Async generator dependencies in FastAPI - Mini Project: Build & Apply

Async Generator Dependencies in FastAPI

📖 Scenario: You are building a simple FastAPI app that needs to manage a resource using an async generator dependency. This pattern is useful for opening and closing connections or sessions cleanly.

🎯 Goal: Create an async generator dependency that yields a resource string, then use it in a FastAPI route to return the resource.

📋 What You'll Learn

Create an async generator function called get_resource that yields the string 'resource_opened'

Use yield inside get_resource to provide the resource

Create a FastAPI app instance called app

Create a GET route /resource that uses get_resource as a dependency

Return the yielded resource string from the route handler

💡 Why This Matters

🌍 Real World

Async generator dependencies are used in FastAPI to manage resources like database connections, sessions, or files that need setup and cleanup around a request.

💼 Career

Understanding async generator dependencies is important for backend developers working with FastAPI to write clean, efficient, and maintainable code that manages resources properly.

1

Create the async generator dependency

Create an async generator function called get_resource that yields the string 'resource_opened'.

FastAPI

# Your code here

Hint

Use async def to define the function and yield to provide the resource.

2

Create the FastAPI app instance

Import FastAPI from fastapi and create an app instance called app.

FastAPI

async def get_resource():
    yield 'resource_opened'

# Your code here

Hint

Use from fastapi import FastAPI and then app = FastAPI().

3

Create a GET route using the async generator dependency

Create a GET route /resource using @app.get("/resource"). Define an async function read_resource that takes a parameter resource with a dependency on get_resource using Depends. Return the resource string from the function.

FastAPI

from fastapi import FastAPI
from fastapi import Depends

async def get_resource():
    yield 'resource_opened'

app = FastAPI()

# Your code here

Hint

Use @app.get("/resource") and Depends(get_resource) to inject the dependency.

4

Complete the FastAPI app with dependency injection

Ensure the full code includes the import of Depends, the async generator get_resource, the app instance app, and the GET route /resource that returns the yielded resource string.

FastAPI

from fastapi import FastAPI
from fastapi import Depends

async def get_resource():
    yield 'resource_opened'

app = FastAPI()

@app.get("/resource")
async def read_resource(resource: str = Depends(get_resource)):
    return resource

# Your code here if needed

Hint

Check that all parts are included and correctly connected.

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