MigrationWriter silently serializes Python 3.15 frozendict values as tuples of keys

[amanjain]

On Python 3.15.0, Django's migration serializer treats built-in frozendict instances as generic iterables, preserving only their keys and silently discarding their values.

Reproduced against Django main at d430084791.

Minimal reproducer:

from django.db.migrations.writer import MigrationWriter

MigrationWriter.serialize(frozendict(answer=42))

Actual result:

("('answer',)", set())

Expected behavior: preserve the mapping's keys and values instead of serializing it as a tuple of keys.

This affects field defaults:

models.JSONField(default=frozendict(answer=42))

The generated migration contains:

models.JSONField(default=('answer',))

The database consequence was reproduced on SQLite: executing a generated AddField migration against an existing row stores ["answer"] instead of {"answer": 42}.

The serializer registry handles dict explicitly, but frozendict is not a dict subclass and falls through to IterableSerializer.

A proposed patch adds a dedicated serializer, regression tests in the migration writer and operation suites, and documentation noting that generated migrations containing frozendict require Python 3.15+.

Validation:

• Python 3.15: 75 writer/regression tests passed.
• Python 3.15: 181 migration operation tests ran successfully, with 2 skipped.
• Python 3.14: 75 writer/regression tests ran successfully, with 3 skipped.
• Formatting and whitespace checks passed.
• The full Django test suite has not been run.

AI disclosure: OpenAI Codex (GPT-6) assisted with investigation, implementation, regression tests, documentation, and commit preparation.

[amanjain]

Proposed patch: ​https://github.com/django/django/pull/22157

Regression tests reproduce both the incorrect serialization and the incorrect JSON stored for existing rows.

[Jacob Walls]

Thanks, I see we need similar special handling just like frozenset. We'll backport this to 6.1 as part of 3.15 support. You don't need a release note, as we caught it before the first release declaring support for 3.15.

[Jacob Walls <jacobtylerwalls@…>]

In 2fff3e9f:

Fixed #37431 -- Added support for serializing frozendicts in migrations.

[Jacob Walls <jacobtylerwalls@…>]

In 9f52f9a:

[6.1.x] Fixed #37431 -- Added support for serializing frozendicts in migrations.

Backport of 2fff3e9f493a9c85fb8a2294358c6751575ee4bf from main.