xml.dom.minidom.Element.ownerDocument is hidden

[krocard]

BPO	23436
Nosy	@loewis, @bitdancer, @florentx

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2015-02-10.19:30:43.712>
labels = ['type-feature', 'library']
title = 'xml.dom.minidom.Element.ownerDocument is hidden'
updated_at = <Date 2020-11-13.06:13:51.337>
user = 'https://bugs.python.org/krocard'

bugs.python.org fields:

activity = <Date 2020-11-13.06:13:51.337>
actor = 'vapier'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2015-02-10.19:30:43.712>
creator = 'krocard'
dependencies = []
files = []
hgrepos = []
issue_num = 23436
keywords = []
message_count = 2.0
messages = ['235700', '236366']
nosy_count = 4.0
nosy_names = ['loewis', 'r.david.murray', 'flox', 'krocard']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue23436'
versions = ['Python 3.4']

[krocard]

Extracted from xml.dom.minidom:

Node(...):
   ...
   ownerDocument = None
   ...

Element(Node):
    \_\_slots__=('ownerDocument', ...)
    ...
\~\~\~

As Element declares an ownerDocument attribute in \_\_slots__, Node's ownerDocument attribute is hidden:

class B: b=1;
class D(B): __slots__={'b'}
D().b -> AttributeError
~~~

This leads to a strange behaviour were accessing a base attribute fails with an attribute error.

Should the Node.ownerDocument attribute not be removed?
Or its name removed from the Element.__slots__ list?
Ie have the attribute in the base or the derivative, but not both.

Independent note: <https://docs.python.org/3/reference/datamodel.html#slots\> says:

When inheriting from a class without __slots__ [Node], the __dict__ attribute of that class will always be accessible, so a __slots__ definition in the subclass [Element] is meaningless.

So as for as I understand Element.__slots__ does not reduce the Element() footprint (it was introduced for that).

[bitdancer]

It looks like the __slots__ declaration on Node was missed when the other slots changes were made (by MvL in 3931f043b79a). Note that the Node base class (xml.dom.Node) has a __slots__ declaration, but it was added after MvL's patch (by Florent in 49c5511b234a).

Changing this now may have backward compatibility issues, though.