name mangling in python

gistfile1.txt

In [29]: class foo():
   ....:     def __init__(self):
   ....:         self.__x = 1
   ....:

In [30]: r1 = foo()

In [31]: r1.__x
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-31-32786e7560ef> in <module>()
----> 1 r1.__x

AttributeError: foo instance has no attribute '__x'

In [32]: r1.__y = 1

In [33]: r1.__y
Out[33]: 1

In [35]: print r1.__y
1
这里的__x __y为什么会有区别呢

In [40]: class foo():
   ....:     def __init__(self):
   ....:         self.__x = 1
   ....:         self.y = 1
   ....:

In [41]: r1 = foo()

In [42]: r1.y
Out[42]: 1

In [43]: r1.__x
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-43-32786e7560ef> in <module>()
----> 1 r1.__x

AttributeError: foo instance has no attribute '__x'

In [44]: r1.__dict__
Out[44]: {'_foo__x': 1, 'y': 1}

In [45]: r1._foo__x
Out[45]: 1

In [46]: r1._foo__x = 2

In [47]: r1._foo__x
Out[47]: 2

In [48]: r1.__z = 1

In [49]: r1.__dict__
Out[49]: {'__z': 1, '_foo__x': 2, 'y': 1}
http://docs.python.org/2/reference/expressions.html#atom-identifiers
https://en.wikipedia.org/wiki/Name_mangling#Name_mangling_in_Python