def f(x):    print 'original'    if x > 0:        return f(x-1)    return 0g = fdef f(x):    print 'new'    return xprint g(5)

结果是:

originalnew4

证明了:

1.when g(5) runs, origninal `f`is called,not new `f`. 2.as 5>0,'return f(x-1)'is executed3.new`f`is called when `f(x-1)` runs. so the result is 4.