xml.etree.ElementTree: get() doesn't return default value, always ATTLIST value

[padremayi]

BPO	46798
Nosy	@scoder, @jacobtylerwalls

^{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 = <Date 2022-03-05.12:28:49.306>
created_at = <Date 2022-02-19.11:31:31.553>
labels = ['expert-XML', 'invalid', 'type-bug', '3.8']
title = "xml.etree.ElementTree: get() doesn't return default value, always ATTLIST value"
updated_at = <Date 2022-03-05.12:28:49.306>
user = 'https://bugs.python.org/padremayi'

bugs.python.org fields:

activity = <Date 2022-03-05.12:28:49.306>
actor = 'scoder'
assignee = 'none'
closed = True
closed_date = <Date 2022-03-05.12:28:49.306>
closer = 'scoder'
components = ['XML']
creation = <Date 2022-02-19.11:31:31.553>
creator = 'padremayi'
dependencies = []
files = []
hgrepos = []
issue_num = 46798
keywords = []
message_count = 6.0
messages = ['413543', '413706', '413780', '413782', '413785', '414556']
nosy_count = 4.0
nosy_names = ['scoder', 'eli.bendersky', 'jacobtylerwalls', 'padremayi']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue46798'
versions = ['Python 3.8']

[padremayi]

XML test file:

<?xml version="1.0"?>
<!DOCTYPE main [
<!ELEMENT main (object+)>
<!ELEMENT object (description, year, manufacturer)>
<!ATTLIST object name CDATA #REQUIRED>
<!ATTLIST object works (yes|no) "yes">
<!ELEMENT description (#PCDATA)>
<!ELEMENT year (#PCDATA)>
<!ELEMENT manufacturer (#PCDATA)>
]>

<main>
<object name="My object">
<description>This is a simple object</description>
<year>2022</year>
<manufacturer>Myself</manufacturer>
</object>
</main>

Python code:
import xml.etree.ElementTree

try:
xml_data = xml.etree.ElementTree.iterparse("test.xml", events=("start", "end"))

    for event, xml_tag in xml_data:
        if event == "end" and xml_tag.tag == "object":
            object_name = xml_tag.get("name")
            object_description = xml_tag.find("description").text
            works = xml_tag.get("works", default="foo")

            print("works value: " + str(works))

            xml_tag.clear()

    print("Done!")

except (NameError, xml.etree.ElementTree.ParseError):
    print("XML error!")

Output:
works value: yes
Done!

Expected behaviour:
works value: foo
Done!

[scoder]

The question here is simply, which is considered more important: the default provided by the document, or the default provided by Python. I don't think it's a clear choice, but the way it is now does not seem unreasonable. Changing it would mean deliberate breakage of existing code that relies on the existing behaviour, and I do not see a reason to do that.

[padremayi]

IMHO if the developer doesn't manage the XML itself it is VERY unreasonable to use the document value and not the developer one. At the moment the developer must predict the future changes on XML structure.

For my point of view if an attribute is not present get() must return None (or the default value passed by developer) AND the document default adding an optional parameter to get() call: if True return 2 values, otherwise return the document one (current behaviour).

In this way the old code continue to work