feat(http-client-python): support P1 inline discriminated unions

[Libba Lawrence (l0lawrence)]

Summary

Implement the Python P1 inline discriminated-union shape described in Azure/typespec-azure#5703 (section 2). This is a scoped Python implementation, not a resolution of the complete cross-language issue.

• Support @discriminated(#{ envelope: "none" }) on unions with an explicit model constraint and nominal model variants declaring matching required literal discriminator properties.
• Preserve named Python union aliases and declared model inheritance. Generate Annotated[Union[...], _DiscriminatedUnion(...)] metadata for union-local deserialization rather than replacing the union with its base model or modifying shared model registrations.
• Honor custom discriminator names and JSON wire names; reuse existing literal-field serialization for inline tags.
• Handle operation request/response boundaries, model properties, optional/container uses, recursion, and cross-namespace aliases. Missing or unknown tags raise DeserializationError rather than selecting the first variant.
• Preserve ordinary untagged unions, direct model behavior, and existing @discriminator hierarchies. Add regression coverage, README documentation, and a feature changelog.

Compatibility and scope

Installed TCGC 0.73.0 exposes discriminatedOptions but not SdkUnionType.baseType. Read the compiler's explicit Union.baseType constraint and match SDK model ancestry against it; do not infer a common ancestor. The sample requires the compiler's experimental union-extends feature.

Initial support targets the default DPG model mode and explicit-base nominal variants. P2 object envelopes, default variants, structural variants, unions without an explicit model constraint, and variants without matching literal tags are unsupported and receive an explicit diagnostic. Existing skipped envelope/Spector scenarios remain untouched.

Validation

• Emitter TypeScript build and Python wheel build passed using installed dependencies (tsc and python -m build --wheel --no-isolation), avoiding dependency bootstrapping/downloads.
• Emitter suite: 40 passed (vitest run --config emitter/vitest.config.ts --testTimeout 20000).
• Focused Python suites: 103 passed, 1 deselected, including 26 new cases covering Azure/unbranded runtimes and generated fixtures.
• Actual emitted/generated Azure and unbranded SDK smoke checks passed for synchronous and asynchronous requests/responses using mocked pipeline responses, including exact inline JSON, variant selection, containers, direct models, and unknown tags.
• Emitter Oxlint, changed generator-source Pylint, package formatting, repository pnpm format, and git diff --check passed.

Baseline caveats

• Deselected test_models_mode_typeddict_does_not_write_empty_models_folder: its existing model-package scaffolding failure was reproduced with baseline CombinedType from HEAD before implementation was committed.
• Direct Pylint of the generated runtime reports pre-existing template findings; rendering and linting the baseline template reproduced the same findings and 9.92/10 score. No new runtime lint findings were observed.
• No full monorepo build/test or live Spector-server run was performed; generated-client smoke checks mocked pipeline responses.

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/@typespec/http-client-python@12285

commit: 02672c4

[github-actions]

Python emitter diff

Baseline gh:57c3a2200daf08f0c558b2ff11fdfd2f5aa353d3 vs this PR.

⚠️ emitter-diff failed (exit 1) — tool/build error, not a diff. See the run.

Informational check (eng/emitter-diff); does not block the PR.

[github-actions]

All changed packages have been documented.

• ✅ @typespec/http-client-python

Show changes

@typespec/http-client-python - feature ✏️

Support inline discriminated unions with an explicit model constraint and nominal,> model variants. Keep the named Python union alias and declared model inheritance,,> and deserialize responses and nested properties using the configured inline,> discriminator. Missing or unknown tags raise DeserializationError. Object,> envelopes and default variants are not supported.,> ,> tsp,> model PetBase { kind: string; name: string; },> model Cat extends PetBase { kind: "cat"; meows: boolean; },> model Dog extends PetBase { kind: "dog"; barks: boolean; },> @discriminated(#{ envelope: "none" }),> union Pet extends PetBase { cat: Cat, dog: Dog },>

[azure-sdk-automation]

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