> It seems to work perfectly on command line though.

If the code is saved in a file, yes, but not in an interactive interpreter.  This is not actually related to IDLE, but to the fact that inspect.getsource merely finds the __file__ attribute of the module object for its argument.  If a module object has no file, the error message indicates that it’s a built-in module (like sys), but this fails to take into account the special __main__ module in an interactive interpreter.

It might be worth it to improve the error message, and in any case the documentation can be improved.