为什么`xml.etree.ElementTree.Element`的实例没有`__dict__`？

Question

from xml.etree.ElementTree import Element
e = Element(None)
e.__dict__

->AttributeError: 'xml.etree.ElementTree.Element' object has no attribute '__dict'

将 C:\Python39\Lib\xml\etree\ElementTree.py 复制到其他地方：

from ElementTree import Element as Element2
e2 = Element2(None)
e2.__dict__

->{'tag': None, 'attrib': {}, '_children': []}

Answer 1

xml.etree.ElementTree 有一个optimised C implementation which replaces the Python implementation if available：

# Import the C accelerators
try:
    # Element is going to be shadowed by the C implementation. We need to keep
    # the Python version of it accessible for some "creative" by external code
    # (see tests)
    _Element_Py = Element

    # Element, SubElement, ParseError, TreeBuilder, XMLParser, _set_factories
    from _elementtree import *
    from _elementtree import _set_factories
except ImportError:
    pass
else:
    _set_factories(Comment, ProcessingInstruction)

值得注意的是，C 扩展类型通常没有__dict__ 插槽。通过复制类的 Python 实现而不加载 C 实现，可以使用原始 Python 实现。

>>> from xml.etree.ElementTree import _Element_Py
>>> e = _Element_Py(None)
>>> e.__dict__
{'tag': None, 'attrib': {}, '_children': []}

Answer 2

This answer 很好地解释了__dict__ 属性：

Python 对象将它们的实例变量存储在属于该对象的字典中。 vars(x) 返回这个字典（和 x.__dict__ 一样）。另一方面，dir(x) 返回 x 的“属性、其类的属性以及递归其类的基类的属性”的字典。

让我们在实践中看看：

import xml.etree.ElementTree as ET

e = ET.Element(None)
print(e.__dict__)
# Raises AttributeError: 'xml.etree.ElementTree.Element' object has no attribute '__dict__'

但是为什么，既然e 确实有属性呢？

import xml.etree.ElementTree as ET

e = ET.Element(None)
print(e.tag)
# Outputs None

e = ET.Element("my-tag")
print(e.tag)
# Outputs my-tag

让我们看看它的类的属性：

import xml.etree.ElementTree as ET

print(ET.Element.__dict__)

这将输出

mappingproxy({'__copy__': <method '__copy__' of 'xml.etree.ElementTree.Element' objects>,
              '__deepcopy__': <method '__deepcopy__' of 'xml.etree.ElementTree.Element' objects>,
              '__delitem__': <slot wrapper '__delitem__' of 'xml.etree.ElementTree.Element' objects>,
              '__doc__': None,
              '__getattribute__': <slot wrapper '__getattribute__' of 'xml.etree.ElementTree.Element' objects>,
              '__getitem__': <slot wrapper '__getitem__' of 'xml.etree.ElementTree.Element' objects>,
              '__getstate__': <method '__getstate__' of 'xml.etree.ElementTree.Element' objects>,
              '__init__': <slot wrapper '__init__' of 'xml.etree.ElementTree.Element' objects>,
              '__len__': <slot wrapper '__len__' of 'xml.etree.ElementTree.Element' objects>,
              '__new__': <built-in method __new__ of type object at 0x92a480>,
              '__repr__': <slot wrapper '__repr__' of 'xml.etree.ElementTree.Element' objects>,
              '__setitem__': <slot wrapper '__setitem__' of 'xml.etree.ElementTree.Element' objects>,
              '__setstate__': <method '__setstate__' of 'xml.etree.ElementTree.Element' objects>,
              '__sizeof__': <method '__sizeof__' of 'xml.etree.ElementTree.Element' objects>,
              'append': <method 'append' of 'xml.etree.ElementTree.Element' objects>,
              'attrib': <attribute 'attrib' of 'xml.etree.ElementTree.Element' objects>,
              'clear': <method 'clear' of 'xml.etree.ElementTree.Element' objects>,
              'extend': <method 'extend' of 'xml.etree.ElementTree.Element' objects>,
              'find': <method 'find' of 'xml.etree.ElementTree.Element' objects>,
              'findall': <method 'findall' of 'xml.etree.ElementTree.Element' objects>,
              'findtext': <method 'findtext' of 'xml.etree.ElementTree.Element' objects>,
              'get': <method 'get' of 'xml.etree.ElementTree.Element' objects>,
              'getchildren': <method 'getchildren' of 'xml.etree.ElementTree.Element' objects>,
              'getiterator': <method 'getiterator' of 'xml.etree.ElementTree.Element' objects>,
              'insert': <method 'insert' of 'xml.etree.ElementTree.Element' objects>,
              'items': <method 'items' of 'xml.etree.ElementTree.Element' objects>,
              'iter': <method 'iter' of 'xml.etree.ElementTree.Element' objects>,
              'iterfind': <method 'iterfind' of 'xml.etree.ElementTree.Element' objects>,
              'itertext': <method 'itertext' of 'xml.etree.ElementTree.Element' objects>,
              'keys': <method 'keys' of 'xml.etree.ElementTree.Element' objects>,
              'makeelement': <method 'makeelement' of 'xml.etree.ElementTree.Element' objects>,
              'remove': <method 'remove' of 'xml.etree.ElementTree.Element' objects>,
              'set': <method 'set' of 'xml.etree.ElementTree.Element' objects>,
              'tag': <attribute 'tag' of 'xml.etree.ElementTree.Element' objects>,
              'tail': <attribute 'tail' of 'xml.etree.ElementTree.Element' objects>,
              'text': <attribute 'text' of 'xml.etree.ElementTree.Element' objects>})

所以tag 不是实例变量，它是类变量，这就是为什么它出现在类的__dict__ 中，而不是出现在e 的__dict__ 中。

其实ET.Element(e)的实例没有任何实例变量，只有ET.Element中定义的类变量。

---

我不确定为什么库类没有__dict__ 而用户定义的类有，但我认为它与this answer 有关：

[...] 这些通常是本机编码的（即用 C 编写的）——并且 __dict__ 属性必须被显式创建——因此与用 Python 编码的类不同——必须不遗余力地允许 __dict__ 进入这些类，而对于 Python 代码，绕道是避免 __dict__。