Fastapi Patterns

FastAPI Patterns

Intro

FastAPI leans on type hints, Pydantic models, and Depends() to make endpoints declarative. Define schemas as BaseModel subclasses, inject collaborators with Depends, and use async def only when the work is genuinely I/O-bound.

Overview

Routes and routers

Use HTTP method decorators (@app.get, @app.post, @app.put, @app.delete, @app.patch). Path parameters are typed function arguments (user_id: int); query parameters are arguments with defaults (skip: int = 0, limit: int = 100). Group related endpoints with APIRouter(prefix="/users", tags=["users"]) and set explicit status codes via status_code=201.

Pydantic models

Define request bodies and responses as BaseModel subclasses. Keep Create, Update, and Response schemas distinct so PATCH semantics and ORM round-trips don't bleed together. Set model_config = ConfigDict(from_attributes=True) on response schemas to allow construction from ORM instances. Use response_model=... on the route to filter what leaks out, and Field(min_length=..., gt=...) for inline validation.

Dependency injection

Dependencies are plain functions or callables resolved by Depends(). Yield-based dependencies (yield session; session.close()) handle cleanup. Reuse them with Annotated: DbDep = Annotated[Session, Depends(get_db)]. Apply app-wide dependencies via FastAPI(dependencies=[Depends(verify_api_key)]). Dependencies chain naturally — get_current_user depends on oauth2_scheme, which extracts the header.

Async vs sync handlers

Use async def for I/O-bound work (async DB drivers, httpx, aiofiles). Use plain def for CPU-bound or sync-only libraries — FastAPI runs them in a thread pool automatically. Never put blocking calls inside an async def handler; they stall the event loop. Use asyncio.gather() to fan out concurrent async calls inside a single handler.

Middleware and CORS

Register middleware with app.add_middleware(...). Common ones: CORSMiddleware, TrustedHostMiddleware, GZipMiddleware. Custom middleware can be a function decorated with @app.middleware("http") or a BaseHTTPMiddleware subclass. Order matters: the last middleware added runs first (outermost).

Error handling

Raise HTTPException(status_code=..., detail=...) for client errors. Validation errors return 422 automatically with field-level detail. Customize via @app.exception_handler(...) or by overriding RequestValidationError. Aim for a consistent error envelope: {"detail": "...", "code": "..."}.

Testing

Use Starlette's TestClient(app) for sync tests and httpx.AsyncClient for async ones. Override dependencies with app.dependency_overrides[get_db] = mock_db and clear the overrides in teardown. WebSocket tests use the client.websocket_connect("/ws") context manager.

Gotchas

Agent-specific failure modes — provider-neutral pause-and-self-check items:

• Calling blocking I/O from an async def handler. A requests.get(...) or synchronous database call inside async def stalls the entire event loop and serializes all concurrent requests behind it. Use async def only with async-native libraries (httpx, async SQLAlchemy drivers, aiofiles); use plain def for synchronous work and let FastAPI run it in a thread pool.
• Sharing one schema model for create, update, and response. A single model leaks server-generated fields (id, created_at) into create payloads and loses PATCH semantics (all fields required vs. all optional). Define XxxCreate, XxxUpdate, and XxxResponse as separate Pydantic models.
• Sharing a single database Session across requests. A session shared between requests bleeds state from one request into another and causes race conditions in concurrent usage. Always inject a per-request session via a Depends(get_db) yield dependency.
• Not setting response_model= on routes. Without response_model, FastAPI returns the raw object including internal fields (passwords, internal IDs, server-only metadata) that were never meant to be serialized. Always set response_model= explicitly.
• Forgetting to clear app.dependency_overrides between tests. A dependency override set in one test leaks into subsequent tests, causing mysterious failures that depend on test execution order. Clear overrides in teardown.
• Returning errors with status 200 and an error payload. Embedding error information in a 200 response breaks client error handling, monitoring, and the framework's automatic error envelope. Raise HTTPException for client errors; let FastAPI format the response.
• Not setting model_config = ConfigDict(from_attributes=True) on response schemas. Without this, Pydantic V2 cannot construct a response model from a SQLAlchemy ORM instance, causing a ValidationError at runtime rather than a type error at authoring time.

Full reference

CRUD endpoint pattern

from fastapi import APIRouter, Depends, HTTPException, status
from sqlalchemy.orm import Session
from typing import Annotated

router = APIRouter(prefix="/items", tags=["items"])
DbDep = Annotated[Session, Depends(get_db)]

@router.post("/", status_code=status.HTTP_201_CREATED, response_model=ItemResponse)
def create_item(item: ItemCreate, db: DbDep):
    db_item = Item(**item.model_dump())
    db.add(db_item)
    db.commit()
    db.refresh(db_item)
    return db_item

@router.get("/", response_model=list[ItemResponse])
def list_items(db: DbDep, skip: int = 0, limit: int = 20):
    return db.query(Item).offset(skip).limit(limit).all()

@router.get("/{item_id}", response_model=ItemResponse)
def get_item(item_id: int, db: DbDep):
    item = db.get(Item, item_id)
    if not item:
        raise HTTPException(status_code=404, detail="Item not found")
    return item

@router.put("/{item_id}", response_model=ItemResponse)
def update_item(item_id: int, data: ItemUpdate, db: DbDep):
    item = db.get(Item, item_id)
    if not item:
        raise HTTPException(status_code=404, detail="Item not found")
    for key, val in data.model_dump(exclude_unset=True).items():
        setattr(item, key, val)
    db.commit()
    db.refresh(item)
    return item

@router.delete("/{item_id}", status_code=status.HTTP_204_NO_CONTENT)
def delete_item(item_id: int, db: DbDep):
    item = db.get(Item, item_id)
    if not item:
        raise HTTPException(status_code=404, detail="Item not found")
    db.delete(item)
    db.commit()

Schema separation

from pydantic import BaseModel, ConfigDict, Field
from datetime import datetime

class ItemCreate(BaseModel):
    name: str = Field(min_length=1, max_length=200)
    description: str | None = None
    price: float = Field(gt=0)
    tags: list[str] = []

class ItemUpdate(BaseModel):
    name: str | None = None
    description: str | None = None
    price: float | None = Field(default=None, gt=0)

class ItemResponse(BaseModel):
    model_config = ConfigDict(from_attributes=True)
    id: int
    name: str
    description: str | None
    price: float
    created_at: datetime

Cursor-based pagination

class PaginatedResponse(BaseModel):
    items: list[ItemResponse]
    next_cursor: str | None
    has_more: bool

@router.get("/", response_model=PaginatedResponse)
def list_items(db: DbDep, cursor: str | None = None, limit: int = 20):
    query = db.query(Item).order_by(Item.id)
    if cursor:
        query = query.filter(Item.id > int(cursor))
    items = query.limit(limit + 1).all()
    has_more = len(items) > limit
    items = items[:limit]
    return PaginatedResponse(
        items=items,
        next_cursor=str(items[-1].id) if has_more else None,
        has_more=has_more,
    )

File upload

from fastapi import UploadFile, File

@router.post("/upload")
async def upload_file(file: UploadFile = File(...)):
    if file.size > 10 * 1024 * 1024:  # 10 MB limit
        raise HTTPException(413, "File too large")
    contents = await file.read()
    return {"filename": file.filename, "size": len(contents)}

JWT authentication

from fastapi import Security
from fastapi.security import OAuth2PasswordBearer, OAuth2PasswordRequestForm
from jose import jwt, JWTError

oauth2_scheme = OAuth2PasswordBearer(tokenUrl="token")

async def get_current_user(token: str = Depends(oauth2_scheme), db: DbDep):
    try:
        payload = jwt.decode(token, SECRET_KEY, algorithms=["HS256"])
        user_id: int = payload.get("sub")
    except JWTError:
        raise HTTPException(status_code=401, detail="Invalid token")
    user = db.get(User, user_id)
    if not user:
        raise HTTPException(status_code=401, detail="User not found")
    return user

CurrentUser = Annotated[User, Depends(get_current_user)]

@app.post("/token")
def login(form: OAuth2PasswordRequestForm = Depends(), db: DbDep):
    user = authenticate(db, form.username, form.password)
    if not user:
        raise HTTPException(400, "Incorrect credentials")
    token = jwt.encode({"sub": user.id}, SECRET_KEY, algorithm="HS256")
    return {"access_token": token, "token_type": "bearer"}

Always hash passwords with bcrypt; never store plaintext. Use Security(get_current_user, scopes=["admin"]) for scope-gated routes.

WebSockets

from fastapi import WebSocket, WebSocketDisconnect

class ConnectionManager:
    def __init__(self):
        self.connections: list[WebSocket] = []

    async def connect(self, ws: WebSocket):
        await ws.accept()
        self.connections.append(ws)

    def disconnect(self, ws: WebSocket):
        self.connections.remove(ws)

    async def broadcast(self, message: str):
        for conn in self.connections:
            await conn.send_text(message)

manager = ConnectionManager()

@app.websocket("/ws")
async def websocket_endpoint(ws: WebSocket):
    await manager.connect(ws)
    try:
        while True:
            data = await ws.receive_text()
            await manager.broadcast(f"Message: {data}")
    except WebSocketDisconnect:
        manager.disconnect(ws)

Background tasks

Inject BackgroundTasks and call background_tasks.add_task(send_email, to=email). Tasks run after the response is flushed, so they cannot block the user. They share the same process as the request, so failures are isolated to the task. For heavy or long-running work, hand off to Celery or ARQ instead.

Streaming responses

from fastapi.responses import StreamingResponse
import asyncio

@router.get("/stream")
async def stream_data():
    async def generate():
        for i in range(100):
            yield f"data: {i}\n\n"
            await asyncio.sleep(0.1)
    return StreamingResponse(generate(), media_type="text/event-stream")

Test patterns

from fastapi.testclient import TestClient

client = TestClient(app)
app.dependency_overrides[get_db] = lambda: test_session

def test_create_item():
    resp = client.post("/items/", json={"name": "Widget", "price": 9.99})
    assert resp.status_code == 201
    assert resp.json()["name"] == "Widget"

def test_item_not_found():
    resp = client.get("/items/9999")
    assert resp.status_code == 404

def test_validation_error():
    resp = client.post("/items/", json={"name": "", "price": -1})
    assert resp.status_code == 422

Always clear app.dependency_overrides between tests so state from one test cannot bleed into another.

Anti-patterns

• Calling blocking I/O from async def — stalls the event loop
• Mixing request and response schemas in one model — leaks server fields like id and created_at into create payloads
• Catching Exception in handlers and returning 500 manually — defeats FastAPI's error envelope
• Reading the request body twice — wrap with a custom middleware if you must
• Sharing a single Session across requests — always inject per request via Depends

Further reading

• FastAPI Documentation
• Pydantic V2 Documentation