fix: MultipleChoiceField use ordered sort

[fbozhang]

This modification has the following advantages:

1. Maintain the input order.
2. Prevent the output from being a set, which would cause issues with JSON serialization (e.g., when passing parameters to certain third-party SDKs that may internally use json.dumps, and I cannot modify the third party's encoder).

[auvipy]

please also add unit tests to verify this and make the CI green

[fbozhang]

please also add unit tests to verify this and make the CI green

I’ve added the tests and the CI is green now.

[browniebroke]

Seems sensible to me 👍🏻

However, it will have to wait for 3.17 as I see this as potentially disruptive for some downstream users. Left a small suggestion

[fbozhang]

Seems sensible to me 👍🏻

However, it will have to wait for 3.17 as I see this as potentially disruptive for some downstream users. Left a small suggestion

Good point — waiting for 3.17 sounds like the right call. Appreciate the suggestion!

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[Copilot]

Pull request overview

This PR modifies MultipleChoiceField to return ordered lists instead of sets, ensuring JSON serialization compatibility and maintaining the order of input values. The change uses dict.fromkeys() to deduplicate values while preserving insertion order.

Key Changes

• Changed to_internal_value() and to_representation() methods to return lists using list(dict.fromkeys([...]))
• Updated all test expectations from sets to lists
• Added JSON serialization tests to prevent regression

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
rest_framework/fields.py	Modified MultipleChoiceField methods to return ordered lists instead of sets using dict.fromkeys() for deduplication
tests/test_fields.py	Updated test expectations from sets to lists, added order-preserving test case, and added JSON serialization validation tests
tests/test_serializer_nested.py	Updated nested serializer test assertions to expect lists instead of sets

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.