pyoas 0.2.0

Generate Pydantic v2 models and FastAPI routers from OpenAPI 3.0/3.1 specs

pyoas

Generate Pydantic v2 models and FastAPI routers from an OpenAPI spec. Organized as a uv workspace with independent, installable packages.

Packages

Package	Purpose
pyoas	Spec loading, ref resolution, tag extraction, Jinja2 rendering, Pydantic v2 model generation, CLI
pyoas[fastapi]	FastAPI router generation + service stubs + test scaffolding (adds FastAPI dependency)
pyoas[claude]	Claude Code skill generation (optional, no extra runtime dependencies)

pyoas[fastapi] and pyoas[claude] both extend the base pyoas package.

Quick start

# Install
uv add pyoas[fastapi]

# Create a minimal config
cat > pyoas.yaml << 'EOF'
spec: openapi.yaml
output:
  models: src/generated/models
  routers: src/generated/routers
EOF

# Generate Pydantic models and FastAPI routers
uv run pyoas generate

# Optionally scaffold service stubs and test files
uv run pyoas scaffold services

A more complete config with all optional features:

spec: openapi.yaml
output:
  models: src/generated/models
  routers: src/generated/routers
services:
  generate: true
  output: src/services
  import_path: myapp.services  # Python import path for service module
tests:
  generate: true
  output: tests/generated
  not_found_exception: "HTTPException(status_code=404, detail='Not found')"
skills:
  generate: true  # requires pyoas[claude]

Then:

uv run pyoas generate   # models + routers + service stubs + tests + skills

Configuration reference (pyoas.yaml)

spec

Path to the OpenAPI 3.0/3.1 spec file (YAML or JSON). Resolved relative to the config file. Required.

output

Key	Default	Description
models	src/generated/models	Output directory for generated model files
routers	src/generated/routers	Output directory for generated router files
models_import	(derived)	Python import path for models; derived from models path if omitted
routers_import	(derived)	Python import path for routers; derived from routers path if omitted
source_root	src	Filesystem prefix stripped when deriving Python import paths

default_tag

Default: "default". Operations with no tag are grouped under this name.

model_config

Key	Default	Description
extra	"ignore"	Pydantic extra setting for response/shared models
request_extra	"forbid"	Pydantic extra setting for request-only models
frozen	false	Makes generated models immutable
populate_by_name	true	Allow populating fields by Python name as well as alias

fields

Key	Default	Description
snake_case	true	Convert camelCase field names to snake_case with an alias
enums_as_literals	true	Render small enums as Literal[...] instead of Enum subclasses

format

Key	Default	Description
enabled	true	Run ruff format on generated files after writing

templates

Key	Default	Description
models	null	Path to a directory of custom Jinja2 templates overriding model templates
routers	null	Path to a directory of custom Jinja2 templates overriding router templates

services

Key	Default	Description
generate	false	Scaffold service stub files
output	src/services	Output directory for service files
overwrite	false	Overwrite existing service files on re-run
import_path	""	Python import path used by routers to import the service (e.g. myapp.services)

tests

Key	Default	Description
generate	false	Scaffold pytest test stub files
output	tests/generated	Output directory for test files
overwrite	false	Overwrite existing test files on re-run (default: append new test classes only)
not_found_exception	null	Exception expression used in test_not_found stubs (e.g. HTTPException(status_code=404))

skills

Requires pyoas[claude] to be installed.

Key	Default	Description
generate	false	Generate Claude Code skill files
output	.claude/commands	Output directory for skill files
overwrite	false	Overwrite existing skill files on re-run

Generated output

Models (pyoas)

One file per tag: {models_output}/{tag}/models.py. Schemas referenced by multiple tags go to {models_output}/shared/models.py.

src/generated/models/
  __init__.py
  pets/
    models.py      # Pet, PetCreate, PetList, ...
  shared/
    models.py      # schemas used by more than one tag

Routers (pyoas[fastapi])

One file per tag: {routers_output}/{tag}/router.py. An __init__.py at the root re-exports all routers.

src/generated/routers/
  __init__.py      # from .pets import router as pets_router; ...
  pets/
    router.py      # APIRouter with typed endpoint stubs

Service stubs

One file per tag: {services_output}/{tag}.py. Scaffolded once; never overwritten by default.

src/services/
  pets.py          # PetsService class with async method stubs

Test scaffolding

One test file per tag plus a shared conftest.py with model factories.

tests/generated/
  conftest.py      # make_pet(), make_pet_list(), ...
  test_pets.py     # TestListPets, TestCreatePet, TestGetPet, ...

Each test class covers one endpoint and includes:

• test_endpoint_exists — verifies the route returns something other than 404/405
• Validation tests for required fields, numeric bounds, string constraints, enum violations
• test_not_found — verifies 404 when the service raises the configured exception
• test_success — happy-path stub (auto-implemented for GET/DELETE, stubbed for others)

CLI reference

pyoas models        # generate Pydantic models only
pyoas fastapi       # generate FastAPI routers only
pyoas generate      # generate models + routers (+ services/tests/skills if configured)
pyoas scaffold services  # scaffold service stubs (skips existing files)

All commands accept:

• --config PATH — path to config file (default: pyoas.yaml)
• --tags TAG1,TAG2 — limit generation to specific tags
• --clean — purge output directory before generating

Claude Code integration (pyoas[claude])

Install pyoas[claude] and set skills.generate: true in your config. Running pyoas generate will write Claude Code skill files to .claude/commands/:

Skill	Invocation	Purpose
implement-tests.md	/implement-tests tests/generated/test_pets.py	Implement all pytest.skip("implement me") stubs in a test file
add-test-case.md	/add-test-case tests/generated/test_pets.py "scenario"	Add a new test method for the described scenario
review-generated.md	/review-generated	Cross-reference generated code against the OpenAPI spec and flag issues

Development

# Install in editable mode with all extras
uv sync --extra fastapi --extra claude

# Run all tests
uv run pytest

# Run tests for a single area
uv run pytest tests/fastapi/

# Update snapshots
uv run pytest tests/fastapi/ --snapshot-update

# Lint and type-check
uv run ruff check src/
uv run mypy src/