PermissionError: Permission denied

Python PermissionError on Windows is frequently caused by file locking rather than traditional permission restrictions. Windows allows only one process to hold an exclusive write lock on a file at a time. If another application — Excel with an open .csv, an antivirus scanner mid-scan, or Windows Search indexing a directory — holds a lock on your target file, Python's open('file.csv', 'w') raises PermissionError even though you own the file and have write permission in the NTFS sense. This contrasts with Linux and macOS, where advisory locking allows multiple processes to open and write to the same file (with potential corruption but no PermissionError). Python code developed on Linux/macOS can fail specifically on Windows due to this lock behavior difference, creating hard-to-reproduce cross-platform bugs. The diagnostic: Sysinternals Process Explorer's Handle search finds which process holds the lock. The workaround is retry logic for short-lived locks, or using tempfile.NamedTemporaryFile with delete=False and renaming after writing — an atomic write pattern that avoids lock contention. On Linux and macOS, PermissionError in Python reflects real Unix permission issues. Check ls -la on the target file and its parent directory. In Docker containers running as non-root users, host-mounted volumes (Docker Desktop bind mounts) may have root-owned permissions that block the container user from writing — fix by setting explicit permissions in the Dockerfile or using named volumes instead of bind mounts.