Async generator dependencies in FastAPI - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an async generator dependency in FastAPI?

An async generator dependency is a function that uses async and yield to provide a resource, like a database session, and then clean it up after use. It allows setup and teardown around a request.

beginner

How do you define an async generator dependency in FastAPI?

You define it as an async function that uses yield to provide a value. After the yield, you put cleanup code that runs after the request finishes.

intermediate

Why use async generator dependencies instead of normal functions in FastAPI?

Because async generator dependencies let you run code before and after a request asynchronously, like opening and closing a database connection safely.

beginner

What happens after the yield statement in an async generator dependency?

The code after yield runs after the request is done. It usually cleans up resources like closing connections or files.

beginner

Can async generator dependencies be used with FastAPI's Depends function?

Yes, you use Depends to declare async generator dependencies in your path operation functions. FastAPI handles calling and cleanup automatically.

What keyword is used to provide a value in an async generator dependency?

A yield

B return

C await

D async

The yield keyword provides the value and pauses the function until cleanup is needed.

What does FastAPI do after the yield in an async generator dependency?

A Ignores the rest of the function

B Starts a new request

C Runs cleanup code

D Returns the value again

FastAPI runs the code after yield to clean up resources after the request.

Why use async generator dependencies in FastAPI?

A To manage resources with setup and cleanup

B To speed up the server startup

C To avoid using async functions

D To block other requests

They help manage resources like database connections safely during requests.

How do you declare an async generator dependency in a path operation?

A Using a global variable

B Using Depends with the async generator function

C Using return statement

D Using a class

You use Depends to tell FastAPI to use the async generator dependency.

Can async generator dependencies run synchronous code after yield?

A Only if decorated

B No, it must be async only

C No, it must be sync only

D Yes, cleanup code can be sync or async

Cleanup code after yield can be synchronous or asynchronous.

Explain how async generator dependencies work in FastAPI and why they are useful.

Think about setup and teardown around a request.

Describe a real-life example where you would use an async generator dependency in FastAPI.

Imagine opening and closing a door for each visitor.

Practice

1. What is the main purpose of using an async generator dependency in FastAPI?

easy

A. To manage setup and cleanup actions around a request automatically

B. To speed up synchronous code execution

C. To replace all regular functions with asynchronous ones

D. To create infinite loops inside API endpoints

Solution

1. Step 1: Understand async generator dependencies

   Async generator dependencies use async def with yield to run setup code before yielding and cleanup code after.

2. Step 2: Identify their purpose in FastAPI

   This pattern is mainly used to manage resources safely, like opening and closing database connections around a request.

3. Final Answer:

   To manage setup and cleanup actions around a request automatically -> Option A

4. Quick Check:

   Async generator dependencies = setup + cleanup [OK]

Hint: Think setup before yield, cleanup after yield [OK]

Common Mistakes:

• Confusing async generators with normal async functions
• Thinking they speed up synchronous code
• Assuming they replace all functions

2. Which of the following is the correct syntax to define an async generator dependency in FastAPI?

easy

A. async def resource(): yield 'data'

B. def resource(): yield 'data'

C. async def resource(): return 'data'

D. def resource(): return 'data'

Solution

1. Step 1: Identify async generator syntax

   An async generator uses async def and yield inside the function body.

2. Step 2: Compare options

   async def resource(): yield 'data' uses async def and yield, which is correct. Options A and B are synchronous, and C uses return instead of yield.

3. Final Answer:

   async def resource(): yield 'data' -> Option A

4. Quick Check:

   Async + yield = async generator [OK]

Hint: Async generator = async def + yield inside [OK]

Common Mistakes:

• Using return instead of yield
• Forgetting async keyword
• Using synchronous def for async generator

3. Given this FastAPI async generator dependency, what will be printed when the endpoint is called?

async def resource():
    print('Setup')
    yield 'data'
    print('Cleanup')

@app.get('/')
async def read(resource=Depends(resource)):
    print('Inside endpoint')
    return resource

medium

A. Inside endpoint only

B. Inside endpoint, Setup, Cleanup

C. Setup, Cleanup, Inside endpoint

D. Setup, Inside endpoint, Cleanup

Solution

1. Step 1: Understand async generator flow

   When the endpoint is called, FastAPI runs the async generator up to the yield (prints 'Setup'), then runs the endpoint code (prints 'Inside endpoint'), then after response finishes, runs code after yield (prints 'Cleanup').

2. Step 2: Order of prints

   The prints happen in order: 'Setup', 'Inside endpoint', 'Cleanup'.

3. Final Answer:

   Setup, Inside endpoint, Cleanup -> Option D

4. Quick Check:

   Yield pauses, so Setup before, Cleanup after endpoint [OK]

Hint: Setup prints before yield, cleanup after endpoint runs [OK]

Common Mistakes:

• Assuming cleanup runs before endpoint
• Thinking yield returns immediately
• Ignoring async generator lifecycle

4. Identify the error in this async generator dependency code:

async def resource():
    connection = await connect_db()
    yield connection
    await connection.close()

@app.get('/')
async def read(conn=Depends(resource)):
    return {'status': 'ok'}

medium

A. Missing await before yield

B. Cannot yield inside async def

C. Not using try-finally to ensure cleanup

D. Depends should not be used with async generators

Solution

1. Step 1: Check resource cleanup safety

   Without try-finally, if an exception occurs, await connection.close() might not run, causing resource leaks.

2. Step 2: Proper pattern for async generator cleanup

   Use try-finally around yield to guarantee cleanup runs even on errors.

3. Final Answer:

   Not using try-finally to ensure cleanup -> Option C

4. Quick Check:

   Always use try-finally for cleanup in async generators [OK]

Hint: Use try-finally to guarantee cleanup after yield [OK]

Common Mistakes:

• Forgetting try-finally causes resource leaks
• Thinking yield needs await
• Believing Depends can't use async generators

5. You want to create an async generator dependency that opens a database connection, yields it, and ensures it closes even if the endpoint raises an error. Which code snippet correctly implements this?

hard

A. async def db():
conn = await open_db()
yield conn
await conn.close()

B. async def db():
conn = await open_db()
try:
yield conn
finally:
await conn.close()

C. async def db():
conn = await open_db()
yield conn
conn.close()

D. async def db():
conn = open_db()
yield conn
await conn.close()

Solution

1. Step 1: Ensure proper async resource management

   Opening the connection must be awaited, and closing must happen even if errors occur.

2. Step 2: Use try-finally around yield

   Only async def db(): conn = await open_db() try: yield conn finally: await conn.close() uses try-finally to guarantee await conn.close() runs after yield, even on exceptions.

3. Final Answer:

   async def db(): conn = await open_db() try: yield conn finally: await conn.close() -> Option B

4. Quick Check:

   Try-finally ensures cleanup after yield [OK]

Hint: Wrap yield in try-finally to always close resources [OK]

Common Mistakes:

• Not awaiting open_db()
• Not using try-finally for cleanup
• Calling close() without await