The part about startup files in my last message is probably wrong.  If one starts IDLE by 'import idlelib.idle' after setting 'sys.ps1 = 'me ', then the setting is recognized.  However, a startup file that runs in the user process will not affect sys.ps1 in the IDLE process.  We could add a command line option -p "prompt", but I don't see this as sufficiently important.

At least some shells and consoles recognize special characters in a prompt string to create the prompt.  For instance, the default setting on Windows is "$P$G", which I presume stands for 'working directory path' and 'greater than symbol', with a result such as "F:/dev/3x>".  (I have no idea where this is documented.)  Python and IDLE will print the literal string instead.  Custom prompts therefore seem like an advanced feature, not one aimed at beginners.