def f(x):

    print 'original'

    if x > 0:

        return f(x-1)

    return 0

g = f

def f(x):

    print 'new'

    return x

print g(5)

结果是:

original

new

4

证明了:

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

3.new`f`is called when `f(x-1)` runs. so the result is 4.