Fix msgspec enum discriminator tag

[koxudaxi]

Fixes #3152

Summary by CodeRabbit

Release Notes

• Bug Fixes

  • Fixed discriminator tag assignment in msgspec.Struct code generation when handling single literal enum values in OpenAPI schemas.

• Tests

  • Added test coverage for msgspec.Struct discriminator generation with single-value enums.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

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Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: 38b7f7be-78cf-4249-aae8-77b2e98fb765

📥 Commits

Reviewing files that changed from the base of the PR and between fcb57e6 and 9b08b55.

⛔ Files ignored due to path filters (1)

• tests/data/openapi/discriminator_enum_single_value_msgspec.yaml is excluded by !tests/data/**/*.yaml and included by none

📒 Files selected for processing (3)

• src/datamodel_code_generator/parser/base.py
• tests/data/expected/main/openapi/discriminator/enum_single_value_msgspec.py
• tests/main/openapi/test_main_openapi.py

✅ Files skipped from review due to trivial changes (1)

• tests/data/expected/main/openapi/discriminator/enum_single_value_msgspec.py

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📝 Walkthrough

Walkthrough

Sets msgspec.Struct variant tag to the literal representation (repr(expected_value)) for single-value enum discriminators and adds a test with expected generated Msgspec models validating the change.

Changes

Enum Discriminator Literal Resolution

Layer / File(s)	Summary
Discriminator tag value resolution in msgspec.Struct src/datamodel_code_generator/parser/base.py	In the msgspec.Struct discriminator path, when a single literal matches the expected value, the base-class kwarg tag is now set to repr(expected_value) instead of using discriminator_field.represented_default.
Test case and expected output tests/main/openapi/test_main_openapi.py, tests/data/expected/main/openapi/discriminator/enum_single_value_msgspec.py	Adds test_main_openapi_discriminator_enum_single_value_msgspec which generates msgspec.Struct output for a discriminator with single-value enums and compares against the new golden file defining StartedEvent and StoppedEvent tagged Structs and an Event union alias.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~12 minutes

Possibly related PRs

• koxudaxi/datamodel-code-generator#3070: Both PRs modify src/datamodel_code_generator/parser/base.py’s OpenAPI discriminator value handling—Fix non-string OpenAPI discriminator literals #3070 changes how expected_value/discriminator candidates are derived for non-string literals, and the main PR adjusts the emitted msgspec.Struct discriminator tag based on that expected_value.
• koxudaxi/datamodel-code-generator#3115: Both PRs modify src/datamodel_code_generator/parser/base.py discriminator handling to correctly compute discriminator literals/tag values (main: msgspec single-literal default uses repr(expected_value); retrieved: /reuse discriminator subclasses recover the proper literal instead of injecting/duplicating the wrong one).

Suggested labels

breaking-change-analyzed

Poem

🐰
I hopped through enums to find the right name,
Repr of the literal put tags in their frame.
No more None in the class declaration,
Msgspec structs now match expectation. 🎉

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 75.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title 'Fix msgspec enum discriminator tag' is clear and directly addresses the main change—fixing how msgspec.Struct discriminator tags are generated for single-value enum fields.
Linked Issues check	✅ Passed	The PR addresses issue #3152 by modifying the discriminator tag generation logic to use repr(expected_value) instead of discriminator_field.represented_default for msgspec.Struct single-literal discriminators [#3152].
Out of Scope Changes check	✅ Passed	All changes are directly related to fixing msgspec discriminator tag generation: the core fix in parser/base.py, test data file, and new test case all focus on the discriminator tag issue.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix-msgspec-enum-discriminator-tag

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[github-actions]

📚 Docs Preview: https://pr-3170.datamodel-code-generator.pages.dev

[codspeed-hq]

Merging this PR will not alter performance

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

✅ 11 untouched benchmarks
⏩ 98 skipped benchmarks¹

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_{Comparing fix-msgspec-enum-discriminator-tag (9b08b55) with main (4d0d9a9)}

Footnotes

1. 98 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. ↩

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (4d0d9a9) to head (9b08b55).

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #3170   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           93        93           
  Lines        20025     20027    +2     
  Branches      2338      2338           
=========================================
+ Hits         20025     20027    +2     

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

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[github-actions]

Breaking Change Analysis

Result: No breaking changes detected

Reasoning: The PR contains two categories of changes: (1) A bug fix for msgspec discriminator tag generation (parser/base.py), changing from discriminator_field.represented_default to repr(expected_value). This corrects previously incorrect generated code for single-value enum discriminators with msgspec.Struct output — the tag value was wrong before and is now correct. This is a correctness fix, not a breaking change. (2) Removal of _schema_requires_model_type and _allof_requires_model_type methods from parser/jsonschema.py, which reverts changes introduced in the immediate parent commit (53ba11d). That parent commit is NOT included in any released version (it sits between 0.57.0 and this PR), so users upgrading from 0.57.0 would never have seen the reverted behavior. Therefore, neither change constitutes a breaking change for released users.

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This analysis was performed by Claude Code Action

[github-actions]

🎉 Released in 0.58.0

This PR is now available in the latest release. See the release notes for details.