def average(*args):

    sum = 0.0
    if len(args) == 0:
        print sum
    else:
        for i in args:
            sum += i
        print sum/len(args)
       
print average()
print average(1, 2)
print average(1, 2, 2, 3, 4)

打印结果：

0.0
None
1.5
None
2.4
None

为什么会多出三行None?请大神指教