python none代表什么

mylist = ['a', 'b', 'c']

>>> assert len(mylist) is not None # 用assert判断列表不为空，正确无返回

>>> assert len(mylist) is None # 用assert判断列表为空

a = None

if a:

    print "a is not None"

else:

    print "a is None"

def add1(a,b):

    return a+b

a1=add1(1,2)

print a1

#会输出3，因为有return，则有返回值

def add2(a,b):

    print a+b

a2 = add2(1,2)

print a2

#会输出None,因为没有return，则add2为None