aio-libs / multidict / 37198908438

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Keep the items when a multidict is re-initialized from itself with kwargs (#1666)

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## What do these changes do?

`d.__init__(d, key=value)` and `d.__init__(proxy_of_d, key=value)`
emptied `d` on the C extension and left only the keyword arguments. With
keyword arguments the clone fast path is skipped, so
`_multidict_tp_init()` called `md_init()`, which replaced the table, and
then copied "from itself" out of the now empty table. The fix keeps the
table in that case and only reserves room for the keyword arguments
before adding them.

The pure-Python implementation snapshots its own items first and was
already correct, so it is unchanged; the new test runs on both backends.

## Are there changes in behavior for the user?

Yes, on the C extension only: re-initializing a multidict from itself
with keyword arguments keeps the existing items, as the pure-Python
implementation does.

## Is it a substantial burden for the maintainers to support this?

No.

## Related issue number

N/A

## Checklist

- [x] I think the code is well written
- [x] Unit tests for the changes exist
- [x] Documentation reflects the changes (N/A, no documented behaviour
changed)
- [ ] If you provide code modification, please add yourself to
`CONTRIBUTORS.txt` (N/A)
- [x] Add a new news fragment into the `CHANGES/` folder
- [x] `make doc-spelling` passes and any new technical words are added
to `docs/spelling_wordlist.txt`

Drafted with Claude Code (Claude Fable 5.1); not yet reviewed by a
human.

<details>
<summary>Agent run details (optional, for reviewers)</summary>

The new test failed on the C extension before the fix (4 of 8 cases) and
passes after it.

- C extension, 3.14.7: `pytest tests -q --no-cov`, 4075 passed, 179
skipped
- C extension, 3.10.17: 4079 passed, 175 skipped
- C extension, 3.14.7t: 4060 passed, 194 skipped
- Pure Python, 3.14.7, clean tree: `MULTIDICT_NO_EXTENSIONS=1 pytest
tests -q --no-cov --no-c-extensions`, 1604 p... (continued)

792 of 1584 branches covered (50.0%)

Branch coverage included in aggregate %.

15 of 20 new or added lines in 2 files covered. (75.0%)

9315 of 10167 relevant lines covered (91.62%)

1.83 hits per line