Async generator dependencies in FastAPI - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this FastAPI async dependency?

Consider this FastAPI async generator dependency. What will the endpoint return when called?

FastAPI

from fastapi import FastAPI, Depends

app = FastAPI()

async def async_dep():
    yield "hello"

@app.get("/test")
async def test(dep: str = Depends(async_dep)):
    return {"message": dep}

A {"message": null}

B {"message": "hello"}

C TypeError: 'async_generator' object is not subscriptable

D 500 Internal Server Error

💡 Hint

Remember that async generator dependencies yield values that are passed to the endpoint parameter.

The async generator dependency yields the string "hello", which is passed as the value of the 'dep' parameter. The endpoint returns it in a JSON response.

❓ lifecycle

intermediate

When is the cleanup code in an async generator dependency executed?

Given this async generator dependency, when will the cleanup code after the yield run?

FastAPI

from fastapi import FastAPI, Depends

app = FastAPI()

async def resource_dep():
    print("Setup resource")
    yield "resource"
    print("Cleanup resource")

@app.get("/use")
async def use_resource(res: str = Depends(resource_dep)):
    return {"used": res}

A Only if the endpoint raises an exception

B Before the endpoint function starts executing

C Never, the cleanup code is ignored

D Immediately after the endpoint function finishes executing

💡 Hint

Think about how async generators work with yield and what happens after yield.

In FastAPI, the code after yield in an async generator dependency runs after the endpoint function completes, allowing cleanup of resources.

📝 Syntax

advanced

Which option correctly defines an async generator dependency in FastAPI?

Select the option that correctly defines an async generator dependency that yields a database session.

A

async def get_db():
    db = Session()
    return db
    db.close()

B

def get_db():
    db = Session()
    yield db
    db.close()

C

async def get_db():
    db = Session()
    yield db
    db.close()

D

async def get_db():
    db = Session()
    yield db
    await db.close()

💡 Hint

Async generator dependencies must use yield and can have cleanup code after yield.

Option C correctly defines an async generator dependency with yield and cleanup after yield. Option C is not async, so FastAPI treats it as a normal function, which is invalid for async cleanup. Option C uses return instead of yield, so no cleanup runs. Option C incorrectly awaits a synchronous close method.

🔧 Debug

advanced

Why does this async generator dependency cause a runtime error?

Examine the code and select the reason for the runtime error when calling the endpoint.

FastAPI

from fastapi import FastAPI, Depends

app = FastAPI()

async def broken_dep():
    resource = open("file.txt")
    yield resource
    resource.close()

@app.get("/read")
async def read_file(f = Depends(broken_dep)):
    return {"content": f.read()}

A Using a synchronous file object in an async dependency causes blocking and runtime error

B The file is closed before reading, causing a ValueError

C The dependency must be a normal function, not async

D Missing await before resource.read() causes a TypeError

💡 Hint

Think about mixing synchronous blocking calls inside async functions.

Opening a file synchronously inside an async generator blocks the event loop, which can cause runtime warnings or errors in async frameworks like FastAPI.

🧠 Conceptual

expert

What happens if an async generator dependency raises an exception after yield?

In FastAPI, if an async generator dependency yields a value but then raises an exception during cleanup code after yield, what is the effect on the endpoint response?

A The endpoint response is replaced by a 500 Internal Server Error due to the exception

B The endpoint response is sent normally, but the exception is logged and does not affect the response

C The exception is silently ignored and no logs are produced

D The endpoint never completes and hangs indefinitely

💡 Hint

Consider how FastAPI handles exceptions in dependency cleanup after the endpoint returns.

If an exception occurs in the cleanup code after yield, FastAPI treats it as an error and returns a 500 Internal Server Error response, interrupting normal response flow.

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