哪位大神能解释一下吗，越详细越好

Question

import time

def performance(f):

    def fn (*args,**kw):

        t1=time.time()

        r=f(*args,**kw)

        t2=time.time()

        print 'call %s()in %fs'%(f.__name__,(t2-t1))

        return r

    return fn    

@performance

def factorial(n):

    return reduce(lambda x,y: x*y, range(1, n+1))

print factorial(10)