[kushaldave002]

Summary

Allow Table.setdefault(key) and InlineTable.setdefault(key) to retrieve existing values without requiring an explicit default, matching dict and TOMLDocument behavior.

Missing-key calls still reject unsupported None values without modifying the document.

Add eight regression cases covering falsy values, normal and inline tables, formatting preservation, and failed insertion.

Validation:

• 1,443 tests pass.
• All configured pre-commit hooks pass.
• Mypy reports the same 33 errors as unchanged upstream; no new errors.

Duplicate Check

• I searched the open PRs and found none that already fixes this issue.
• Related open PRs (if any, and why this PR is still needed): None found in repository searches for setdefault.

Agent Drafting Metadata

• Agent: OpenAI Codex
• Model: Exact deployment identifier unavailable.
• Notes: Assisted with implementation, regression tests, and verification. The contributor confirmed review and approval.

[human-attestation]

✅ Human Attestation: @kushaldave002 attested this PR at commit 7caff0d (2026-11-10T15:01:45.544Z).

I confirm the code in this PR was written by a human. / 我确认此 PR 中的代码均由人类编写。

Later pushes to this PR stay attested. Attestation page