os.path.join (folder, filename) ensures the path is correct for the system. On Windows, it uses backslashes (reports\summary.txt); on Linux and macOS it uses forward slashes (reports/summary.txt).

As a buddy of mine always says "the nice thing about standards is that there's so many to choose from". Take CSV files for example. CSV, of course, stands for "Comma Separated Values", more often than ...