Description

tobiaspal

opened on Aug 17, 2026

Issue body actions

Bug report

Bug description:

• Location: Lib/xml/dom/domreg.py:73-76

cpython/Lib/xml/dom/domreg.py

Lines 72 to 78 in cbe3a7f

	for creator in well_known_implementations.keys():
	try:
	dom = getDOMImplementation(name = creator)
	except Exception: # typically ImportError, or AttributeError
	continue
	if _good_enough(dom, features):
	return dom

• Problem: When called with no explicit implementation name, tries each well-known implementation (minidom, 4DOM) in turn under except Exception: continue (comment: "typically ImportError, or AttributeError"). A genuine bug in a candidate's getDOMImplementation() factory (e.g. a real AttributeError from a typo) is caught identically to "this implementation isn't available" and silently skipped.
• Reproducer (verified): registered a fake well-known implementation whose getDOMImplementation() deliberately raises AttributeError to simulate a real bug (not a missing-module ImportError); the fallback loop silently moved on to minidom with no trace of the failure.
• Suggested fix: Catch (ImportError, AttributeError) specifically, matching what the comment already says is "typical."
• Difficulty: Trivial. Note: low real-world impact since well_known_implementations is a tiny, fixed, mostly-dead-code dict — good for a first-time contributor precisely because it's low-stakes.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-155944: Catch only expected errors in getDOMImplementation fallback #155948
• gh-155944: narrow xml.dom fallback discovery to ImportError and AttributeError #157980

Activity

1. tobiaspal

   added

   type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

   on Aug 17, 2026
2. picnixz commented on Aug 17, 2026

   

   picnixz

   on Aug 17, 2026

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   A genuine bug in a candidate's getDOMImplementation() factory (e.g. a real AttributeError from a typo) is caught identically to "this implementation isn't available" and silently skipped.

   I don't think we should use it then. A faulty implementation is not a good enough implementation.

   Reacted by Stan Ulbrych and Paul Tobias
3. picnixz

   added

   pendingThe issue will be closed if no feedback is providedThe issue will be closed if no feedback is provided

   on Aug 17, 2026
4. picnixz commented on Aug 17, 2026

   

   picnixz

   on Aug 17, 2026

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   We should just hope that getDOMImplementation is not faulty there. In particular, with sufficient test coverage, it should be fine. As such, if there's not enough coverage, you can add tests instead.
5. picnixz

   added

   testsTests in the Lib/test dirTests in the Lib/test dir

   type-featureA feature request or enhancementA feature request or enhancement

   pendingThe issue will be closed if no feedback is providedThe issue will be closed if no feedback is provided

   and removed

   type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

   pendingThe issue will be closed if no feedback is providedThe issue will be closed if no feedback is provided

   on Aug 17, 2026
6. shaneraphel

   added a commit that references this issue on Sep 24, 2026

   pythongh-155944: skip a failing DOM candidate without hiding a later …

   a2a7795
7. kevinprimeraquecreo-stack

   added this to Project_VKon Oct 18, 2026
8. github-project-automation

   moved this to In progress in Project_VKon Oct 18, 2026
9. kevinprimeraquecreo-stack

   added this to Project_VKon Oct 18, 2026

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