_relative_path.py

#!/usr/bin/python
# -*- coding: utf-8 -*-

import pathlib

def create_relative_path(initial_path: pathlib.Path, new_root: str) -> pathlib.Path:
    """Creates a new path from the initial path.

    Example:
    your_path = pathlib.Path('/foo/bar/baz/bang')
    create_relative_path(your_path, 'bar') returns -> Windows|UnixPath('bar/baz/bang')

    If `new_root` does not exist, `create_relative_path` will return `None`.

    """
    stringified_initial_path = initial_path.__str__()
    new_root_index = stringified_initial_path.index(new_root)
    if new_root_index > 0:
        return pathlib.Path(stringified_initial_path[new_root_index:])
    else:
        return None


my_path = pathlib.Path('/foo/bar/baz/bang')
new_path = create_relative_path(my_path, 'baz')

import os
if os.name == 'nt':
    assert new_path.__str__() == 'baz\\bang'
else:
    assert new_path.__str__() == 'baz/bang'

print('new_path="{}"'.format(new_path))