Function!

函数定义有两种写法：
1.def print_two(*args):
    arg1, arg2 = args
    print "arg1: %r, arg2: %r" % (arg1, arg2)
2.def print_two_again(arg1, arg2):
    print "arg1: %r, arg2: %r" % (arg1, arg2)

也可以无参数：
def print_none():
    print "I got nothin'."

参数可以是变量：
1.cheese_and_crackers(20, 30)

2.amount_of_cheese = 10
 amount_of_crackers = 50
 cheese_and_crackers(amount_of_cheese, amount_of_crackers)

返回值：
def add(a, b):
    print "ADDING %d + %d" % (a, b)
    return a + b