FileNotFoundError: No such file or directory

Python FileNotFoundError is nearly always a working directory mismatch. Scripts executed directly (python script.py) run with the calling shell's current directory as os.getcwd(). Scripts executed through IDEs, cron jobs, or imported as modules inherit whatever directory the parent process used. A path like open('data/input.csv') only works if the process's current directory contains a data/ folder — true when running from the project root, false when called from elsewhere. The portable fix is to build paths relative to the script file itself using __file__: os.path.join(os.path.dirname(os.path.abspath(__file__)), 'data', 'input.csv'). This works regardless of launch directory because __file__ is the absolute path of the script itself. The modern equivalent using pathlib: pathlib.Path(__file__).parent / 'data' / 'input.csv'. In VS Code, the debugger's cwd setting in .vscode/launch.json controls where the Python process runs, while the integrated terminal uses the workspace root — these can differ. For notebooks, %pwd (IPython magic) shows the notebook's working directory, which may not be where you expect. Using os.getcwd() as a debug print at the top of your script immediately reveals which directory Python considers current, eliminating guesswork about relative path failures.