default value for __init__ doesn't work

[人言落日是天涯，望极天涯不见家]

Please look at below code snippet:
class test():
    def __init__(self, a, dic={}):
        self.a = a
        self.dic = dic
        print('__init__ params:',a, dic)

    def get(self):
        self.dic[1] = 2
        self.dic[4] = 5

def foo():
    print('in foo function')
    bar = test(1)
    bar.get()

if __name__ == '__main__':
    foo()
    foo()
-----------------------
Result:
in foo function
__init__ params: 1 {}
in foo function
__init__ params: 1 {1: 2, 4: 5}

But my expect result is :
in foo function
__init__ params: 1 {}
in foo function
__init__ params: 1 {}

it seems that the default value for dic doesn't work on the second
call for the class test.
It's wired. Who can give a explaination for this scenario?