The tests around unicode filenames use both FileIO directly and open so kept using open for them / validates that unicode filenames passed through open() match behavior of FileIO called directly with an encoded name.

For the append tests it looks like FileIO directly is closer to what was intended, there is test_io that has append tests around self.open directly, this is test_fileio so intent I think was to test FileIO directly.

• Issue: Use implemenation-specific I/O open / constructors in tests #133982