>>> class sample:
...     def __init__(self, arr=[]):
...             self.arr = arr
...
>>> s1 = sample()
>>> s2 = sample()
>>> s1.arr
[]
>>> s2.arr
[]
>>> s2.arr.append(2)
>>> s2.arr
[2]
>>> s1.arr
[2]
>>> s3 = sample([3])
>>> s3.arr
[3]
>>> s2.arr
[2]
>>> s1.arr
[2]
>>> id(s1.arr) == id(s2.arr)
True