fastapi-dependency-injection

# 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.

```python
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.

```python
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 o