在内的内部定义并使用，外部无法访问，以双下划线作为前作，定义后被python转为_classname__变量名了

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In [1]: class aa:
...: __x = 12 #私有变量_ _x
...: def px(self):
...: print 'private __x', aa.__x #内部访问
...:

In [2]: a = aa()

In [3]: a.px()
private __x 12

In [4]: dir(a)
Out[4]: ['__doc__', '__module__', '_aa__x', 'px'] # map成_classname__变量名了

In [5]: a.__x = 13#实例对象a的实例变量

In [6]: dir(a)
Out[6]: ['__doc__', '__module__', '__x', '_aa__x', 'px']

In [7]: print a.__x
13

In [8]:In [8]: print aa.__x
---------------------------------------------------------------------------
AttributeError Traceback (most recent call last)
in ()
----> 1 print aa.__x

AttributeError: class aa has no attribute '__x'

In [9]:
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