有两种类型的回调函数：
blocking callbacks (also known as synchronous callbacks or just callbacks)
deferred callbacks (also known as asynchronous callbacks)
那么，在python中如何实现回调函数呢，看代码：
def my_callback(input):
print "function my_callback was called with %s input" % (input,)
def caller(input, func):
func(input)
for i in range(5):
caller(i, my_callback)