关于 Python 中 private function 的疑惑

Question

>>>classMyClass:
defPublicMethod(self):
print'publicmethod'
def__PrivateMethod(self):
print'thisisprivate!'
>>>obj=MyClass()
>>>obj.PublicMethod()
publicmethod
>>>obj.__PrivateMethod()
Traceback(mostrecentcalllast):
File"",line1,in
AttributeError:MyClassinstancehasnoattribute'__PrivateMethod'
>>>dir(obj)
['_MyClass__PrivateMethod','__doc__','__module__','PublicMethod']
>>>obj._MyClass__PrivateMethod()
thisisprivate!
如上的执行，为什么到了obj.__PrivateMethod()就会出错，为何会这样？