Fix crash when parsing enum containing only null value

[koxudaxi]

Summary

• Fix black.parsing.InvalidInput crash when parsing OpenAPI schema with enum: [null] and nullable: true
• Generate RootModel[None] instead of empty Enum class for null-only enums

Fixes #1969

Summary by CodeRabbit

• New Features

  • Added support for null-only and empty enums in OpenAPI schemas, generating appropriate nullable type models.

• Tests

  • Added test coverage for empty enum parsing and OpenAPI null-only enum code generation scenarios.

_{✏️ Tip: You can customize this high-level summary in your review settings.}

[coderabbitai]

Walkthrough

This change adds support for empty enum schemas in OpenAPI/JSON Schema by modifying the enum parsing logic to generate either a null type reference or a synthetic model with a null-type root field, preventing the generation of invalid empty Enum classes that cause Black parsing failures.

Changes

Cohort / File(s)	Summary
Empty enum handling logic src/datamodel_code_generator/parser/jsonschema.py	Modified parse_enum to branch on empty enum_fields: returns Types.null if not nullable, or creates a synthetic enum-like model with null-type root field if nullable/openable.
Expected output and OpenAPI spec tests/data/expected/main/openapi/null_only_enum.py, tests/data/openapi/null_only_enum.yaml	Added new OpenAPI schema definition with null-only enum and corresponding expected Pydantic model output featuring __root__: None = None field.
Test coverage tests/main/openapi/test_main_openapi.py, tests/parser/test_jsonschema.py	Added two new test functions: end-to-end OpenAPI generation test and unit test for empty non-nullable enum parsing.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~15 minutes

• Focus areas: Verify the conditional logic in parse_enum correctly differentiates between nullable and non-nullable empty enums; confirm the synthetic model creation produces valid output; validate test assertions match expected behavior for the edge case.

Poem

🐰 An enum that's empty, quite strange indeed,
Now generates code that won't make Black bleed!
A null-type model, so clever and neat,
Makes edge cases tidy—the fix is complete! ✨

Pre-merge checks and finishing touches

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title 'Fix crash when parsing enum containing only null value' directly and specifically describes the main fix: preventing crashes when handling null-only enums in OpenAPI schemas.
Linked Issues check	✅ Passed	The PR addresses all coding requirements from issue #1969: fixes the crash by generating valid RootModel[None] code instead of empty enums, and includes test coverage for the fix.
Out of Scope Changes check	✅ Passed	All changes are directly related to fixing null-only enum parsing: core logic changes in jsonschema.py, test input file, expected output file, and corresponding test cases.
Docstring Coverage	✅ Passed	Docstring coverage is 83.33% which is sufficient. The required threshold is 80.00%.

✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch fix/null-only-enum-crash

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.51%. Comparing base (f42c49f) to head (7b77df6).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2674   +/-   ##
=======================================
  Coverage   99.51%   99.51%           
=======================================
  Files          79       79           
  Lines       10986    11001   +15     
  Branches     1316     1318    +2     
=======================================
+ Hits        10933    10948   +15     
  Misses         32       32           
  Partials       21       21           

Flag	Coverage Δ
unittests	99.51% <100.00%> (+<0.01%)	⬆️

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[codspeed-hq]

CodSpeed Performance Report

Merging #2674 will not alter performance

_{Comparing fix/null-only-enum-crash (7b77df6) with main (f42c49f)}

Summary

✅ 50 untouched
⏩ 3 skipped¹

Footnotes

1. 3 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[coderabbitai]

Actionable comments posted: 0

🧹 Nitpick comments (1)

tests/parser/test_jsonschema.py (1)

926-931: Good test coverage for the non-nullable empty enum case.

The test correctly validates that an empty enum with no nullable flag returns a None type. Consider adding a complementary test for the nullable case to ensure both branches are unit-tested:

def test_parse_enum_empty_enum_nullable() -> None:
    """Test parse_enum creates root model when enum_fields is empty and nullable."""
    parser = JsonSchemaParser("")
    obj = JsonSchemaObject.parse_obj({"type": "string", "enum": [None], "nullable": True})
    result = parser.parse_enum("NullEnum", obj, ["NullEnum"])
    assert result.reference is not None
    assert len(parser.results) == 1

📜 Review details

Configuration used: defaults

Review profile: CHILL

Plan: Pro

📥 Commits

Reviewing files that changed from the base of the PR and between f42c49f and 7b77df6.

📒 Files selected for processing (5)

• src/datamodel_code_generator/parser/jsonschema.py (1 hunks)
• tests/data/expected/main/openapi/null_only_enum.py (1 hunks)
• tests/data/openapi/null_only_enum.yaml (1 hunks)
• tests/main/openapi/test_main_openapi.py (1 hunks)
• tests/parser/test_jsonschema.py (1 hunks)

🧰 Additional context used

🧬 Code graph analysis (2)

tests/parser/test_jsonschema.py (1)

src/datamodel_code_generator/parser/jsonschema.py (2)

• JsonSchemaObject (186-450)
• parse_enum (2507-2689)

src/datamodel_code_generator/parser/jsonschema.py (3)

src/datamodel_code_generator/parser/openapi.py (1)

• get_data_type (381-390)

src/datamodel_code_generator/model/types.py (1)

• get_data_type (100-106)

src/datamodel_code_generator/reference.py (2)

• reference (77-79)
• add (787-841)

🪛 Checkov (3.2.334)

tests/data/openapi/null_only_enum.yaml

[high] 1-14: Ensure that the global security field has rules defined

(CKV_OPENAPI_4)

🔇 Additional comments (4)

tests/data/expected/main/openapi/null_only_enum.py (1)

1-11: LGTM!

The expected output correctly generates a RootModel with __root__: None = None for the null-only enum case. This avoids the Black parsing error that occurred with empty Enum classes and properly represents a type that only accepts None.

src/datamodel_code_generator/parser/jsonschema.py (1)

2570-2608: Well-structured fix for the null-only enum crash.

The implementation correctly handles two cases:

1. Empty enum that is not nullable → returns a simple None type
2. Empty enum that is nullable → creates a synthetic root model with a null-type field

This prevents the generation of empty Enum classes that caused Black parsing failures.

tests/data/openapi/null_only_enum.yaml (1)

1-12: LGTM!

This test fixture correctly reproduces the minimal case from issue #1969 - a schema with type: string, enum: [null], and nullable: true. The Checkov warning about missing security rules is irrelevant for a test fixture focused on schema parsing.

tests/main/openapi/test_main_openapi.py (1)

4177-4185: LGTM! Well-structured regression test for null-only enum handling.

The test appropriately validates the fix for issue #1969 by ensuring that OpenAPI schemas with enum: [null] and nullable: true no longer cause a crash and generate the expected output (RootModel[None] instead of an empty Enum class).