`xml.dom.domreg.getDOMImplementation`: fallback discovery masks genuine bugs, not just "not installed"

[tobiaspal]

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

[picnixz]

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.

[picnixz]

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.