Python类如何定义私有变量

> class demo(object):
...   __a =1
...   b = 2
...
> d = demo()
> print(d.b)
> d.__a
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
AttributeError: 'demo' object has no attribute '__a'

> d._demo__a
1

> class fun(object):
...   def __a(self):
...       print('聚焦学院AAA')
...   def b(self):
...       print('聚焦学院BBB')
...
> f = fun()
> f.b()
聚焦学院BBB
> f.__a()
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
AttributeError: 'fun' object has no attribute '__a'
> f._fun__a()
聚焦学院AAA