The io doc says for IOBase flush()
    Flush the write buffers of the stream if applicable. This does nothing for read-only and non-blocking streams.

and for BufferedWriter flush()
    Force bytes held in the buffer into the raw stream. A BlockingIOError should be raised if the raw stream blocks.

On 3.x, open(filename, "wb"), used in writefile(), returns a BufferedWriter.  So it seems than an exception is possible, which would crash IDLE without try-except.

Serhiy, please read the previous message(s).  Do you remember if you intended to remove the f.flush in writefile(), which Guido proposes to restore?