python 奇怪参数传值问题

class MyCls():
    def __init__(self,users=[]):
        self.users=users

def add(a=None,b=None):
    a+=b
    return a

a=1
b=2
print(add(a,b),a,b)##3 1 2

a=(1,2)
b=(3,4)
print(add(a,b),a,b)##(1, 2, 3, 4) (1, 2) (3, 4)

a=[1,2]
b=[3,4]
print(add(a,b),a,b)##[1, 2, 3, 4] [1, 2, 3, 4] [3, 4]

my1=MyCls(["Aa","cc"])
print(my1.users) ##['Aa', 'cc']
my2=MyCls()
my2.users.append("dd")
print(my2.users) ##['dd']
my3=MyCls()
my3.users.append("ee")
print(my3.users)##['dd', 'ee']
print(my2.users)##['dd', 'ee']