Startup and shutdown events in FastAPI - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when the FastAPI app starts?

Consider this FastAPI app with a startup event handler. What will be printed to the console when the app starts?

FastAPI

from fastapi import FastAPI

app = FastAPI()

@app.on_event("startup")
async def startup_event():
    print("App is starting")

@app.get("/")
async def read_root():
    return {"message": "Hello"}

A Nothing is printed until the first request is received

B The app raises an error because startup events must be synchronous

C The message 'App is starting' is printed every time a request is received

D The message 'App is starting' is printed once when the app starts

💡 Hint

Think about when startup events run in FastAPI lifecycle.

Startup event handlers run once when the app starts, before handling any requests. They can be async functions.

❓ component_behavior

intermediate

What happens on app shutdown with multiple handlers?

Given this FastAPI app with two shutdown event handlers, what will be printed when the app shuts down?

FastAPI

from fastapi import FastAPI

app = FastAPI()

@app.on_event("shutdown")
async def shutdown_event_1():
    print("Shutdown handler 1")

@app.on_event("shutdown")
async def shutdown_event_2():
    print("Shutdown handler 2")

A Both 'Shutdown handler 1' and 'Shutdown handler 2' are printed, order not guaranteed

B Only 'Shutdown handler 2' is printed

C No output is printed because shutdown handlers must be synchronous

D Only 'Shutdown handler 1' is printed

💡 Hint

Multiple shutdown handlers can be registered and all run on shutdown.

FastAPI runs all registered shutdown event handlers on app shutdown. The order is not guaranteed.

📝 Syntax

advanced

Identify the syntax error in this startup event handler

Which option shows the correct way to define a startup event handler in FastAPI without syntax errors?

FastAPI

from fastapi import FastAPI

app = FastAPI()

@app.on_event("startup")
def startup_event():
    print("Starting")

A

@app.on_event("startup")
async def startup_event():
    print("Starting")

B

@app.on_event("startup")
def startup_event
    print("Starting")

C

@app.on_event("startup")
async startup_event():
    print("Starting")

D

@app.on_event("startup")
def startup_event():
print("Starting")

💡 Hint

Check function definition syntax and async keyword.

Option A correctly defines an async function with proper syntax. Others have syntax errors like missing colon or indentation.

❓ state_output

advanced

What is the value of the counter after app shutdown?

This FastAPI app increments a counter on startup and shutdown events. What is the final value of counter after the app starts and then shuts down?

FastAPI

from fastapi import FastAPI

app = FastAPI()
counter = 0

@app.on_event("startup")
async def startup_event():
    global counter
    counter += 1

@app.on_event("shutdown")
async def shutdown_event():
    global counter
    counter += 2

A 0

B 3

C 2

D 1

💡 Hint

Consider the order and increments during startup and shutdown.

Counter starts at 0, increments by 1 on startup, then by 2 on shutdown, total 3.

🔧 Debug

expert

Why does this shutdown event handler not run?

This FastAPI app has a shutdown event handler but it never prints its message when the app stops. Why?

FastAPI

from fastapi import FastAPI

app = FastAPI()

@app.on_event("shutdown")
def shutdown_event():
    print("App is shutting down")

@app.get("/")
async def read_root():
    return {"msg": "Hello"}

A The shutdown event handler is missing the @app.shutdown decorator

B The shutdown event handler is synchronous but must be async to run

C The app must be run with a server that supports lifespan events for shutdown to trigger

D The shutdown event handler must be called manually inside a route

💡 Hint

Think about how FastAPI apps are run and how shutdown events are triggered.

Shutdown events only run if the server supports lifespan events, e.g., uvicorn with proper shutdown. Running app without server or with improper server prevents shutdown events.

Practice

1. What is the main purpose of using @app.on_event("startup") in a FastAPI application?

easy

A. To define API routes for the application.

B. To handle HTTP requests from clients.

C. To shut down the server immediately.

D. To run code when the application starts, like initializing resources.

Solution

1. Step 1: Understand the startup event role

   The @app.on_event("startup") decorator marks a function to run when the app starts, useful for setup tasks.

2. Step 2: Differentiate from other app parts

   Handling requests or shutting down are different concerns; startup is specifically for initialization.

3. Final Answer:

   To run code when the application starts, like initializing resources. -> Option D

4. Quick Check:

   Startup event = run code at app start [OK]

Hint: Startup event runs code once when app launches [OK]

Common Mistakes:

• Confusing startup with request handling
• Thinking startup runs multiple times per request
• Mixing startup with shutdown event

2. Which of the following is the correct syntax to define a shutdown event handler in FastAPI?

easy

A. @app.on_event("start")
def cleanup():
print("Cleaning up")

B. @app.shutdown_event
def cleanup():
print("Cleaning up")

C. @app.on_event("shutdown")
def cleanup():
print("Cleaning up")

D. @app.event("shutdown")
def cleanup():
print("Cleaning up")

Solution

1. Step 1: Recall correct decorator for shutdown

   The correct decorator is @app.on_event("shutdown") to run code when the app stops.

2. Step 2: Check syntax correctness

   Options A, B, and D use incorrect decorator names or syntax.

3. Final Answer:

   @app.on_event("shutdown")\ndef cleanup():\n print("Cleaning up") -> Option C

4. Quick Check:

   Shutdown event decorator = @app.on_event("shutdown") [OK]

Hint: Shutdown event uses @app.on_event("shutdown") exactly [OK]

Common Mistakes:

• Using wrong event name like "start" instead of "shutdown"
• Missing @app.on_event decorator
• Using non-existent decorators like @app.shutdown_event

3. Consider this FastAPI code snippet:

from fastapi import FastAPI
app = FastAPI()

@app.on_event("startup")
async def startup_event():
    print("Starting app")

@app.on_event("shutdown")
async def shutdown_event():
    print("Stopping app")

@app.get("/")
async def read_root():
    return {"message": "Hello"}

What will be printed when the server starts and then stops?

medium

A. Stopping app (on start), Starting app (on shutdown)

B. Starting app (on start), Stopping app (on shutdown)

C. Only Starting app when server starts

D. No output printed at all

Solution

1. Step 1: Identify startup and shutdown prints

   The startup_event prints "Starting app" when the app starts, and shutdown_event prints "Stopping app" when the app stops.

2. Step 2: Understand event timing

   These events run once each: startup at launch, shutdown at server stop.

3. Final Answer:

   Starting app (on start), Stopping app (on shutdown) -> Option B

4. Quick Check:

   Startup prints "Starting app", shutdown prints "Stopping app" [OK]

Hint: Startup prints on launch, shutdown prints on stop [OK]

Common Mistakes:

• Swapping startup and shutdown outputs
• Expecting prints on every request
• Thinking no output occurs without explicit call

4. This FastAPI code is intended to print a message on shutdown but does not work:

from fastapi import FastAPI
app = FastAPI()

def shutdown_event():
    print("App is stopping")

app.on_event("shutdown", shutdown_event)

What is the problem?

medium

A. The decorator syntax is incorrect; should use @app.on_event("shutdown") above the function.

B. FastAPI does not support shutdown events.

C. The function name must be shutdown_handler, not shutdown_event.

D. The function must be async to work as an event handler.

Solution

1. Step 1: Check decorator usage

   The code uses app.on_event("shutdown", shutdown_event) which incorrectly passes two arguments to on_event; it expects only the event name and returns a decorator to apply to a function.

2. Step 2: Identify common mistake

   FastAPI expects the decorator syntax @app.on_event("shutdown") placed above the function definition for clarity and proper registration.

3. Final Answer:

   The decorator syntax is incorrect; should use @app.on_event("shutdown") above the function. -> Option A

4. Quick Check:

   Use @app.on_event("shutdown") decorator syntax [OK]

Hint: Always use @app.on_event("shutdown") decorator syntax [OK]

Common Mistakes:

• Assuming function must be async (it can be sync)
• Using wrong function names
• Believing FastAPI lacks shutdown support

5. You want to open a database connection when your FastAPI app starts and close it when it stops. Which code correctly uses startup and shutdown events to do this?

from fastapi import FastAPI
app = FastAPI()
db = None

# Option A
@app.on_event("startup")
async def connect_db():
    global db
    db = "Connected"

@app.on_event("shutdown")
async def close_db():
    global db
    db = None

# Option B
@app.on_event("startup")
def connect_db():
    db = "Connected"

@app.on_event("shutdown")
def close_db():
    db = None

# Option C
@app.on_event("startup")
async def connect_db():
    db = "Connected"

@app.on_event("shutdown")
async def close_db():
    db = None

# Option D
@app.on_event("startup")
async def connect_db():
    global db
    db = None

@app.on_event("shutdown")
async def close_db():
    global db
    db = "Connected"

hard

A. Option A correctly manages the global db connection on startup and shutdown.

B. Option B correctly manages db without global keyword.

C. Option C correctly manages db asynchronously without global keyword.

D. Option D reverses connection and disconnection logic.

Solution

1. Step 1: Understand global variable usage

   Since db is defined outside functions, to modify it inside functions, global db is needed.

2. Step 2: Check startup and shutdown logic

   The correct version uses global db in both async event handlers, setting db = "Connected" on startup and db = None on shutdown. Versions without global db create local variables. One version reverses the connection and disconnection logic.

3. Final Answer:

   Option A correctly manages the global db connection on startup and shutdown. -> Option A

4. Quick Check:

   Use global to modify external variables in event handlers [OK]

Hint: Use global keyword to modify external variables inside event functions [OK]

Common Mistakes:

• Forgetting global keyword causes local variable shadowing
• Reversing startup and shutdown logic
• Assuming async is required for all event handlers