Return a single key from key() for a single-element list (fixes #430)

[tienvantranspk-tech]

Fixes #430.

Problem

key(["my_key"]) builds a single-element DottedKey, which behaves differently from key("my_key") (a SingleKey). For example:

import tomlkit
doc = tomlkit.document()
doc.append(tomlkit.key(["my_key"]), "value")
"my_key" in doc        # -> False  (key("my_key") gives True)

Fix

In key(), when the iterable yields exactly one key, return that SingleKey instead of wrapping it in a DottedKey. Multi-element lists are unchanged (still dotted keys).

Tests

Added a regression test. The existing suite passes (333 passed, excluding the toml-test git submodule which is not initialised in my checkout), and ruff check / ruff format are clean.

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Fixes creation of TOML keys so that a one-element key list behaves like a plain SingleKey (instead of a single-element DottedKey), aligning behavior with issue #430 and expected document interactions.

Changes:

• Update tomlkit.key() to return SingleKey when the iterable contains exactly one element.
• Add regression coverage ensuring one-element key lists serialize and behave like plain keys, while multi-element lists remain dotted.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
tomlkit/api.py	Adjusts key() behavior for single-element iterables to avoid creating single-element dotted keys.
tests/test_api.py	Adds a regression test covering the new single-element iterable behavior and preserves dotted behavior for multi-element lists.

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