security.py

import os


class SuspiciousFileOperation(Exception):
    pass


def path_traversal_check(unsafe_path, known_safe_path):
    known_safe_path = os.path.abspath(known_safe_path)
    unsafe_path = os.path.abspath(unsafe_path)

    if (os.path.commonprefix([known_safe_path, unsafe_path]) != known_safe_path):
        raise SuspiciousFileOperation("{} is not safe".format(unsafe_path))

    # Passes the check
    return unsafe_path