为什么我的TreeBuilder对象不包含TreeBuilder的私有属性？

Question

当我实例化xml.etree.ElementTree.TreeBuilder对象时，我看不到任何TreeBuilder的私有属性。

Python 3.4.2 (v3.4.2:ab2c023a9432, Oct  6 2014, 22:15:05) on win32
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>>>
>>> import xml.etree.ElementTree as etree
>>> etree.__file__
'C:\\Python34\\Lib\\xml\\etree\\ElementTree.py'
>>> b = etree.TreeBuilder()
>>> b._data
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'xml.etree.ElementTree.TreeBuilder' object has no attribute '_data'
>>>

即使_data应该在那里（摘自C:\Python34\Lib\xml\etree\ElementTree.py，第1384行）：

def __init__(self, element_factory=None):
    self._data = [] # data collector
    self._elem = [] # element stack
    self._last = None # last element
    self._tail = None # true if we're after an end tag
    if element_factory is None:
        element_factory = Element
    self._factory = element_factory

目标

我希望子类TreeBuilder添加对XML注释元素的支持，但需要获取其中一个私有属性来附加注释元素。