thebushidocollective FastAPI Dependency Injection
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
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
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
Chain dependencies - Build complex dependencies from simpler sub-dependencies to create composable, testable, and maintainable code structures
Class-based for state - Use classes for dependencies that maintain state or configuration, leveraging
__call__method for callable instancesType hints everywhere - Always add type hints for better editor support, automatic validation, and improved documentation generation
Override for testing - Use dependency_overrides to inject mocks during testing without modifying production code
Global for cross-cutting - Apply common dependencies (logging, auth, rate limiting) globally or at router level to avoid repetition
Async when possible - Use async dependencies for I/O operations to maximize performance and concurrency benefits
Separate concerns - Keep authentication, authorization, and business logic in separate dependencies for better testability and reusability
Document dependencies - Add docstrings to explain complex dependency chains, especially when building multi-level hierarchies
Use Security utilities - Leverage FastAPI's security utilities like OAuth2PasswordBearer and HTTPBearer for authentication patterns
Validate early - Place validation dependencies early in the chain to fail fast and provide clear error messages
Keep dependencies pure - Dependencies should have minimal side effects; use background tasks for non-critical operations
Use context managers - Wrap dependencies in context managers when dealing with transactions or resource pools
Dependency composition - Compose larger dependencies from smaller, focused ones rather than creating monolithic dependencies
FastAPI DI Common Pitfalls
Forgetting yield - Not using yield means resources won't be cleaned up properly, leading to connection leaks and resource exhaustion
Circular dependencies - Creating dependency cycles causes infinite loops and stack overflow errors; design dependencies in a directed acyclic graph
Not using Depends() - Forgetting Depends() wrapper means function is called directly instead of being injected, breaking the dependency resolution
Overusing use_cache=False - Disabling cache unnecessarily hurts performance by creating multiple instances of the same dependency per request
Heavy dependencies - Putting too much logic in dependencies instead of services makes them hard to test and violates single responsibility
Not testing overrides - Forgetting to test with dependency_overrides means tests may use production resources instead of mocks
Mixing sync and async - Incorrectly mixing synchronous and asynchronous dependencies can block the event loop or cause runtime errors
Global state issues - Not properly managing singleton dependencies leads to shared state bugs in concurrent requests
Exception handling - Not handling exceptions in dependencies properly can leave resources in inconsistent states or leak connections
Type hint mistakes - Missing or incorrect type hints break dependency injection and automatic validation
Ignoring dependency order - Dependencies are executed in the order they appear in function signature; incorrect order can cause issues
Not cleaning test overrides - Forgetting to reset app.dependency_overrides after tests causes subsequent tests to fail
Overusing global dependencies - Applying too many dependencies globally can hurt performance and make debugging difficult
Memory leaks with generators - Not properly closing resources in finally block of generator dependencies causes memory leaks
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_
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