[issue28236] In xml.etree.ElementTree Element can be created with empty and None tag

[Raymond Hettinger]

Raymond Hettinger  added the comment:

Marking this as closed.

Gordon, thank you for showing an interest in this tracker item.  While fixing 
bugs is of interest, altering long standing intentional design decisions is not 
useful.  The time to do that is before a module is released, not a decade later.

--
resolution:  -> not a bug
stage:  -> resolved
status: open -> closed

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[issue28236] In xml.etree.ElementTree Element can be created with empty and None tag

[Stefan Behnel]

Stefan Behnel  added the comment:

>That said, I personally question the implementation decision to
>represent things like treating comments as an Element with a tag of a
>Comment function. 

This is not going to change.

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[issue28236] In xml.etree.ElementTree Element can be created with empty and None tag

[Gordon P. Hemsley]

Gordon P. Hemsley  added the comment:

Issues of potential relevance to this discussion:
* Issue28237 - In xml.etree.ElementTree bytes tag or attributes raises on 
serialization
* Issue5166 - ElementTree and minidom don't prevent creation of not well-formed 
XML

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[issue28236] In xml.etree.ElementTree Element can be created with empty and None tag

[Gordon P. Hemsley]

Gordon P. Hemsley  added the comment:

To be clear, we are talking about the Element class of the ElementTree module, 
which is distinct from the ElementTree class of the same module.

That said, I personally question the implementation decision to represent 
things like treating comments as an Element with a tag of a Comment function. 
The XML standard is pretty clear that neither comments nor processing 
instructions are in fact elements, and I don't see it as a Good Thing that 
arbitrary objects are allowed as the value of tag, unless there is a 
requirement that such objects are subclasses of str.

Note also that the documentation makes no mention of tag being anything other 
than a string. And there is inconsistency with where bytes are supposedly 
allowed (according to the documentation) and where they're actually allowed 
(according to the code). Given this, I think it's hard to say what user code is 
expected to make use of.

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[issue28236] In xml.etree.ElementTree Element can be created with empty and None tag

[Stefan Behnel]

Stefan Behnel  added the comment:

I also consider it an actual feature of ElementTree to allow arbitrary objects 
as its tags, even if it's not one of the most prominent. lxml cannot copy this 
because it is based on C libraries internally, but that shouldn't prevent ET 
from allowing it.

The fact that None tags disappear is also definitely a feature. It's an easy 
way to delete tags from trees without requiring any restructuring.

OTOH, whether an empty string should be serialised in the way the OP shows is 
not so clear. The output is not XML. I can't see any use case for this, but it 
feels like a potential source of bugs. I think it would be better to have the 
serialiser explicitly reject this than letting it silently generate broken 
output.

Not something to change in Py3.6, though.

--
versions: +Python 3.7 -Python 3.6

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[issue28236] In xml.etree.ElementTree Element can be created with empty and None tag

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

I concur with Raymond. Supporting non-string tags is a feature of ElementTree 
that is used internally (for comments, etc) and can be used in user code. And 
the C implementation intentionally reproduces this feature.

--
nosy: +eli.bendersky, scoder, serhiy.storchaka

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[issue28236] In xml.etree.ElementTree Element can be created with empty and None tag

[Gordon P. Hemsley]

Gordon P. Hemsley  added the comment:

I disagree. This library is meant to be an interface onto XML syntax, and XML 
has pretty strict requirements on syntax. As msg277125 shows, you're liable to 
get very far downstream before the error becomes apparent.

In addition, I'm finding a number of internal inconsistencies, both between the 
docs and the code and between the Python code and the C code, that demonstrate 
that doing these type checks up front would be beneficial to the entire 
library. (Note: The C code also does not do them.)

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[issue28236] In xml.etree.ElementTree Element can be created with empty and None tag

[Raymond Hettinger]

Raymond Hettinger  added the comment:

I don't think this is worth fixing.  The package is under no obligation to make 
early type checks for arguments.  It is typical in the Python world to let 
those kinds of input errors surface downstream when they are used.  In 
contrast, C code typically does the checks when the arguments are passed in.

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nosy: +rhettinger

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[issue28236] In xml.etree.ElementTree Element can be created with empty and None tag

[Gordon P. Hemsley]

Gordon P. Hemsley  added the comment:

I decided to take a look at this, since it seems easy...

At first glance, this would appear to be a straightforward change--the docs 
state in multiple places that Element() takes a string as its tag argument.

But it turns out that a lot of internal functionality depends on passing in 
non-strings as the tag value.

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nosy: +gphemsley

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[issue28236] In xml.etree.ElementTree Element can be created with empty and None tag

[py.user]

New submission from py.user:

It is possible to create and serialize an Element instance with empty string 
tag value:

>>> import xml.etree.ElementTree as etree
>>>
>>> root = etree.Element('')
>>> elem = etree.SubElement(root, '')
>>>
>>> root

>>> elem

>>>
>>> etree.tostring(root)
b'<>< />'
>>> etree.dump(root)
<>< />
>>>


It is possible to create and serialize an Element instance with None tag value:

>>> import xml.etree.ElementTree as etree
>>>
>>> root = etree.Element(None)
>>> elem = etree.SubElement(root, None)
>>>
>>> root

>>> root[0]

>>> len(root)
1
>>> etree.tostring(root)
b''
>>> etree.dump(root)

>>>


And same try with site package lxml raises an exception both for empty string 
and for None:

>>> import lxml.etree
>>>
>>> lxml.etree.Element('')
Traceback (most recent call last):
  File "", line 1, in 
  File "lxml.etree.pyx", line 2809, in lxml.etree.Element 
(src/lxml/lxml.etree.c:61393)
  File "apihelpers.pxi", line 87, in lxml.etree._makeElement 
(src/lxml/lxml.etree.c:13390)
  File "apihelpers.pxi", line 1446, in lxml.etree._getNsTag 
(src/lxml/lxml.etree.c:25978)
  File "apihelpers.pxi", line 1481, in lxml.etree.__getNsTag 
(src/lxml/lxml.etree.c:26304)
ValueError: Empty tag name
>>>
>>> lxml.etree.Element(None)
Traceback (most recent call last):
  File "", line 1, in 
  File "lxml.etree.pyx", line 2809, in lxml.etree.Element 
(src/lxml/lxml.etree.c:61393)
  File "apihelpers.pxi", line 87, in lxml.etree._makeElement 
(src/lxml/lxml.etree.c:13390)
  File "apihelpers.pxi", line 1446, in lxml.etree._getNsTag 
(src/lxml/lxml.etree.c:25978)
  File "apihelpers.pxi", line 1464, in lxml.etree.__getNsTag 
(src/lxml/lxml.etree.c:26114)
  File "apihelpers.pxi", line 1342, in lxml.etree._utf8 
(src/lxml/lxml.etree.c:24770)
TypeError: Argument must be bytes or unicode, got 'NoneType'
>>>

--
components: Library (Lib), XML
messages: 277125
nosy: py.user
priority: normal
severity: normal
status: open
title: In xml.etree.ElementTree Element can be created with empty and None tag
type: behavior
versions: Python 3.6

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