Here is the bug-triggered code snippet in the file uploaded

class A:
    def __init__(self, b=[]):
        print('b = ', b)
        self.a = b 

for i in range(3):
    a = A()
    a.a.append(1)

    print(a.a)

It seems that when I pass a list "b" to __init__ function with default value empty list. Every time I create a new instance of this class and append one new variable to "self.a", the default value of "b" changed at the next time I create another instance of class A.

The outcome of this code snippet is 
a =  []
[1]
a =  [1]
[1, 1]
a =  [1, 1]
[1, 1, 1]

I am new to python. Is it a legal behavior in python? If yes, what is the principle beneath it? Thanks in advance!