sys.excepthook () and sys.unraisablehook () now explicitly flush the file (usually sys.stderr). If file.flush () fails, sys.excepthook () silently ignores the error, whereas sys.unraisablehook () logs ...

Passing 639 raises the ValueError, so 640 is the lower limit as documented: "The lowest limit that can be configured is 640 digits as provided in sys.int_info.str_digits_check_threshold.