Overview

Master FastAPI's dependency injection system for building modular, testable APIs with reusable dependencies.

Basic Dependencies

Simple dependency injection patterns in FastAPI.

from fastapi import Depends, FastAPI

app = FastAPI()

def get_db():
    db = Database()
    try:
        yield db
    finally:
        db.close()

@app.get('/users')
async def get_users(db = Depends(get_db)):
    return await db.query('SELECT * FROM users')

# Simple function dependency
def common_parameters(q: str = None, skip: int = 0, limit: int = 100):
    return {'q': q, 'skip': skip, 'limit': limit}

@app.get('/items')
async def read_items(commons: dict = Depends(common_parameters)):
    return commons

# Async dependencies
async def get_async_db():
    db = await AsyncDatabase.connect()
    try:
        yield db
    finally:
        await db.disconnect()

@app.get('/async-users')
async def get_async_users(db = Depends(get_async_db)):
    return await db.fetch_all('SELECT * FROM users')

Dependency Scopes

Understand dependency lifecycle and caching.

from fastapi import Depends

# Request-scoped dependency (default, cached per request)
def get_current_user(token: str = Depends(oauth2_scheme)):
    user = decode_token(token)
    return user

# Multiple uses in same request reuse the same instance
@app.get('/profile')
async def get_profile(
    user1 = Depends(get_current_user),
    user2 = Depends(get_current_user)  # Same instance as user1
):
    assert user1 is user2  # True
    return user1

# No caching (use_cache=False)
def get_fresh_data(use_cache: bool = False):
    return fetch_from_api()

@app.get('/data')
async def get_data(data = Depends(get_fresh_data, use_cache=False)):
    return data  # Fetches fresh data each time

# Singleton pattern (application scope)
class Settings:
    def __init__(self):
        self.app_name = "MyApp"
        self.admin_email = "admin@example.com"

settings = Settings()  # Created once at startup

def get_settings():
    return settings

@app.get('/settings')
async def read_settings(settings: Settings = Depends(get_settings)):
    return settings

Sub-Dependencies

Build complex dependency chains.

from fastapi import Depends, HTTPException, status

# Sub-dependency chain
def get_db():
    db = Database()
    try:
        yield db
    finally:
        db.close()

def get_current_user(
    token: str = Depends(oauth2_scheme),
    db = Depends(get_db)
):
    user = db.query_one('SELECT * FROM users WHERE token = ?', token)
    if not user:
        raise HTTPException(status_code=401, detail='Invalid token')
    return user

def get_current_active_user(
    current_user = Depends(get_current_user)
):
    if not current_user.is_active:
        raise HTTPException(status_code=400, detail='Inactive user')
    return current_user

def get_admin_user(
    current_user = Depends(get_current_active_user)
):
    if not current_user.is_admin:
        raise HTTPException(status_code=403, detail='Not authorized')
    return current_user

# Use in endpoint
@app.delete('/users/{user_id}')
async def delete_user(
    user_id: int,
    admin = Depends(get_admin_user),
    db = Depends(get_db)
):
    await db.execute('DELETE FROM users WHERE id = ?', user_id)
    return {'status': 'deleted'}

Class-Based Dependencies

Use classes for stateful dependencies.

from fastapi import Depends

class Database:
    def __init__(self, connection_string: str):
        self.connection_string = connection_string
        self.connection = None

    async def connect(self):
        self.connection = await create_connection(self.connection_string)
        return self

    async def disconnect(self):
        if self.connection:
            await self.connection.close()

    async def fetch_all(self, query: str):
        return await self.connection.fetch_all(query)

# Callable class (using __call__)
class DatabaseDependency:
    def __init__(self):
        self.db = None

    async def __call__(self):
        if not self.db:
            self.db = Database('postgresql://localhost/db')
            await self.db.connect()
        return self.db

db_dependency = DatabaseDependency()

@app.get('/users')
async def get_users(db = Depends(db_dependency)):
    return await db.fetch_all('SELECT * FROM users')

# Class with initialization parameters
class Pagination:
    def __init__(self, skip: int = 0, limit: int = 100):
        self.skip = skip
        self.limit = limit

@app.get('/items')
async def get_items(pagination: Pagination = Depends()):
    return {'skip': pagination.skip, 'limit': pagination.limit}

# Advanced class-based dependency with configuration
class ServiceClient:
    def __init__(
        self,
        api_key: str,
        timeout: int = 30,
        max_retries: int = 3
    ):
        self.api_key = api_key
        self.timeout = timeout
        self.max_retries = max_retries
        self.client = None

    async def __call__(self):
        if not self.client:
            self.client = await create_http_client(
                api_key=self.api_key,
                timeout=self.timeout
            )
        return self.client

# Factory function for configurable class-based dependency
def create_service_dependency(api_key: str):
    return ServiceClient(api_key=api_key, timeout=60)

service = create_service_dependency('my-api-key')

@app.get('/external-data')
async def get_external_data(client = Depends(service)):
    return await client.fetch('/data')

Generator Dependencies for Cleanup

Use generator functions to ensure proper resource cleanup.

from contextlib import asynccontextmanager
from fastapi import Depends
import httpx

# Database connection with cleanup
async def get_db_connection():
    connection = await Database.connect('postgresql://localhost/db')
    try:
        yield connection
    finally:
        await connection.close()
        print('Database connection closed')

# HTTP client with cleanup
async def get_http_client():
    async with httpx.AsyncClient(timeout=30.0) as client:
        yield client
    # Client automatically closed after yield

@app.get('/external-api')
async def call_external_api(client = Depends(get_http_client)):
    response = await client.get('https://api.example.com/data')
    return response.json()

# File handle with cleanup
async def get_file_writer(filename: str):
    file = await aiofiles.open(filename, mode='a')
    try:
        yield file
    finally:
        await file.close()

# Multiple resources with cleanup
async def get_resources():
    db = await Database.connect('postgresql://localhost/db')
    cache = await Redis.connect('redis://localhost')
    logger = Logger('app')

    try:
        yield {'db': db, 'cache': cache, 'logger': logger}
    finally:
        await cache.close()
        await db.close()
        logger.shutdown()

@app.post('/process')
async def process_data(
    data: dict,
    resources = Depends(get_resources)
):
    db = resources['db']
    cache = resources['cache']
    logger = resources['logger']

    logger.info('Processing data')
    result = await db.save(data)
    await cache.invalidate('data_cache')
    return result

# Context manager as dependency
@asynccontextmanager
async def transaction_context(db = Depends(get_db)):
    async with db.begin() as transaction:
        try:
            yield transaction
            await transaction.commit()
        except Exception:
            await transaction.rollback()
            raise

async def get_transaction(db = Depends(get_db)):
    async with transaction_context(db) as txn:
        yield txn

@app.post('/transfer')
async def transfer_funds(
    from_account: int,
    to_account: int,
    amount: float,
    txn = Depends(get_transaction)
):
    await txn.execute(
        'UPDATE accounts SET balance = balance - ? WHERE id = ?',
        amount, from_account
    )
    await txn.execute(
        'UPDATE accounts SET balance = balance + ? WHERE id = ?',
        amount, to_account
    )
    return {'status': 'success'}

Dependency Chains and Sub-Dependencies

Build complex dependency hierarchies.

from fastapi import Depends, HTTPException
from typing import Optional

# Level 1: Configuration
def get_config():
    return {
        'database_url': 'postgresql://localhost/db',
        'redis_url': 'redis://localhost',
        'secret_key': 'super-secret'
    }

# Level 2: Infrastructure (depends on config)
def get_db(config: dict = Depends(get_config)):
    db = Database(config['database_url'])
    try:
        yield db
    finally:
        db.close()

def get_cache(config: dict = Depends(get_config)):
    cache = Redis(config['redis_url'])
    try:
        yield cache
    finally:
        cache.close()

# Level 3: Authentication (depends on infrastructure)
def get_token_from_header(authorization: str = Header(None)):
    if not authorization:
        raise HTTPException(status_code=401, detail='Missing token')
    scheme, token = authorization.split()
    if scheme.lower() != 'bearer':
        raise HTTPException(status_code=401, detail='Invalid scheme')
    return token

def verify_token(
    token: str = Depends(get_token_from_header),
    config: dict = Depends(get_config)
):
    try:
        payload = jwt.decode(
            token,
            config['secret_key'],
            algorithms=['HS256']
        )
        return payload
    except JWTError:
        raise HTTPException(status_code=401, detail='Invalid token')

def get_current_user(
    payload: dict = Depends(verify_token),
    db = Depends(get_db)
):
    user_id = payload.get('user_id')
    user = db.query_one('SELECT * FROM users WHERE id = ?', user_id)
    if not user:
        raise HTTPException(status_code=404, detail='User not found')
    return user

# Level 4: Authorization (depends on authentication)
def get_active_user(user = Depends(get_current_user)):
    if not user.is_active:
        raise HTTPException(status_code=403, detail='Inactive user')
    return user

def require_permission(permission: str):
    def permission_checker(user = Depends(get_active_user)):
        if permission not in user.permissions:
            raise HTTPException(
                status_code=403,
                detail=f'Permission {permission} required'
            )
        return user
    return permission_checker

# Level 5: Business logic (depends on authorization)
def get_user_service(
    db = Depends(get_db),
    cache = Depends(get_cache),
    current_user = Depends(get_active_user)
):
    return UserService(db=db, cache=cache, user=current_user)

# Use in endpoint
@app.post('/users/{user_id}/deactivate')
async def deactivate_user(
    user_id: int,
    admin = Depends(require_permission('users.deactivate')),
    service = Depends(get_user_service)
):
    result = await service.deactivate_user(user_id)
    return result

OAuth2 Dependencies

Implement OAuth2 authentication with dependencies.

from fastapi import Depends, HTTPException, status
from fastapi.security import (
    OAuth2PasswordBearer,
    OAuth2PasswordRequestForm,
    HTTPBearer,
    HTTPAuthorizationCredentials
)
from datetime import datetime, timedelta
from jose import JWTError, jwt

# OAuth2 with password flow
oauth2_scheme = OAuth2PasswordBearer(tokenUrl='token')

SECRET_KEY = 'your-secret-key-here'
ALGORITHM = 'HS256'
ACCESS_TOKEN_EXPIRE_MINUTES = 30

def create_access_token(data: dict, expires_delta: timedelta = None):
    to_encode = data.copy()
    if expires_delta:
        expire = datetime.utcnow() + expires_delta
    else:
        expire = datetime.utcnow() + timedelta(minutes=15)
    to_encode.update({'exp': expire})
    encoded_jwt = jwt.encode(to_encode, SECRET_KEY, algorithm=ALGORITHM)
    return encoded_jwt

async def get_current_user_from_token(
    token: str = Depends(oauth2_scheme),
    db = Depends(get_db)
):
    credentials_exception = HTTPException(
        status_code=status.HTTP_401_UNAUTHORIZED,
        detail='Could not validate credentials',
        headers={'WWW-Authenticate': 'Bearer'},
    )
    try:
        payload = jwt.decode(token, SECRET_KEY, algorithms=[ALGORITHM])
        user_id: str = payload.get('sub')
        if user_id is None:
            raise credentials_exception
    except JWTError:
        raise credentials_exception

    user = await db.get_user_by_id(user_id)
    if user is None:
        raise credentials_exception
    return user

# OAuth2 with bearer token
bearer_scheme = HTTPBearer()

async def verify_bearer_token(
    credentials: HTTPAuthorizationCredentials = Depends(bearer_scheme)
):
    token = credentials.credentials
    try:
        payload = jwt.decode(token, SECRET_KEY, algorithms=[ALGORITHM])
        return payload
    except JWTError:
        raise HTTPException(
            status_code=status.HTTP_401_UNAUTHORIZED,
            detail='Invalid authentication credentials'
        )

# OAuth2 with scopes
from fastapi.security import OAuth2PasswordBearer, SecurityScopes

oauth2_scheme_scopes = OAuth2PasswordBearer(
    tokenUrl='token',
    scopes={
        'users:read': 'Read user information',
        'users:write': 'Modify user information',
        'admin': 'Admin access'
    }
)

async def get_current_user_with_scopes(
    security_scopes: SecurityScopes,
    token: str = Depends(oauth2_scheme_scopes),
    db = Depends(get_db)
):
    if security_scopes.scopes:
        authenticate_value = f'Bearer scope="{security_scopes.scope_str}"'
    else:
        authenticate_value = 'Bearer'

    credentials_exception = HTTPException(
        status_code=status.HTTP_401_UNAUTHORIZED,
        detail='Could not validate credentials',
        headers={'WWW-Authenticate': authenticate_value},
    )

    try:
        payload = jwt.decode(token, SECRET_KEY, algorithms=[ALGORITHM])
        user_id: str = payload.get('sub')
        if user_id is None:
            raise credentials_exception
        token_scopes = payload.get('scopes', [])
    except JWTError:
        raise credentials_exception

    user = await db.get_user_by_id(user_id)
    if user is None:
        raise credentials_exception

    for scope in security_scopes.scopes:
        if scope not in token_scopes:
            raise HTTPException(
                status_code=status.HTTP_403_FORBIDDEN,
                detail='Not enough permissions',
                headers={'WWW-Authenticate': authenticate_value},
            )
    return user

# Use with scopes
@app.get('/users/me', dependencies=[Security(
    get_current_user_with_scopes,
    scopes=['users:read']
)])
async def read_users_me(
    current_user = Depends(get_current_user_with_scopes)
):
    return current_user

@app.put('/users/me', dependencies=[Security(
    get_current_user_with_scopes,
    scopes=['users:write']
)])
async def update_user_me(
    user_update: UserUpdate,
    current_user = Depends(get_current_user_with_scopes),
    db = Depends(get_db)
):
    updated_user = await db.update_user(current_user.id, user_update)
    return updated_user

Database Dependencies

Manage database connections with dependencies.

from fastapi import Depends
from sqlalchemy.ext.asyncio import AsyncSession, create_async_engine
from sqlalchemy.orm import sessionmaker

# SQLAlchemy setup
DATABASE_URL = 'postgresql+asyncpg://user:pass@localhost/db'
engine = create_async_engine(DATABASE_URL, echo=True)
AsyncSessionLocal = sessionmaker(engine, class_=AsyncSession, expire_on_commit=False)

# Dependency
async def get_db() -> AsyncSession:
    async with AsyncSessionLocal() as session:
        try:
            yield session
            await session.commit()
        except Exception:
            await session.rollback()
            raise
        finally:
            await session.close()

# Usage
from sqlalchemy import select

@app.get('/users/{user_id}')
async def get_user(user_id: int, db: AsyncSession = Depends(get_db)):
    result = await db.execute(select(User).where(User.id == user_id))
    user = result.scalar_one_or_none()
    if not user:
        raise HTTPException(status_code=404, detail='User not found')
    return user

# With transaction
@app.post('/users')
async def create_user(user_data: UserCreate, db: AsyncSession = Depends(get_db)):
    user = User(**user_data.dict())
    db.add(user)
    await db.flush()  # Get the ID before commit
    return user

Authentication Dependencies

Implement authentication patterns.

from fastapi import Depends, HTTPException, status
from fastapi.security import OAuth2PasswordBearer, OAuth2PasswordRequestForm
from jose import JWTError, jwt
from passlib.context import CryptContext

oauth2_scheme = OAuth2PasswordBearer(tokenUrl='token')
pwd_context = CryptContext(schemes=['bcrypt'], deprecated='auto')

SECRET_KEY = 'your-secret-key'
ALGORITHM = 'HS256'

# Token verification
async def get_current_user(
    token: str = Depends(oauth2_scheme),
    db = Depends(get_db)
):
    credentials_exception = HTTPException(
        status_code=status.HTTP_401_UNAUTHORIZED,
        detail='Could not validate credentials',
        headers={'WWW-Authenticate': 'Bearer'},
    )
    try:
        payload = jwt.decode(token, SECRET_KEY, algorithms=[ALGORITHM])
        email: str = payload.get('sub')
        if email is None:
            raise credentials_exception
    except JWTError:
        raise credentials_exception

    user = await db.fetch_one('SELECT * FROM users WHERE email = ?', email)
    if user is None:
        raise credentials_exception
    return user

# Active user check
async def get_current_active_user(
    current_user = Depends(get_current_user)
):
    if not current_user.is_active:
        raise HTTPException(status_code=400, detail='Inactive user')
    return current_user

# Role-based access
def require_role(required_role: str):
    async def role_checker(current_user = Depends(get_current_active_user)):
        if current_user.role != required_role:
            raise HTTPException(
                status_code=403,
                detail=f'Role {required_role} required'
            )
        return current_user
    return role_checker

@app.get('/admin')
async def admin_route(user = Depends(require_role('admin'))):
    return {'message': 'Admin access granted'}

Caching Dependencies

Implement caching patterns with dependencies.

from fastapi import Depends
from functools import lru_cache
import redis.asyncio as redis

# In-memory cache
@lru_cache()
def get_settings():
    return Settings()

@app.get('/config')
async def get_config(settings: Settings = Depends(get_settings)):
    return settings

# Redis cache dependency
class RedisCache:
    def __init__(self):
        self.redis = None

    async def get_connection(self):
        if not self.redis:
            self.redis = await redis.from_url('redis://localhost')
        return self.redis

    async def get(self, key: str):
        conn = await self.get_connection()
        value = await conn.get(key)
        return value.decode() if value else None

    async def set(self, key: str, value: str, expire: int = 3600):
        conn = await self.get_connection()
        await conn.set(key, value, ex=expire)

cache = RedisCache()

async def get_cache():
    return cache

@app.get('/cached-data/{key}')
async def get_cached_data(
    key: str,
    cache: RedisCache = Depends(get_cache)
):
    value = await cache.get(key)
    if value:
        return {'value': value, 'cached': True}

    # Fetch and cache
    value = fetch_expensive_data(key)
    await cache.set(key, value)
    return {'value': value, 'cached': False}

Background Task Dependencies

Use dependencies with background tasks.

from fastapi import BackgroundTasks, Depends
import asyncio

async def send_email(email: str, message: str):
    # Simulate sending email
    await asyncio.sleep(2)
    print(f'Email sent to {email}: {message}')

def get_email_service():
    # Could return configured email service
    return send_email

@app.post('/users')
async def create_user(
    user: UserCreate,
    background_tasks: BackgroundTasks,
    email_service = Depends(get_email_service),
    db = Depends(get_db)
):
    user_obj = await db.create_user(user)
    background_tasks.add_task(
        email_service,
        user.email,
        'Welcome to our service!'
    )
    return user_obj

# Complex background task with dependencies
class EmailService:
    def __init__(self, db = Depends(get_db)):
        self.db = db

    async def send_welcome_email(self, user_id: int):
        user = await self.db.get_user(user_id)
        await send_email(user.email, f'Welcome {user.name}!')

@app.post('/users/v2')
async def create_user_v2(
    user: UserCreate,
    background_tasks: BackgroundTasks,
    db = Depends(get_db)
):
    user_obj = await db.create_user(user)
    email_service = EmailService(db)
    background_tasks.add_task(
        email_service.send_welcome_email,
        user_obj.id
    )
    return user_obj

# Background task with cleanup
async def process_file_with_cleanup(file_path: str):
    try:
        # Process file
        await process_file(file_path)
    finally:
        # Cleanup
        os.remove(file_path)

@app.post('/upload')
async def upload_file(
    file: UploadFile,
    background_tasks: BackgroundTasks
):
    file_path = f'/tmp/{file.filename}'
    with open(file_path, 'wb') as f:
        f.write(await file.read())

    background_tasks.add_task(process_file_with_cleanup, file_path)
    return {'status': 'processing'}

WebSocket Dependencies

Use dependencies with WebSocket connections.

from fastapi import WebSocket, Depends, WebSocketDisconnect
from typing import List

class ConnectionManager:
    def __init__(self):
        self.active_connections: List[WebSocket] = []

    async def connect(self, websocket: WebSocket):
        await websocket.accept()
        self.active_connections.append(websocket)

    def disconnect(self, websocket: WebSocket):
        self.active_connections.remove(websocket)

    async def broadcast(self, message: str):
        for connection in self.active_connections:
            await connection.send_text(message)

manager = ConnectionManager()

def get_connection_manager():
    return manager

# WebSocket with authentication
async def get_ws_current_user(
    websocket: WebSocket,
    token: str = Query(...),
    db = Depends(get_db)
):
    try:
        payload = jwt.decode(token, SECRET_KEY, algorithms=[ALGORITHM])
        user_id = payload.get('sub')
        user = await db.get_user_by_id(user_id)
        if not user:
            await websocket.close(code=1008)
            raise HTTPException(status_code=401, detail='Invalid token')
        return user
    except JWTError:
        await websocket.close(code=1008)
        raise HTTPException(status_code=401, detail='Invalid token')

@app.websocket('/ws/{client_id}')
async def websocket_endpoint(
    websocket: WebSocket,
    client_id: int,
    manager: ConnectionManager = Depends(get_connection_manager),
    user = Depends(get_ws_current_user)
):
    await manager.connect(websocket)
    try:
        while True:
            data = await websocket.receive_text()
            message = f'User {user.name}: {data}'
            await manager.broadcast(message)
    except WebSocketDisconnect:
        manager.disconnect(websocket)
        await manager.broadcast(f'User {user.name} left the chat')

# WebSocket with database
@app.websocket('/ws/messages/{room_id}')
async def message_websocket(
    websocket: WebSocket,
    room_id: int,
    db = Depends(get_db),
    user = Depends(get_ws_current_user)
):
    await websocket.accept()

    # Send message history
    messages = await db.get_room_messages(room_id)
    await websocket.send_json(messages)

    try:
        while True:
            data = await websocket.receive_text()
            # Save to database
            message = await db.create_message(
                room_id=room_id,
                user_id=user.id,
                content=data
            )
            await websocket.send_json(message)
    except WebSocketDisconnect:
        pass

Custom Dependency Providers

Create custom dependency injection patterns.

from typing import Callable, Type, TypeVar, Generic
from fastapi import Depends

T = TypeVar('T')

# Dependency factory
class DependencyFactory(Generic[T]):
    def __init__(self, dependency_class: Type[T], **kwargs):
        self.dependency_class = dependency_class
        self.kwargs = kwargs

    def __call__(self) -> T:
        return self.dependency_class(**self.kwargs)

# Service locator pattern
class ServiceLocator:
    def __init__(self):
        self._services = {}

    def register(self, name: str, service):
        self._services[name] = service

    def get(self, name: str):
        return self._services.get(name)

    def create_dependency(self, name: str):
        def get_service():
            service = self.get(name)
            if service is None:
                raise ValueError(f'Service {name} not registered')
            return service
        return get_service

# Initialize service locator
locator = ServiceLocator()
locator.register('db', Database('postgresql://localhost/db'))
locator.register('cache', Redis('redis://localhost'))

# Use in endpoint
get_db_from_locator = locator.create_dependency('db')

@app.get('/items')
async def get_items(db = Depends(get_db_from_locator)):
    return await db.fetch_all('SELECT * FROM items')

# Dependency provider with context
class ContextualDependency:
    def __init__(self, request: Request):
        self.request = request
        self.context = {}

    def set(self, key: str, value):
        self.context[key] = value

    def get(self, key: str):
        return self.context.get(key)

async def get_request_context(request: Request):
    context = ContextualDependency(request)
    context.set('request_id', str(uuid.uuid4()))
    context.set('timestamp', datetime.utcnow())
    return context

@app.get('/context-example')
async def context_example(context = Depends(get_request_context)):
    return {
        'request_id': context.get('request_id'),
        'timestamp': context.get('timestamp')
    }

# Lazy dependency loading
class LazyDependency:
    def __init__(self, factory: Callable):
        self.factory = factory
        self._instance = None

    def __call__(self):
        if self._instance is None:
            self._instance = self.factory()
        return self._instance

def create_expensive_service():
    print('Creating expensive service...')
    return ExpensiveService()

expensive_service = LazyDependency(create_expensive_service)

@app.get('/expensive')
async def use_expensive(service = Depends(expensive_service)):
    return service.do_work()

Scoped Dependencies Per Request

Manage request-scoped state and lifecycle.

from contextvars import ContextVar
from fastapi import Depends, Request

# Request-scoped storage using context variables
request_id_var: ContextVar[str] = ContextVar('request_id')

async def set_request_id(request: Request):
    request_id = request.headers.get('X-Request-ID', str(uuid.uuid4()))
    request_id_var.set(request_id)
    return request_id

async def get_request_id():
    return request_id_var.get()

# Request-scoped logger
class RequestLogger:
    def __init__(self, request_id: str = Depends(get_request_id)):
        self.request_id = request_id

    def info(self, message: str):
        print(f'[{self.request_id}] INFO: {message}')

    def error(self, message: str):
        print(f'[{self.request_id}] ERROR: {message}')

@app.get('/scoped-logging')
async def scoped_logging(
    request_id: str = Depends(set_request_id),
    logger: RequestLogger = Depends()
):
    logger.info('Processing request')
    # Do work
    logger.info('Request completed')
    return {'request_id': request_id}

# Request-scoped unit of work pattern
class UnitOfWork:
    def __init__(self, db = Depends(get_db)):
        self.db = db
        self.repositories = {}

    def get_repository(self, model_class):
        if model_class not in self.repositories:
            self.repositories[model_class] = Repository(self.db, model_class)
        return self.repositories[model_class]

    async def commit(self):
        await self.db.commit()

    async def rollback(self):
        await self.db.rollback()

@app.post('/complex-transaction')
async def complex_transaction(
    data: TransactionData,
    uow: UnitOfWork = Depends()
):
    try:
        user_repo = uow.get_repository(User)
        order_repo = uow.get_repository(Order)

        user = await user_repo.create(data.user)
        order = await order_repo.create(data.order, user_id=user.id)

        await uow.commit()
        return {'user': user, 'order': order}
    except Exception:
        await uow.rollback()
        raise

Global Dependencies with app.dependency_overrides

Use global dependency management and overrides.

from fastapi import FastAPI, Depends

app = FastAPI()

# Original dependencies
def get_production_db():
    return ProductionDatabase('postgresql://prod/db')

def get_production_cache():
    return RedisCache('redis://prod')

# Default app setup
@app.get('/data')
async def get_data(
    db = Depends(get_production_db),
    cache = Depends(get_production_cache)
):
    cached = await cache.get('data')
    if cached:
        return cached

    data = await db.fetch_all('SELECT * FROM data')
    await cache.set('data', data)
    return data

# Override for testing environment
if os.getenv('ENVIRONMENT') == 'test':
    def get_test_db():
        return TestDatabase(':memory:')

    def get_test_cache():
        return InMemoryCache()

    app.dependency_overrides[get_production_db] = get_test_db
    app.dependency_overrides[get_production_cache] = get_test_cache

# Override for development
if os.getenv('ENVIRONMENT') == 'development':
    def get_dev_db():
        return DevDatabase('postgresql://localhost/dev')

    app.dependency_overrides[get_production_db] = get_dev_db

# Dynamic override based on request
async def override_db_by_tenant(request: Request):
    tenant_id = request.headers.get('X-Tenant-ID')
    if tenant_id:
        return TenantDatabase(tenant_id)
    return get_production_db()

# Conditional override
def setup_dependencies(app: FastAPI, config: dict):
    if config.get('use_mock_db'):
        app.dependency_overrides[get_production_db] = lambda: MockDB()

    if config.get('use_local_cache'):
        app.dependency_overrides[get_production_cache] = lambda: LocalCache()

# Clear overrides
def clear_overrides():
    app.dependency_overrides = {}

Testing with Dependencies

Override dependencies for testing.

from fastapi.testclient import TestClient

# Original dependency
def get_db():
    db = ProductionDB()
    try:
        yield db
    finally:
        db.close()

app.dependency_overrides = {}

# Test with mock database
def test_get_users():
    def override_get_db():
        return MockDB()

    app.dependency_overrides[get_db] = override_get_db

    client = TestClient(app)
    response = client.get('/users')

    assert response.status_code == 200

    # Cleanup
    app.dependency_overrides = {}

# Pytest fixture for dependency override
import pytest

@pytest.fixture
def client():
    def override_get_db():
        return MockDB()

    app.dependency_overrides[get_db] = override_get_db
    client = TestClient(app)
    yield client
    app.dependency_overrides = {}

def test_with_fixture(client):
    response = client.get('/users')
    assert response.status_code == 200

Global Dependencies

Apply dependencies to all routes.

from fastapi import FastAPI, Depends

# Logging dependency
async def log_request(request: Request):
    print(f'{request.method} {request.url}')

# Rate limiting
async def rate_limit(request: Request):
    client_ip = request.client.host
    # Check rate limit
    if is_rate_limited(client_ip):
        raise HTTPException(status_code=429, detail='Too many requests')

# Apply globally
app = FastAPI(dependencies=[Depends(log_request), Depends(rate_limit)])

# Apply to router
router = APIRouter(dependencies=[Depends(get_current_user)])

@router.get('/protected-resource')
async def protected_route():
    return {'message': 'This requires authentication'}

app.include_router(router)

When to Use This Skill

Use fastapi-dependency-injection when building modern, production-ready applications that require advanced patterns, best practices, and optimal performance.

FastAPI DI Best Practices

1. Use yield for cleanup - Always use yield for resources that need cleanup like database connections, file handles, and network connections to ensure proper resource management

2. Leverage caching - Use dependency caching (enabled by default) to avoid redundant work within a request; multiple uses of the same dependency in one request share the same instance

3. Chain dependencies - Build complex dependencies from simpler sub-dependencies to create composable, testable, and maintainable code structures

4. Class-based for state - Use classes for dependencies that maintain state or configuration, leveraging __call__ method for callable instances

5. Type hints everywhere - Always add type hints for better editor support, automatic validation, and improved documentation generation

6. Override for testing - Use dependency_overrides to inject mocks during testing without modifying production code

7. Global for cross-cutting - Apply common dependencies (logging, auth, rate limiting) globally or at router level to avoid repetition

8. Async when possible - Use async dependencies for I/O operations to maximize performance and concurrency benefits

9. Separate concerns - Keep authentication, authorization, and business logic in separate dependencies for better testability and reusability

10. Document dependencies - Add docstrings to explain complex dependency chains, especially when building multi-level hierarchies

11. Use Security utilities - Leverage FastAPI's security utilities like OAuth2PasswordBearer and HTTPBearer for authentication patterns

12. Validate early - Place validation dependencies early in the chain to fail fast and provide clear error messages

13. Keep dependencies pure - Dependencies should have minimal side effects; use background tasks for non-critical operations

14. Use context managers - Wrap dependencies in context managers when dealing with transactions or resource pools

15. Dependency composition - Compose larger dependencies from smaller, focused ones rather than creating monolithic dependencies

FastAPI DI Common Pitfalls

1. Forgetting yield - Not using yield means resources won't be cleaned up properly, leading to connection leaks and resource exhaustion

2. Circular dependencies - Creating dependency cycles causes infinite loops and stack overflow errors; design dependencies in a directed acyclic graph

3. Not using Depends() - Forgetting Depends() wrapper means function is called directly instead of being injected, breaking the dependency resolution

4. Overusing use_cache=False - Disabling cache unnecessarily hurts performance by creating multiple instances of the same dependency per request

5. Heavy dependencies - Putting too much logic in dependencies instead of services makes them hard to test and violates single responsibility

6. Not testing overrides - Forgetting to test with dependency_overrides means tests may use production resources instead of mocks

7. Mixing sync and async - Incorrectly mixing synchronous and asynchronous dependencies can block the event loop or cause runtime errors

8. Global state issues - Not properly managing singleton dependencies leads to shared state bugs in concurrent requests

9. Exception handling - Not handling exceptions in dependencies properly can leave resources in inconsistent states or leak connections

10. Type hint mistakes - Missing or incorrect type hints break dependency injection and automatic validation

11. Ignoring dependency order - Dependencies are executed in the order they appear in function signature; incorrect order can cause issues

12. Not cleaning test overrides - Forgetting to reset app.dependency_overrides after tests causes subsequent tests to fail

13. Overusing global dependencies - Applying too many dependencies globally can hurt performance and make debugging difficult

14. Memory leaks with generators - Not properly closing resources in finally block of generator dependencies causes memory leaks

15. Security misconfiguration - Using weak or missing security dependencies exposes endpoints to unauthorized access

Advanced Caching Patterns

Implement sophisticated caching strategies with dependencies.

from fastapi import Depends
import hashlib
import json

# Multi-layer cache with fallback
class CacheLayer:
    def __init__(
        self,
        memory_cache = Depends(get_memory_cache),
        redis_cache = Depends(get_redis_cache)
    ):
        self.memory = memory_cache
        self.redis = redis_cache

    async def get(self, key: str):
        # Try memory first
        value = self.memory.get(key)
        if value:
            return value

        # Try Redis
        value = await self.redis.get(key)
        if value:
            # Populate memory cache
            self.memory.set(key, value)
            return value

        return None

    async def set(self, key: str, value, ttl: int = 3600):
        self.memory.set(key, value, ttl=min(ttl, 300))
        await self.redis.set(key, value, ttl=ttl)

# Cache key generation
def create_cache_key(*args, **kwargs):
    key_data = json.dumps({'args': args, 'kwargs': kwargs}, sort_keys=True)
    return hashlib.md5(key_data.encode()).hexdigest()

# Dependency with automatic caching
def cached_dependency(ttl: int = 3600):
    async def dependency(
        params: dict,
        cache: CacheLayer = Depends()
    ):
        cache_key = create_cache_key(**params)
        cached_value = await cache.get(cache_key)

        if cached_value:
            return cached_value

        # Compute expensive value
        value = await compute_expensive_value(params)
        await cache.set(cache_key, value, ttl=ttl)
        return value

    return dependency

@app.get('/cached-endpoint')
async def cached_endpoint(
    result = Depends(cached_dependency(ttl=1800))
):
    return result

# Cache invalidation dependency
class CacheInvalidator:
    def __init__(self, cache: CacheLayer = Depends()):
        self.cache = cache
        self.invalidation_queue = []

    def invalidate(self, pattern: str):
        self.invalidation_queue.append(pattern)

    async def flush(self):
        for pattern in self.invalidation_queue:
            await self.cache.redis.delete_pattern(pattern)
        self.invalidation_queue.clear()

@app.post('/users')
async def create_user(
    user: UserCreate,
    db = Depends(get_db),
    invalidator: CacheInvalidator = Depends()
):
    new_user = await db.create_user(user)
    invalidator.invalidate('users:*')
    await invalidator.flush()
    return new_user

Middleware-Style Dependencies

Use dependencies for cross-cutting concerns.

from fastapi import Depends, Request
from time import time

# Request timing dependency
async def measure_request_time(request: Request):
    start_time = time()
    yield
    duration = time() - start_time
    print(f'{request.method} {request.url.path} took {duration:.3f}s')

# Request ID tracking
async def track_request_id(request: Request):
    request_id = request.headers.get('X-Request-ID', str(uuid.uuid4()))
    request.state.request_id = request_id
    yield request_id

def get_request_id(request: Request):
    return request.state.request_id

# Rate limiting per user
class RateLimiter:
    def __init__(self):
        self.requests = {}

    async def check_rate_limit(
        self,
        user = Depends(get_current_user),
        cache = Depends(get_cache)
    ):
        key = f'rate_limit:{user.id}'
        count = await cache.incr(key)

        if count == 1:
            await cache.expire(key, 60)

        if count > 100:
            raise HTTPException(
                status_code=429,
                detail='Rate limit exceeded'
            )
        return True

rate_limiter = RateLimiter()

@app.get('/protected')
async def protected_endpoint(
    rate_limit_ok = Depends(rate_limiter.check_rate_limit)
):
    return {'message': 'Success'}

# Request validation
async def validate_content_type(request: Request):
    content_type = request.headers.get('Content-Type')
    if not content_type or 'application/json' not in content_type:
        raise HTTPException(
            status_code=415,
            detail='Content-Type must be application/json'
        )
    return True

@app.post('/data', dependencies=[Depends(validate_content_type)])
async def post_data(data: dict):
    return data

Resources

Official FastAPI documentation and guides:

• FastAPI Dependencies Core dependency injection concepts and basic usage
• Advanced Dependencies Advanced patterns including dependency overrides and custom providers
• Sub-Dependencies Building complex dependency chains and hierarchies
• Dependencies in Path Operations Using dependencies at the decorator level
• Global Dependencies Applying dependencies to entire applications or routers
• Testing Dependencies Overriding dependencies for testing purposes
• Security and OAuth2 Security utilities and OAuth2 implementation with dependencies
• SQL Databases Database session management with dependency injection
• Async SQL Async database patterns with SQLAlchemy

Additional resources:

• FastAPI GitHub Source code and examples
• FastAPI Discussions Community questions and patterns
• Dependency Injection Design Pattern General DI concepts and theory