open('w') on a path whose parent directory doesn't exist

Question

Accepted Answer

Many developers assume 'w' mode is purely creative - if nothing exists, Python builds it. That's true only for the file itself. If the parent directory is missing, open() raises FileNotFoundError regardless of mode. The distinction matters in pipelines: 'w' on 'reports/output.txt' when 'reports/' doesn't exist fails immediately. The fix is to create the directory first: Path('/nonexistent/dir').mkdir(parents=True, exist_ok=True) before calling open(). The parents=True flag creates every missing ancestor, and exist_ok=True makes it idempotent so you don't have to check first. This is a common source of bugs in ETL jobs that write to date-partitioned paths like 'output/2024/04/23/result.csv' - the directory tree must be created before the file.