# Python Generator > Design of the ejected Redocly `python` client generator. Read it, and update it, before changing generators/python/. - Skill: `redocly/python-generator` (Agent Skill) - Install (CLI): `npx skillmds@latest add redocly/python-generator` - Raw SKILL.md: https://api.skillmd.com/api/skills/redocly/python-generator/raw - Safety review: pending - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: Coding & Dev Tools - Author: redocly (https://skillmd.com/u/redocly) - Updated: 2026-09-17 - Page: https://skillmd.com/skills/redocly/python-generator --- # The `python` generator — its skill This file is the DESIGN of your ejected `python` generator (`generators/python/`): **to change the generator, edit this skill first, then make the code match it** — a diff to `generators/python/` that has no covering sentence here is incomplete. ## What it emits One self-contained `.py`: typed dataclass models, a sync `Client` and an async `AsyncClient`, and the embedded runtime. Python ≥ 3.9; the only dependency is [httpx](https://www.python-httpx.org/) (`pip install httpx`). ## Design decisions that must hold - **The file name is an importable module name.** The `--output` stem follows the TypeScript convention (`openapi.client.ts`), and `openapi.client.py` cannot be imported by name — nor can hyphens or a leading digit. The stem is converted with `identifierFor(stem, snake)`, so `rebilly-core.client.ts` emits `rebilly_core_client.py` and `import rebilly_core_client` just works. - **Models are dataclasses by default**, required fields first (a dataclass constraint), optionals `Optional[T] = None`. Wire names live in a `_field_map: ClassVar[Dict[str, str]]`; decode/encode is reflective (`_decode.py`, `get_type_hints`) — no per-model codecs. - **`models: pydantic` emits `BaseModel` classes instead**, for the FastAPI-shaped half of the ecosystem that expects them. A wire name becomes `Field(alias=…)` with `populate_by_name=True`, so `_field_map` is not emitted in this mode — the alias is the mapping. Everything else is unchanged: the same class names, the same field names, the same `Optional[T] = None`, the same enums and union aliases, the same client and runtime. Switching modes must not change a call site. - **A discriminated union carries its discriminator into the pydantic annotation.** The decoder hands a whole object tree to `model_validate`, so a union nested in a model is resolved by pydantic and never reaches the `DISCRIMINATORS` table that dataclass mode walks. Pydantic resolves it correctly from `Annotated[Union[...], Field(discriminator=…)]`, which it accepts only when every member types that property as a `Literal` — and the mapping already pins one value per member, so the members get `Literal["cat"]`. Such a union registers no table entry: pydantic owns it at every depth, and the `Literal` makes the decoder's member probe exact. A union whose members never declare the property keeps the plain `Union` and the table entry, and pydantic then matches nested members its own way — the description is what has to change there. - **One runtime serves both model modes.** `_decode.py` dispatches on the target: a class with `model_validate` is validated by pydantic, a dataclass is hydrated reflectively, and `encode` mirrors that with `model_dump(by_alias=True, exclude_none=True, mode="json")`. A second runtime variant per mode would double the surface that has to stay in step, and pydantic's `ValidationError` already subclasses `ValueError`, so union member probing needs no new except clause. - **`models: pydantic` adds a dependency, and the header says so.** The default mode keeps httpx as the only requirement; the pydantic header asks for both. A mode that quietly needed a package the file never named would fail at import with nothing to act on. - **Every parameter is its own argument, so their names share one namespace** with the arguments the method declares itself (`body`, `headers`, `timeout`, `retry`, `idempotency_key`). Build them with `uniqueIdentifiers(..., { taken: … })`: OpenAPI lets one operation use a name in two locations (`id` in the path AND in the query), and a `def` that declared one name twice is a `SyntaxError`. The wire name is untouched, so the request is unchanged. - **Naming:** fields/methods snake*case via `identifierFor(..., RESERVED_WORDS.python)`; reserved words get a trailing underscore (`class*`); `+1`/`-1`become`plus_1`/`minus_1`. - **Enums** are `class X(str, Enum)` with SCREAMING members; **unions** are `Union[...]` aliases. A DISCRIMINATED union registers its dispatch table in the runtime's `DISCRIMINATORS` registry (`DISCRIMINATORS[Pet] = ("petType", {"cat": Cat, ...})`), and `decode()` routes through it — `isinstance` narrowing works on decoded members. Undiscriminated unions decode by trying each member in order (the first that hydrates wins — see `_decode.py`). **allOf** is flattened via `flattenAllOf`. - **Auth keys match the other languages.** `auth={"apiKey": {...}}` is the documented key — the same spelling TypeScript and PHP use, and the same as the scheme kind — with `api_key` accepted as an alias so a snake_case config keeps working. - **Errors:** `errorMode` maps to raising `ApiError` (default) or returning a `Result` dataclass — the only generator with both modes outside TypeScript. - **Dates:** `dateType: Date` annotates `format: date-time` as `datetime` and `date` as `date`; `_decode.py` parses ISO strings into them and `encode()` writes `isoformat()` back. The default (`string`) keeps the wire shape. - **Response headers:** an operation that DECLARES success-response headers gains a `_with_headers()` variant (sync and async) returning `Envelope[T]` — `data`, `headers` (coerced to int/bool/str with snake_case keys; absent/unparsable values omitted), and the raw `response`. Operations without declared headers get no variant, and the base method stays body-only. - **Servers:** when the description declares servers, a `Servers` class is emitted with one static method per server; server VARIABLES become keyword arguments defaulting to the spec's defaults (`Servers.production(organization_id="org_x")`), so templated base URLs need no manual string building. The client's baked default stays `servers[0]` with variable defaults substituted. - **Parity surface:** auth (bearer/basic/apiKey), retries with `Retry-After` + jittered backoff, timeouts, idempotency keys, middleware, pagination (`_pages()` / `_items()` + `aiter` mirrors), SSE (`iter_sse`/`aiter_sse`), multipart. - The runtime is hand-written in `runtime/*.py` in this folder and embedded as strings at prepare time — generator code never builds runtime logic from templates. Under `--runtime module` the same sources are written as sibling `_*.py` files instead (package-relative imports become sibling imports; the client star-imports each module). - Authored ONLY with the neutral toolkit (`Printer`, naming, schema, pagination helpers) — the dogfooding guard fails otherwise. - **It documents itself.** With `client.docs` (or `--docs`), the `docs` hook writes `.python.md`: the security schemes, then one section per operation with its parameters, body, response type, and behavior notes. The call snippets come from this generator's own `sample` hook, so the page can only show the syntax of the SDK beside it, and the layout comes from `renderReferencePage` in the authoring toolkit — reachable from an ejected copy through `@redocly/client-generator`. Pagination on the page is decided by `paginationRuleFor`, the same helper this generator resolves pagination with. ## The modify loop 1. Edit this skill: state the new behavior or decision. 2. Make `generators/python/` match it. 3. Run `redocly generate-client` and inspect the `git diff` of the generated output — generated files are never hand-edited. Newer built-in versions merge in with `redocly eject-generator python --update`.