os.path.join Ignoring First Argument

Why This Happens

os.path.join() is designed so that if any component is an absolute path, previous components are discarded. This catches developers off guard with user input or config values starting with /.

The Problem

import os
path = os.path.join('/home/user', '/data/file.txt')
# Result: '/data/file.txt'

The Fix

import os
path = os.path.join('/home/user', 'data/file.txt')
# Result: '/home/user/data/file.txt'

# Strip leading slash:
subpath = '/data/file.txt'.lstrip('/')
path = os.path.join('/home/user', subpath)

Step-by-Step Fix

1. 1

   Remove leading slashes

   Strip leading / from relative path components.

2. 2

   Use pathlib

   pathlib.Path('/home') / 'data' / 'file.txt' is clearer.

3. 3

   Validate user input

   Always strip leading separators from relative parts.