If src is effectively blank, compile(src, filename, mode) raises SyntaxError if mode is 'single' but not if it is 'exec'.  I believe IDLE compiles with 'single', but it has the behavior Jim (and I) expect and consider correct, printing '>>>' after effectively blank lines.  This is because IDLE uses code.InteractiveInterpreter, which uses codeop.compile_command, which uses codeop._maybe_compile, which replaces effectively blank statements with 'pass'.  compile('pass', '', 'single') returns a do-nothing code object.  The C-coded interactive interpreter is doing something else.