Show Puzzle about Python Class Attributes and Instance Attributes

Class Puzzle

>>> class Case(object):
...     a = []
...     def show_attribute(self):
...         print self.a

>>> Case.a
[]
>>> case = Case()
>>> case.a
[]
>>> case.a = 123
>>> case.a
123
>>> case.show_attribute()
123
>>> case_2 = Case()
>>> case_2.a
[]
>>> case_2.show_attribute()
[]
>>> case_2.a.append(123)
>>> case_2.show_attribute()
[123]
>>> Case.a
[123]
>>> case_3 = Case()
>>> case_3.a
[123]