Fix allow_null=True fields skipped from validated_data when partial=False

[suportly]

Description

Fixes an issue where serializer fields with allow_null=True and required=False (but no explicit default) are silently omitted from validated_data when partial=False, effectively allowing unintended partial updates.

refs #9501

Root Cause

In Field.validate_empty_values(), when input data is missing (empty), partial=False, and required=False, the method calls self.get_default(). Since allow_null=True does not set an explicit default value, get_default() raises SkipField, and the field is silently omitted from validated_data — identical to partial=True behavior.

Fix

In validate_empty_values(), catch SkipField from get_default() and return (True, None) when the field has allow_null=True. This ensures nullable fields appear in validated_data during full (non-partial) updates. For non-nullable fields, existing behavior is preserved (re-raise SkipField).

# Before
return (True, self.get_default())

# After
try:
    return (True, self.get_default())
except SkipField:
    if self.allow_null:
        return (True, None)
    raise

Test plan

• Added 6 new test cases in TestAllowNullNotRequiredInclusions:
  • Missing nullable field on create → included as None
  • Missing nullable field on full update → included as None
  • Missing nullable field on partial update → still skipped (preserved)
  • Explicit null value → works as before
  • Explicit default takes precedence over None
  • Non-nullable required=False → still skipped (preserved)
• All 1476 existing tests pass with no regressions
• All 6 new tests pass

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This pull request fixes a bug where serializer fields with allow_null=True and required=False (but no explicit default) were being silently omitted from validated_data during full updates (partial=False), causing unintended partial update behavior. The fix ensures these nullable fields are included in validated_data with a None value during full updates, while preserving the existing behavior for partial updates.

Changes:

• Modified Field.validate_empty_values() to catch SkipField exceptions and return None for nullable fields during full updates
• Added comprehensive test coverage with 6 new test cases covering create, update, partial update, explicit null, explicit default, and non-nullable scenarios

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
rest_framework/fields.py	Added try-except block in validate_empty_values() to handle SkipField from get_default() and return (True, None) for fields with allow_null=True
tests/test_serializer.py	Added TestAllowNullNotRequiredInclusions test class with 6 comprehensive test cases validating the fix across different scenarios

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[browniebroke]

Thanks for raising this old issue to my attention again. Closing as per: #9501 (comment)