How to get the relative path between two absolute paths in Python using pathlib?

This is trivially os.path.relpath

import os.path
from pathlib import Path

origin      = Path('middle-earth/gondor/minas-tirith/castle').resolve()
destination = Path('middle-earth/gondor/osgiliath/tower').resolve()

assert os.path.relpath(destination, start=origin) == '..\\..\\osgiliath\\tower'

If you'd like your own Python function to convert an absolute path to a relative path:

def absolute_file_path_to_relative(start_file_path, destination_file_path):
    return (start_file_path.count("/") + start_file_path.count("\\") + 1) * (".." + ((start_file_path.find("/") > -1) and "/" or "\\")) + destination_file_path

This assumes that:

1) start_file_path starts with the same root folder as destination_file_path.

2) Types of slashes don't occur interchangably.

3) You're not using a filesystem that permits slashes in the file name.

Those assumptions may be an advantage or disadvantage, depending on your use case.

Disadvantages: if you're using pathlib, you'll break that module's API flow in your code by mixing in this function; limited use cases; inputs have to be sterile for the filesystem you're working with.

Advantages: runs 202x faster than @AdamSmith's answer (tested on Windows 7, 32-bit)