After the recent change that assignments to __debug__ are disallowed, I noticed that IDLE stops working (see SF bug report), since it was assigning to __debug__. Simply commenting-out the assignment (to zero) did no good: Inside the __debug__ blocks, IDLE would try to perform print statements, which would write to the re-assigned sys.stdout, which would invoke the code that had the __debug__, which would give up thanks to infinite recursion. So essentially, you either have to remove the __debug__ blocks, or rewrite them to writing to save_stdout - in which case all the ColorDelegator debug message appear in the terminal window. So anybody porting to Python 2.1 will essentially have to remove all __debug__ blocks that were previously disabled by assigning 0 to __debug__. I think this is undesirable. As I recall, in the original description of __debug__, being able to assign to it was reported as one of its main features, so that you still had a run-time option (unless the interpreter was running with -O, which eliminates the __debug__ blocks). So in short, I think this change should be reverted. Regards, Martin P.S. What was the motivation for that change, anyway?