relpath.py

import os
import py

def _relpath(path, *, relto):
    # TODO py35+ commonpath = os.path.commonpath([path, relto])
    commonpath = str(py.path.local(path).common(py.path.local(relto)))

    def _commonpath_to(_path):
        return _path[len(commonpath):].strip(os.path.sep)

    relto_to_commonpath = os.path.sep.join(
        os.path.pardir
        for component in _commonpath_to(relto).split(os.path.sep)
        if component
    )
    return os.path.join(relto_to_commonpath, _commonpath_to(path))