Pydantic

The FastAPI and Django Ninja targets write a Pydantic 2 model for each schema of your contract. Pydantic checks the request before your handler runs.

Used by: FastAPI and Django Ninja.

Set up

Nothing to add: the pypi:seampact-fastapi or pypi:seampact-ninja target writes the models. Its one option for validation is rules.

# seampact.config.yaml, under targets
  - id: notes
    out: notes
    use: pypi:seampact-fastapi
    api: notes
    options: { rules: seampact-consistent }

The models land in schemas.py of the generated folder, for example generated/notes/schemas.py.

What you get

For the CreateTask schema of the overview:

class CreateTask(_pydantic.BaseModel):
    model_config = _MODEL_CONFIG

    title: _typing.Annotated[str, _pydantic.Field(min_length=1, max_length=200)]

An optional member is Omittable[...] = None. A uuid is WireUuid, a date-time is WireDateTime, and an enum is a StrEnum. The helpers come from seampact.codegen.runtime.pydantic.

Use them in your code

from generated.notes.schemas import CreateTask

task = CreateTask.model_validate({"title": "Buy milk"})

When a check fails

Each Pydantic error becomes an issue of the 400 body, named by the contract rule, not by Pydantic’s error type.

Native or consistent rules

With the default rules: native, the models use Pydantic’s own Field arguments, as above. With rules: seampact-consistent, they use seampact’s checks, equal on every target (Switch the validator):

class CreateTask(_pydantic.BaseModel):
    model_config = _MODEL_CONFIG

    title: _typing.Annotated[str, _seampact.length(min_length=1, max_length=200)]
    effort: _seampact.Omittable[_typing.Annotated[_seampact.WholeNumber, _seampact.bounds(minimum="0")]] = None

Limits

The Python generators refuse by name a oneOf, a map schema, an open or narrowed enum and a property default (Generate fails).

Next

• FastAPI or Django Ninja.
• The 400 body: When a check fails.