Async generator dependencies in FastAPI - Step-by-Step Execution

Concept Flow - Async generator dependencies

Request received

↓

Call async generator dependency

↓

Yield resource (e.g., DB session)

↓

Use resource in endpoint

↓

After response sent

↓

Run cleanup code after yield

↓

Close resource

↓

Request complete

Shows how FastAPI calls an async generator dependency, yields a resource for use, then runs cleanup after response.

Execution Sample

FastAPI

async def get_db():
    db = create_db_session()
    try:
        yield db
    finally:
        await db.close()

Defines an async generator dependency that yields a DB session and closes it after use.

Execution Table

Step	Action	State Before	State After	Notes
1	Request starts, call get_db()	No DB session	DB session created	DB session object created but not yielded yet
2	Yield DB session	DB session created	DB session yielded	Endpoint receives DB session to use
3	Endpoint uses DB session	DB session yielded	DB session in use	Endpoint runs with DB session
4	Response sent	DB session in use	Response sent	Client gets response
5	Run finally block after yield	Response sent	Closing DB session	Cleanup starts
6	Await db.close()	Closing DB session	DB session closed	DB connection closed
7	Request complete	DB session closed	No active DB session	Dependency cleanup done

💡 Request lifecycle ends after cleanup code runs post-yield

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 5	After Step 6	Final
db	None	DB session created	DB session yielded	DB session yielded (still open)	DB session closing	DB session closed

Key Moments - 3 Insights

Why does the async generator use yield instead of return?

Because FastAPI needs to pause the function to give the resource to the endpoint, then resume after the response to run cleanup. See execution_table steps 2 and 5.

When does the cleanup code after yield run?

It runs after the response is sent to the client, as shown in execution_table step 5, ensuring resources are properly closed.

What happens if the endpoint raises an error while using the yielded resource?

The finally block still runs after yield, so cleanup happens regardless of errors, ensuring no resource leaks.

Visual Quiz - 3 Questions

Look at the execution_table, what is the state of 'db' after step 2?

A DB session yielded and ready for use

B DB session closed

C No DB session created yet

D DB session is closing

💡 Hint

Check the 'State After' column for step 2 in execution_table

At which step does the cleanup code start running?

A Step 3

B Step 1

C Step 5

D Step 7

💡 Hint

Look for 'Run finally block after yield' in execution_table

If the endpoint raises an error during DB use, what happens to the cleanup?

A Cleanup code does not run

B Cleanup code runs after response

C DB session stays open

D Request hangs indefinitely

💡 Hint

Refer to key_moments about error handling and finally block

Concept Snapshot

Async generator dependencies in FastAPI:
- Use async def with yield to provide resources
- Yield pauses function, lets endpoint use resource
- After response, code after yield runs for cleanup
- Ensures resources like DB sessions close properly
- Works even if endpoint errors occur

Full Transcript

This visual trace shows how FastAPI handles async generator dependencies. When a request comes in, FastAPI calls the async generator function, which creates a resource like a database session. It then yields this resource to the endpoint function, pausing the generator. The endpoint uses the resource to process the request. After the response is sent to the client, FastAPI resumes the generator to run the cleanup code after the yield, such as closing the database session. This ensures resources are properly managed and closed even if the endpoint raises an error. The variable tracker shows the state of the database session object through each step, from creation to closure. Key moments clarify why yield is used and when cleanup happens. The quiz questions help reinforce understanding by referencing the execution steps and variable states.