Lifespan context manager in FastAPI

Introduction

The lifespan context manager helps you run setup and cleanup code when your FastAPI app starts and stops. It keeps your app organized and efficient.

You want to connect to a database when the app starts and disconnect when it stops.

You need to load some data or configuration once before the app handles requests.

You want to start background tasks or services when the app launches and stop them on shutdown.

You want to clean up resources like files or network connections when the app closes.

Syntax

FastAPI

from fastapi import FastAPI
from contextlib import asynccontextmanager

@asynccontextmanager
async def lifespan(app: FastAPI):
    # setup code here
    yield
    # cleanup code here

app = FastAPI(lifespan=lifespan)

The @asynccontextmanager decorator marks the function as a lifespan manager.

The code before yield runs at startup, and the code after yield runs at shutdown.

Examples

This example prints messages when the app starts and stops.

FastAPI

from fastapi import FastAPI
from contextlib import asynccontextmanager

@asynccontextmanager
async def lifespan(app: FastAPI):
    print('Starting app')
    yield
    print('Stopping app')

app = FastAPI(lifespan=lifespan)

This example connects to a database on startup and disconnects on shutdown.

FastAPI

from fastapi import FastAPI
from contextlib import asynccontextmanager

@asynccontextmanager
async def lifespan(app: FastAPI):
    app.state.db = connect_to_db()
    yield
    app.state.db.disconnect()

Sample Program

This FastAPI app uses the lifespan context manager to print messages when starting and stopping. It also keeps a counter of visits stored in app.state. Each time you visit the root URL, the counter increases.

FastAPI

from fastapi import FastAPI
from contextlib import asynccontextmanager

@asynccontextmanager
async def lifespan(app: FastAPI):
    print('App is starting')
    app.state.counter = 0
    yield
    print('App is stopping')

app = FastAPI(lifespan=lifespan)

@app.get('/')
async def read_root():
    app.state.counter += 1
    return {"visits": app.state.counter}

Important Notes

Use app.state to store data you want to keep during the app's life.

The lifespan function must be async and use yield to separate startup and shutdown code.

FastAPI automatically calls the lifespan manager when the app runs with an ASGI server like Uvicorn.

Summary

The lifespan context manager runs code when your FastAPI app starts and stops.

Use it to set up and clean up resources like databases or background tasks.

It keeps your app organized and efficient by managing app-wide state.

Practice

1. What is the main purpose of the lifespan context manager in a FastAPI application?

easy

A. To manage user authentication and authorization

B. To handle HTTP requests and responses

C. To define API routes and endpoints

D. To run setup code when the app starts and cleanup code when it stops

Solution

1. Step 1: Understand the role of lifespan context manager

   The lifespan context manager is designed to run code at the start and end of the FastAPI app lifecycle.

2. Step 2: Identify its main use

   It is used to set up resources like database connections when the app starts and clean them up when the app stops.

3. Final Answer:

   To run setup code when the app starts and cleanup code when it stops -> Option D

4. Quick Check:

   Lifespan manages startup and shutdown code = A [OK]

Hint: Lifespan runs code at app start and stop [OK]

Common Mistakes:

• Confusing lifespan with route handling
• Thinking lifespan manages HTTP requests
• Assuming lifespan handles user sessions

2. Which of the following is the correct way to define a lifespan context manager in FastAPI?

easy

A. async def lifespan(app): yield

B. def lifespan(): return app

C. async def lifespan(): return app

D. def lifespan(app): yield

Solution

1. Step 1: Recall lifespan signature

   The lifespan function must be async and accept the app parameter to manage startup and shutdown.

2. Step 2: Confirm use of yield

   Using yield inside the async function allows running code before and after the yield for startup and shutdown.

3. Final Answer:

   async def lifespan(app): yield -> Option A

4. Quick Check:

   Async + app param + yield = A [OK]

Hint: Lifespan is async with app param and uses yield [OK]

Common Mistakes:

• Forgetting async keyword
• Missing app parameter
• Using return instead of yield

3. Given this FastAPI lifespan code snippet, what will be printed when the app starts and stops?

async def lifespan(app):
    print('Starting app')
    yield
    print('Stopping app')

medium

A. Only 'Starting app' is printed

B. 'Starting app' prints on start, 'Stopping app' prints on shutdown

C. Only 'Stopping app' is printed

D. Neither message is printed

Solution

1. Step 1: Understand yield in lifespan

   The code before yield runs at startup, and code after yield runs at shutdown.

2. Step 2: Match prints to lifecycle events

   So 'Starting app' prints when app starts, and 'Stopping app' prints when app stops.

3. Final Answer:

   'Starting app' prints on start, 'Stopping app' prints on shutdown -> Option B

4. Quick Check:

   Code before yield = start, after yield = stop [OK]

Hint: Code before yield runs on start, after yield on stop [OK]

Common Mistakes:

• Thinking both prints run immediately
• Assuming yield blocks all prints
• Confusing start and stop timing

4. What is wrong with this lifespan context manager code?

async def lifespan(app):
    print('Starting')
    return
    print('Stopping')

medium

A. The app parameter is not used, causing runtime error

B. Missing async keyword causes syntax error

C. Using return instead of yield prevents shutdown code from running

D. Print statements are not allowed in lifespan functions

Solution

1. Step 1: Identify use of return instead of yield

   The lifespan function must use yield to separate startup and shutdown code.

2. Step 2: Understand effect of return

   Using return exits the function immediately, so shutdown code after it never runs.

3. Final Answer:

   Using return instead of yield prevents shutdown code from running -> Option C

4. Quick Check:

   Return exits early; yield separates start/stop [OK]

Hint: Use yield, not return, to run shutdown code [OK]

Common Mistakes:

• Confusing return and yield in async functions
• Ignoring that shutdown code runs after yield
• Assuming print statements cause errors

5. You want to open a database connection when your FastAPI app starts and close it when the app stops using the lifespan context manager. Which code correctly implements this?

hard

A. async def lifespan(app):
db = await connect_db()
app.state.db = db
yield
await db.close()

B. async def lifespan(app):
db = await connect_db()
yield
app.state.db = db
await db.close()

C. def lifespan(app):
db = connect_db()
app.state.db = db
yield
db.close()

D. async def lifespan():
db = await connect_db()
app.state.db = db
yield
await db.close()

Solution

1. Step 1: Confirm async function with app parameter

   The lifespan function must be async and accept the app parameter to store the db connection.

2. Step 2: Check order of operations

   Connect to the database before yield, store it on app.state, then close it after yield.

3. Step 3: Verify correct use of await and yield

   Await connect_db and db.close, yield separates startup and shutdown code.

4. Final Answer:

   async def lifespan(app): db = await connect_db() app.state.db = db yield await db.close() -> Option A

5. Quick Check:

   Async + app param + yield + await connect/close = D [OK]

Hint: Connect before yield, close after, store in app.state [OK]

Common Mistakes:

• Placing yield before storing db connection
• Missing async or await keywords
• Not passing app parameter to lifespan