Fix dot-notation read on $@ref-backed ConfigParser proxies

[VenkateswarluNagineni]

Description

_ConfigProxy (added in #8858) resolves a dotted key by chaining to get_parsed_content, and falls back to the underlying container when the chained id is not in the resolver:

try:
    return self._chain(key)
except KeyError:
    return getattr(self._value, key)

A proxy backed by a $@ref wraps the parsed value of the referenced node, but that node's children have no ids of their own, so alias::x is absent from the resolver. The fallback then looks x up as a dict attribute rather than a key, and dot-notation fails on a value that bracket-notation returns happily:

parser = ConfigParser(config={"target": {"x": 1, "y": 2}, "alias": "$@target"}, globals={"monai": "monai"})

parser.alias["x"]   # -> 1
parser.alias.x      # -> AttributeError: 'dict' object has no attribute 'x'

This also affects chained refs ("alias": "$@mid", "mid": "$@target").

Ref-backed proxies are already treated as first-class elsewhere: _backing_id() resolves the full $@ref chain for writes, and test_ref_backed_proxy_write_through covers parser.alias["x"] reads and writes. The dot-notation read is the one path that was not covered, and it diverges. It also contradicts the documented precedence rule on the class ("Config keys take precedence over dict/list attributes and methods") — here x is a key of the aliased node, but the dict attribute lookup wins and raises.

Proposed changes

Make __getattr__'s fallback mirror the one __getitem__ already uses: if the chained id is absent but the key exists in the underlying container, return self._value[key]. Keys that are not in the container still fall through to getattr, so container methods (.keys(), .items(), …) are unaffected, as is the existing "config key shadows a same-named dict method" behaviour.

The change is confined to the except KeyError fallback, so any id that resolves today keeps resolving through _chain exactly as before — no behaviour change for non-ref proxies.

How did you test it?

• Added test_ref_backed_proxy_attribute_read and test_chained_ref_backed_proxy_attribute_read next to the existing ref write-through tests. Both fail without the source change (AttributeError) and pass with it; they assert parser.alias.x == parser.alias["x"], and that .keys() still resolves.
• tests/bundle/test_config_parser.py: 34 passed before, 36 passed after (2 new), no regressions.
• Whole tests/bundle/ suite: identical results before and after the change (the only failures are pre-existing network-dependent test_bundle_download cases, unchanged by this PR).
• ruff check / ruff format --check (repo-pinned 0.15.20), black, isort clean on both files; mypy monai/bundle/config_parser.py reports the same single pre-existing yaml.safe_dump error as dev, no new ones.

Notes for the reviewer

The fallback returns the raw value, matching __getitem__'s fallback rather than wrapping it in a new proxy — this keeps the two notations exactly consistent and the change minimal. Happy to wrap the result in _wrap_parsed instead if you'd prefer deeper dot-chaining through refs, though that would make dot- and bracket-notation diverge again in the other direction.

Types of changes

• Non-breaking change (fix or new feature that would not break existing functionality).
• Breaking change (fix or new feature that would cause existing functionality to change).
• New tests added to cover the changes.
• Integration tests passed locally by running ./runtests.sh -f -u --net --coverage.
• Quick tests passed locally by running ./runtests.sh --quick --unittests --disttests. (Ran the affected suites directly with pytest on Windows, plus ruff/black/isort/mypy, rather than runtests.sh; details above.)
• In-line docstrings updated.
• Documentation updated, tested make html command in the docs/ folder.

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📒 Files selected for processing (2)

• monai/bundle/config_parser.py
• tests/bundle/test_config_parser.py

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📝 Walkthrough

Walkthrough

Updated _ConfigProxy.__getattr__ to return literal dictionary entries when chained configuration lookup cannot resolve a key, otherwise preserving attribute fallback behavior. Added tests confirming direct and chained $@ref proxies provide matching dot- and bracket-notation reads, including container method fallback.

Estimated code review effort: 2 (Simple) | ~10 minutes

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 16.67% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately and concisely summarizes the main change: fixing dot-notation reads on $@ref-backed ConfigParser proxies.
Description check	✅ Passed	The description is detailed and follows the template sections, including changes, tests, and change-type checkboxes; only the issue link is left blank.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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