Why don't instances of `xml.etree.ElementTree.Element` have `__dict__`?

Question

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

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

Copy C:\Python39\Lib\xml\etree\ElementTree.py somewhere else:

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

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

Answer 1

This answer explains well the __dict__ attribute:

Python objects store their instance variables in a dictionary that belongs to the object. vars(x) returns this dictionary (as does x.__dict__). dir(x), on the other hand, returns a dictionary of x's "attributes, its class's attributes, and recursively the attributes of its class's base classes."

So let's see this in practice:

import xml.etree.ElementTree as ET

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

But why, since e does have attributes?

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

Let's see the attributes from its class:

import xml.etree.ElementTree as ET

print(ET.Element.__dict__)

This will output

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>})

So tag is not an instance variable, it's a class variable, that's why it's present in the class' __dict__, not in the e's __dict__.

In fact, the instance of ET.Element (e) does not have any instance variables, but only class variables defined in the ET.Element.

---

I'm not sure about why the library class doesn't have a __dict__ and a user defined class has, but I think it's related to this answer:

[...] These are usually natively coded (i.e. written in C) - and the __dict__ attribute would have to be explicitly created - so unlike classes coded in Python - one have to go out of his way to allow __dict__ in these classes, while for Python code, the detour is on avoiding __dict__.

Answer 2

The xml.etree.ElementTree has an 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)

Notably, C extension types generally do not have a __dict__ slot. By copying the Python implementation of the class without loading of the C implementation, one is using the original Python implementation.

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