Fix _ensure_container dropping flags when target is already a config (#580)

[jbbqqf]

Summary

omegaconf._utils._ensure_container silently dropped its flags argument when the input was already a DictConfig or ListConfig. The first two branches construct fresh containers via OmegaConf.create(..., flags=flags) / OmegaConf.structured(..., flags=flags), but the third branch (already-a-config) returned the target unchanged. This PR applies the flags in that path so the contract is uniform.

Fixes #580.

Context

Issue #580 (filed by @Jasha10) shows the divergence:

• _ensure_container({...}, flags={"my_flag": True}) → flag set
• _ensure_container(DataclassUser, flags={"my_flag": True}) → flag set
• _ensure_container(DictConfig({...}), flags={"my_flag": True}) → flag silently dropped

The function is internal but its flags: Optional[Dict[str, bool]] parameter advertises a single post-condition; this fix makes the four input shapes behave consistently.

Changes

• omegaconf/_utils.py — split the not OmegaConf.is_config(target) branch into elif OmegaConf.is_config(target): apply flags + else: raise ValueError. Inline comment points at the issue and explains why the explicit _set_flag loop is needed (the primitive/structured branches never run for already-built configs).
• tests/test_utils.py — new parametrized regression test_ensure_container_applies_flags covering dict, list, DictConfig, ListConfig inputs.
• news/580.bugfix — towncrier entry.

Reproduce BEFORE/AFTER yourself (copy-paste)

# --- one-time setup ---
git clone https://github.com/omry/omegaconf.git /tmp/repro-580 && cd /tmp/repro-580
python -m venv .venv && . .venv/bin/activate
pip install -e . pyyaml attrs pytest pytest-mock pytest-benchmark pytest-timeout setuptools antlr4-tools antlr4-python3-runtime
python setup.py antlr

# Grab the regression test from this PR so BEFORE and AFTER run the same command.
git fetch https://github.com/jbbqqf/omegaconf.git fix/580-ensure-container-flags
git checkout FETCH_HEAD -- tests/test_utils.py

# --- BEFORE (origin/main) ---
git checkout origin/main -- omegaconf/_utils.py
pytest tests/test_utils.py::test_ensure_container_applies_flags -v
# Expected: 2 failures on the `dictconfig` and `listconfig` parametrizations
#           ("AssertionError: assert None is True").

# --- AFTER (this PR) ---
git checkout FETCH_HEAD -- omegaconf/_utils.py
pytest tests/test_utils.py::test_ensure_container_applies_flags -v
# Expected: 4 passed.

What I ran locally

• pytest tests/test_utils.py::test_ensure_container_applies_flags -v → 4 passed on the branch, 2 failed on origin/main (the dictconfig / listconfig parametrizations).
• pytest tests/test_utils.py tests/test_create.py tests/test_basic_ops_dict.py tests/test_basic_ops_list.py -q → 1607 passed, 1 skipped on the branch.

Edge cases tested

#	Scenario	Input	Expected	Verified by
1	Primitive dict	{"name": "bond", "age": 7}	flag applied via OmegaConf.create(..., flags=...) (unchanged path)	test_ensure_container_applies_flags[dict]
2	Primitive list	[1, 2, 3]	flag applied via OmegaConf.create(..., flags=...) (unchanged path)	test_ensure_container_applies_flags[list]
3	Pre-built DictConfig	DictConfig(content={"name": "bond", "age": 7})	flag applied via the new _set_flag loop	test_ensure_container_applies_flags[dictconfig] (was failing on main)
4	Pre-built ListConfig	ListConfig(content=[1, 2, 3])	flag applied via the new _set_flag loop	test_ensure_container_applies_flags[listconfig] (was failing on main)
5	Invalid input	"abc"	unchanged: still raises ValueError	existing test_ensure_container_raises_ValueError still passes

Risk / blast radius

Narrow. All current internal callers of _ensure_container (omegaconf/omegaconf.py lines 430, 472, 1112, 1165) invoke it without flags=, so the new branch is only entered when an external user (or future caller) explicitly passes flags with a pre-built config — exactly the scenario that was silently broken. Mutation is in-place, matching how _set_flag works elsewhere; no new copy semantics.

Release note

Fix `omegaconf._utils._ensure_container` silently dropping the `flags` argument when called with a pre-built `DictConfig` or `ListConfig`.

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PR drafted with assistance from Claude Code. The change was reviewed manually against omry/omegaconf's source and the issue body. The reproducer block above was used during development; it is the same one a reviewer can paste verbatim.