callback——回调函数
link: https://stackoverflow.com/questions/824234/what-is-a-callback-function?utm_medium=organic&utm_source=google_rich_qa&utm_campaign=google_rich_qa
回调函数（callback）应该理解为“call at back”或者“call at after”，随后调用的函数。
def sum(a,b):
return a + b
def times(a,b):
return a * b