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Prominent Features

A guided tour of the flagship features that define Lauren. Each section is a quick conceptual overview; deep-dives live in the Core Concepts and Guides sections.

1. Radix-tree router with O(depth) lookup

Routes are compiled into a radix tree at startup. Static segments take priority over parameters; parameters take priority over wildcards. Per-method dispatch sets the Allow header automatically on 405.

python
@controller("/files")
class FilesController:
    @get("/health")          # static — wins
    async def health(self): ...

    @get("/{name}")          # param — second priority
    async def show(self, name: Path[str]): ...

    @get("/*path")           # wildcard — fallback
    async def deep(self, path: Path[str]): ...

Two routes with the same (method, path) raise RouterConflictError at startup — never silently shadowed.

2. Three-scope dependency injection

python
@injectable(scope=Scope.SINGLETON)   # one per app
class Clock: ...

@injectable(scope=Scope.REQUEST)     # one per request, shared in handler tree
class DbSession: ...

@injectable(scope=Scope.TRANSIENT)   # new every resolve
class IdGen: ...

Scope rules are enforced at startup:

  • SINGLETON may depend on SINGLETON only.
  • REQUEST may depend on SINGLETON or REQUEST.
  • TRANSIENT may depend on anything.

Violations raise DIScopeViolationError — not at runtime, at boot.

3. Protocols, multi-bindings, and list[T] injection

Bind any number of providers to a Protocol, then ask for a single one or all of them:

python
@runtime_checkable
class EmailSender(Protocol):
    def send(self, to: str, msg: str) -> None: ...

@injectable(provides=[EmailSender], multi=True)
class SmtpSender: ...

@injectable(provides=[EmailSender], multi=True)
class SmsSender: ...

@injectable()
class Dispatcher:
    def __init__(self, senders: list[EmailSender]) -> None:
        self._senders = senders   # exactly the senders, in registration order

Multiple providers without multi=True raise ProtocolAmbiguityError at startup. You can't accidentally bind two implementations to the same scalar token.

4. Typed extractors

python
@get("/items/{id}")
async def show(
    self,
    id: Path[int],                                  # path variable
    fields: Query[list[str]] = QueryField(default=[]),
    auth: Header[str] = HeaderField(alias="x-auth"),
    body: Json[CreateItem] = ...,                   # Pydantic-validated
) -> ItemOut: ...

Built-in extractors: Path, Query, Header, Cookie, Json, Form, Bytes, State, Depends, UploadFile, ByteStream. Plus custom extractors (/docs/guide) — implement extract once and use the type as a parameter annotation forever.

5. Modules with explicit imports / exports

python
@module(providers=[Clock], exports=[Clock])
class SharedModule: ...

@module(
    controllers=[UserController],
    providers=[UserRepo, DbSession],
    imports=[SharedModule],         # imports SharedModule's exports
)
class AppModule: ...

Visibility is explicit: a provider is reachable only if declared in this module or imported from another module's exports. Import cycles raise CircularModuleError at startup.

6. Lifecycle hooks in topological order

python
@injectable()
class Db:
    @post_construct
    async def connect(self) -> None: ...

    @pre_destruct
    async def disconnect(self) -> None: ...

@post_construct runs in topological order (deps first). @pre_destruct runs in reverse topological order at shutdown, with bounded timeouts. Failures are collected and reported, never aborting the rest of teardown.

7. Auto-serialization of handler returns

Return whatever feels right; Lauren builds the Response:

python
async def h1(self) -> dict:        return {"ok": True}              # JSON 200
async def h2(self) -> UserOut:     return UserOut(id=1, name="x")   # Pydantic → JSON 200
async def h3(self) -> list[UserOut]: return [u1, u2]                # JSON array
async def h4(self):                return {"id": 1}, 201            # body + status
async def h5(self):                return {"q": True}, 202, {"x-q": "default"}
async def h6(self) -> None:        return None                      # 204 No Content
async def h7(self):                return Response.html("<h1>hi</h1>")  # raw Response

The default JSON encoder handles Pydantic models, enums, datetimes, UUIDs, Decimal, pathlib.Path, sets, and dataclasses out of the box.

8. Strict decorator inheritance

Subclasses of @injectable / @controller / @module / @middleware() classes are not automatically of the same role. You must opt in.

python
@injectable()
class Base: ...

class Child(Base):
    pass    # registering this raises MetadataInheritanceError

@injectable()
class ChildOK(Base):
    pass    # explicit opt-in — fine

This is one of Lauren's most opinionated calls — and one developers thank us for after their first surprise refactor. See Class Inheritance Rules.

9. Onion-model middleware + class/route guards

python
@middleware()
class RequestId:
    async def dispatch(self, request, call_next):
        request.state.rid = uuid.uuid4().hex
        resp = await call_next(request)
        return resp.with_header("x-request-id", request.state.rid)

# Global, controller, or route-level — pick your scope:
app = LaurenFactory.create(AppModule, global_middlewares=[RequestId])

@use_middlewares(AuthMiddleware)
@controller("/private")
class P: ...

Guards work the same way:

python
@use_guards(AdminGuard)
@controller("/admin")
class AdminController:
    @get("/purge")
    @use_guards(SuperAdminGuard)         # composes; AdminGuard runs first
    async def purge(self): ...

10. Interceptors — wrap the handler, not the transport

Interceptors run around the handler (after guards, before the response is sent) and receive a CallHandler so they can observe or mutate both the inbound context and the outbound response. They compose with @use_interceptors at the global, controller, or route level — same scoping rules as guards.

python
from lauren import interceptor, use_interceptors, ExecutionContext, CallHandler, Response

@interceptor()
class AuditLog:
    async def intercept(self, ctx: ExecutionContext, call_handler: CallHandler) -> Response:
        response = await call_handler.handle()
        # response is available here — inspect or wrap it
        print(f"[audit] {ctx.request.method} {ctx.request.path} → {response.status_code}")
        return response

# Global:
app = LaurenFactory.create(AppModule, global_interceptors=[AuditLog])

# Controller or route:
@use_interceptors(AuditLog)
@controller("/api")
class API: ...

Interceptors execute after guards and see the real response, unlike middleware which wraps the entire transport layer. Full guide: Interceptors.

11. Custom exception handlers

Catch domain errors with class-form (DI-injected) or function-form handlers:

python
@exception_handler(NotFoundError, ConflictError)
class DomainErrors:
    def __init__(self, log: Logger) -> None:
        self.log = log
    async def catch(self, exc, request) -> Response:
        return Response.json({"error": str(exc)}, status=400)

@exception_handler(ValueError)
async def handle_value_error(exc, request) -> Response:
    return Response.json({"detail": str(exc)}, status=422)

Attach with @use_exception_handlers(...) per controller / route, or globally via LaurenFactory.create(..., global_exception_handlers=[...]). Full guide: Custom Exception Handlers.

12. Custom providers (NestJS-style recipes)

When @injectable isn't enough — environment-conditional swaps, externally-built objects, alias tokens — Lauren ships the four NestJS recipes:

python
from lauren import use_value, use_class, use_factory, use_existing, Token

DB_URL = Token("DB_URL")

@module(providers=[
    use_value(provide=DB_URL, value="postgres://..."),
    use_class(provide=ConfigService, use=ProductionConfigService),
    use_factory(provide="CONNECTION", factory=make_conn, inject=[DB_URL]),
    use_existing(provide="LegacyLogger", existing=LoggerService),
])
class AppModule: ...

Full guide: Custom Providers.

13. OpenAPI 3.1 generation

python
@get("/users/{id}", response_model=UserOut, operation_id="getUser", tags=["users"])
async def show(self, id: Path[int]) -> UserOut: ...

# Then:
schema = app.openapi()    # dict; serve at /openapi.json or feed to Swagger UI / ReDoc

Field descriptors emit constraints (ge, le, pattern, alias, ...) into the parameter schema. Pydantic response models become components.schemas references.

14. Structured logging — Console or JSON

python
from lauren.logging import default_logger, ConsoleLogger, JsonLogger, LogLevel

# TTY-aware default + LAUREN_LOG_LEVEL / LAUREN_LOG_FORMAT env vars:
app = LaurenFactory.create(AppModule, logger=default_logger())

# Or pick explicitly:
app = LaurenFactory.create(AppModule, logger=JsonLogger(level=LogLevel.INFO))

Per-request traces fire at DEBUG for 2xx/3xx, WARN for 4xx, ERROR for 5xx. Production runs at INFO stay quiet unless something wants attention.

python
[Lauren] 18:22:01.123  INFO  [LaurenFactory]  Starting application (root=AppModule)
[Lauren] 18:22:01.124  INFO  [RouterExplorer] Mapped {GET /users/{id}} → UserController.show
[Lauren] 18:22:01.124  INFO  [LaurenApp]      Application ready (1.2ms)  routes=12
[Lauren] 18:22:01.240  WARN  [Request]        GET /users/999 404 2.1ms → UserController.show
[Lauren] 18:22:01.314  INFO  [Shutdown]       Shutdown complete. Goodbye.

15. Graceful shutdown with signals

python
from lauren.signals import install_signal_handlers, wait_for_shutdown

@app.on_shutdown
async def flush_metrics():
    await metrics_client.flush()

event = install_signal_handlers(app, drain_timeout=30)
await wait_for_shutdown(event)

Four phases, all logged: drain → on_shutdown callbacks → @pre_destruct hooks → goodbye. Idempotent — concurrent calls return once the first drain has completed.

16. WebSockets, SSE, and Socket.IO

  • WebSockets — @ws_controller gateways with @on_connect, @on_message("event"), and @on_disconnect hooks; typed Pydantic frames; BroadcastGroup for room-scoped fan-out.
  • Server-Sent Events — Response.sse(async_iter) or EventStream with keep_alive=N for long-lived browser streams and Last-Event-ID resumability.
  • Socket.IO — Engine.IO v4 / Socket.IO v5 adapter that lets the official socket.io-client connect with no glue.

17. Typed bidirectional streaming

Stream[T] (inbound) and StreamingResponse[T] (outbound) form a symmetrical streaming primitive. The same wire-format vocabulary (text/event-stream, application/x-ndjson, application/json+stream) is honoured in both directions:

python
from lauren import Stream, StreamingResponse

@post("/transcribe")
async def transcribe(
    self, audio: Stream[AudioChunk]
) -> StreamingResponse[Transcript]:
    async def produce():
        async for chunk in audio:         # validated AudioChunk
            yield Transcript(text=chunk.text.upper(), confidence=0.95)
    return produce()

Content negotiation is automatic from the Accept header. The OpenAPI document carries x-streaming: true and lists all three negotiable content types.

18. Lifecycle event bus

SignalBus is an in-process typed pub/sub system that fires at well-known lifecycle points — startup, per-request, background tasks, shutdown — without coupling user code to framework internals:

python
from lauren.signals import RequestComplete

@app.signals.on(RequestComplete)
def on_complete(event: RequestComplete) -> None:
    metrics.record("request.duration", event.duration_s, tags={"status": event.status})

Listeners are sync or async. Errors are logged but never propagate out of emit. A listener on LifecycleEvent receives every event (firehose). MRO dispatch means base-class subscriptions are first-class.

19. AI-ready documentation (llms.txt / llms-full.txt)

Lauren ships an llms.txt-format overview and a complete LLM-ready reference at the package root, also available programmatically:

python
from lauren import docs
print(docs.llms_full_txt())     # ~25 KB — paste into any AI assistant

Coding agents (Claude, Cursor, Aider) can ingest the full reference and produce idiomatic Lauren code on the first try.

Where to dive next

Want to...Go to
Understand modules, controllers, injectablesCore Concepts
Write a custom extractorCustom Extractors
Add an authorization guardCustom Guards
Write request-tracing middlewareCustom Middleware
Add cross-cutting response logicInterceptors
Handle a domain errorCustom Exception Handlers
Compare to FastAPI / Litestar / BlackSheepComparisons
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