Completing the answer supplied by the OP: after setting the breakpoint - you must turn on IDLE's debug mode (using debug --> debugger). When you run the program, press "Go" in the debug window that opens up and IDLE will stop at the breakpoint.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…