fastapi-canon

fastapi-canon is an opinionated composition library for feature-oriented FastAPI applications. A feature groups its routers, Dishka providers, error contracts, exception handlers, and lifespan into one immutable value. The application installs an explicitly ordered set of those values at its composition root.

Dishka is a deliberate part of this canon, not an optional integration. fastapi-canon defines one dependency-injection approach: features contribute Dishka providers, and the composition builds and owns one shared Dishka container. Applications that choose another dependency-injection framework are outside the library's intended architecture.

Contents

• Quick start
• Feature composition
  • Shared API router
  • Dishka
• Error contracts
  • OpenAPI representation
• Success response contracts
  • Server-sent events
  • Binary and PDF streams
  • Streaming boundary
• Installation guarantees
• Requirements
• Development
• Showcase

Quick start

from fastapi import APIRouter, FastAPI
from fastapi_canon import Composition, Feature

projects = APIRouter(prefix="/projects", tags=["projects"])


@projects.get("")
async def list_projects() -> list[str]:
    return []


project_feature = Feature(name="projects", routers=[projects])

app = Composition(project_feature).apply(FastAPI())

Feature composition

Every contribution is optional:

feature = Feature(
    name="projects",
    routers=[router],
    providers=[provider],
    errors=feature_errors,
    exception_handlers=[handler_spec],
    lifespan=feature_lifespan,
)

Mutable sequences passed to Feature are copied to tuples. Installing features preserves declaration order for routes and startup. Shutdown runs in reverse order, including cleanup of features that started before a later feature failed. Feature names are required, unique within a composition, and included in configuration diagnostics.

Shared API router

Use router_factory to create one application router per installed composition. Feature routers are included in that router before it is mounted on the app, so the factory can supply a shared prefix, tags, dependencies, and a custom APIRouter subclass:

from fastapi import APIRouter, Depends


class ApplicationRouter(APIRouter):
    pass


async def require_request_id() -> None: ...


composition = Composition(
    project_feature,
    router_factory=lambda: ApplicationRouter(
        prefix="/api/v1",
        tags=["api"],
        dependencies=[Depends(require_request_id)],
    ),
)

The factory is called during apply() and must return a fresh, empty APIRouter. Omitting it preserves direct router installation.

Dishka

Dishka is a required runtime dependency and the canonical dependency-injection mechanism. A feature accepts provider instances, provider classes, and zero-argument factories:

feature = Feature(
    name="projects",
    providers=[ProjectProvider, lambda: DatabaseProvider(settings.database)],
)

Classes and factories are materialized once per application during apply(); instances are used as supplied. The providers from every feature are validated together and used to build one AsyncContainer.

Composition manages the container lifecycle. It configures Dishka's FastAPI middleware, exposes the container as app.state.dishka_container, and closes it during application shutdown. Existing application lifespans are composed automatically. Applications should treat the exposed container as composition-owned and not close it independently.

Error contracts

Error contracts are implemented directly by fastapi-canon; no separate error library is required. Each feature may expose one ErrorRegistry. Registries are merged and installed once, so runtime RFC 9457 Problem Details and OpenAPI use the same definitions:

from fastapi_canon import Composition, Error, ErrorOptions, ErrorRegistry, Feature


class ProjectNotFound(Exception):
    pass


project_not_found = Error(
    ProjectNotFound,
    status=404,
    code="project_not_found",
    title="Project not found",
    detail="The requested project does not exist.",
)

project_errors = ErrorRegistry(
    name="projects",
    errors=[project_not_found],
)

project_feature = Feature(
    name="projects",
    routers=[projects],
    errors=project_errors,
)

app = Composition(
    project_feature,
    errors=ErrorOptions(
        type_base="https://api.example.com/problems",
    ),
).apply(FastAPI())

Declare endpoint responses from the same registry used at runtime:

@projects.get(
    "/{project_id}",
    responses=project_errors.responses(project_not_found),
)
async def get_project(project_id: str) -> dict[str, str]:
    raise ProjectNotFound(project_id)

Normalized HTTPException responses use the same declaration path:

@projects.get(
    "/private",
    responses=project_errors.responses(http_statuses=[401, 403]),
)
async def private_project() -> dict[str, str]: ...

These contracts document the normalized runtime codes (http_401, http_403, and so on) and application/problem+json. A domain error and a generic HTTP problem may share a status; the generated response then uses the same discriminated oneOf representation.

FastAPI responses={...} entries without canon error declarations require no additional metadata. Success contracts and other media types can be declared alongside problem responses.

OpenAPI representation

Every declared error becomes a reusable schema in components.schemas. The corresponding operation references it as an application/problem+json response:

paths:
  /projects/{project_id}:
    get:
      responses:
        "404":
          description: Project not found
          content:
            application/problem+json:
              schema:
                $ref: "#/components/schemas/ProjectNotFoundProblem"

components:
  schemas:
    ProjectNotFoundProblem:
      type: object
      required: [type, title, status, code]
      properties:
        type:
          type: string
          const: https://api.example.com/problems/project_not_found
        title:
          type: string
          const: Project not found
        status:
          type: integer
          const: 404
        code:
          type: string
          const: project_not_found
        detail:
          type: [string, "null"]

Typed extension fields and documented response headers are added to that same schema and response. If multiple errors share one status code, the response uses oneOf with code as its discriminator. When validation normalization is enabled, FastAPI's default 422 response is replaced by RequestValidationProblem using the same media type.

When all local registries already share a type_base, it is inferred. The ErrorOptions settings include_validation_error, include_http_exceptions, and include_unhandled_error are passed to the integrated error engine and default to True.

Use ExceptionHandlerSpec for a deliberately custom Starlette/FastAPI handler:

from fastapi_canon import ExceptionHandlerSpec

feature = Feature(
    name="projects",
    exception_handlers=[ExceptionHandlerSpec(DomainError, domain_error_handler)],
)

Error.code is the stable client contract. Dynamic detail, extensions, and headers are public response data and should contain only information classified for API exposure. Exception text can be used as detail when it is itself a reviewed public contract. Domain registries should use specific exception types; broad built-ins such as Exception, ValueError, and RuntimeError can also match unrelated programming failures.

Success response contracts

Response describes one successful response using the same responses() call as domain and HTTP errors:

from fastapi_canon import Response


@router.get(
    "/events",
    responses=api_errors.responses(
        http_statuses=[401, 403],
        success=Response.sse(),
    ),
)
async def events() -> StreamingResponse: ...

The generic form accepts a status, media type, OpenAPI schema, description, and response headers:

success = Response(
    status=200,
    media_type="application/example+json",
    schema={"type": "object"},
    description="Example document",
    headers=["X-Request-ID"],
)

The available constructors are:

Constructor	Default contract
Response.json()	application/json with status 200
Response.empty()	No response content with status 204
Response.stream(media_type)	String stream with status 200
Response.binary(media_type)	Binary string stream with status 200
Response.sse()	Response.stream("text/event-stream")

Schemas and header definitions are copied into immutable mappings. Header names may be supplied as a list for standard string-valued header schemas or as a mapping containing complete OpenAPI header definitions. When a success status is not 200, set the same status_code on the FastAPI route.

Server-sent events

from collections.abc import AsyncIterator

from fastapi.responses import StreamingResponse
from fastapi_canon import Response


async def event_chunks() -> AsyncIterator[str]:
    yield "event: ready\ndata: {}\n\n"
    yield 'event: message\ndata: {"id": 1}\n\n'


@router.get(
    "/events",
    responses=api_errors.responses(
        http_statuses=[401, 403],
        success=Response.sse(),
    ),
)
async def events() -> StreamingResponse:
    return StreamingResponse(
        event_chunks(),
        media_type="text/event-stream",
        headers={
            "Cache-Control": "no-cache",
            "X-Accel-Buffering": "no",
        },
    )

Binary and PDF streams

from collections.abc import Iterator
from pathlib import Path
from uuid import UUID

from fastapi.responses import StreamingResponse
from fastapi_canon import Response


def pdf_chunks(path: Path) -> Iterator[bytes]:
    with path.open("rb") as source:
        while chunk := source.read(64 * 1024):
            yield chunk


@router.get(
    "/documents/{document_id}",
    responses=api_errors.responses(
        document_missing,
        http_statuses=[401, 403],
        success=Response.binary(
            "application/pdf",
            headers=["Content-Disposition"],
        ),
    ),
)
async def document(document_id: UUID) -> StreamingResponse:
    path = Path("documents") / f"{document_id}.pdf"
    if not path.is_file():
        raise DocumentMissing
    return StreamingResponse(
        pdf_chunks(path),
        media_type="application/pdf",
        headers={
            "Content-Disposition": f'inline; filename="{document_id}.pdf"',
        },
    )

The generated operation documents only text/event-stream or application/pdf for the successful response. Error responses continue to use application/problem+json.

Streaming boundary

Authentication, authorization, validation, and resource lookup should complete before returning StreamingResponse. Once response headers have been sent, an exception inside the iterator cannot be converted into another HTTP response. SSE protocols can define an application-specific failure event; a failed binary iterator produces an incomplete download.

Installation guarantees

• Feature order is explicit and deterministic.
• Feature names identify conflicting contributions in diagnostics.
• Reinstalling the exact same feature objects with the same options is a no-op.
• A different second installation is rejected.
• Duplicate routers, providers, handlers, and error collisions fail during configuration.
• Known configuration errors are validated against a temporary application before the real application is changed.
• Provider-backed features must be installed before the application starts.
• Disabling a feature means omitting it from Composition, which removes all of its contributions together.

Configuration failures raise FeatureConfigurationError.

Requirements

• CPython 3.12, 3.13, or 3.14
• FastAPI 0.115 or newer, below 1.0
• Dishka 1.10 or newer, below 2.0
• Pydantic 2.9 or newer, below 3.0

Development

Install the development dependencies and run the quality gates:

uv sync --all-groups
uv run pre-commit install --config .pre-commit-config.yml
uv run pre-commit run --config .pre-commit-config.yml --all-files
uv run ruff format --check .
uv run ruff check .
uv run mypy
uv run pytest
uv build

Showcase

See examples/showcase for a runnable two-feature FastAPI application. It keeps error contracts alongside their feature routes and merges them once at the composition root.