After_request hooks in Flask - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What does this Flask after_request hook do?

Consider this Flask app code with an after_request hook. What will be the response header 'X-Hello' value for any request?

Flask

from flask import Flask, Response
app = Flask(__name__)

@app.after_request
def add_header(response: Response):
    response.headers['X-Hello'] = 'World'
    return response

@app.route('/')
def index():
    return 'Hi!'

# Assume app runs normally

A The response will not include any 'X-Hello' header.

B The response will include header 'X-Hello' with value 'Hello'.

C The response will include header 'X-Hello' with value 'World'.

D The code will raise a runtime error because after_request must return a string.

💡 Hint

Remember that after_request modifies the response object before sending it.

The after_request hook modifies the response headers by adding 'X-Hello' with value 'World'. This header will be present in every response.

❓ lifecycle

intermediate

When is the after_request hook called in Flask?

In the Flask request lifecycle, at what point is the after_request function executed?

A Before the view function is called.

B After the view function returns but before the response is sent to the client.

C Only if the view function raises an exception.

D After the response is sent to the client.

💡 Hint

Think about when you want to modify the response before the client sees it.

The after_request hook runs after the view function returns a response but before Flask sends it to the client. It allows modifying the response.

🔧 Debug

advanced

Why does this after_request hook cause an error?

Examine this code snippet. Why will it cause an error when a request is made?

Flask

from flask import Flask
app = Flask(__name__)

@app.after_request
def faulty_hook(response):
    return 'Not a response object'

@app.route('/')
def home():
    return 'Hello'

A Because after_request must return a response object, not a string.

B Because the after_request function must accept no arguments.

C Because the home route returns a string instead of a response object.

D Because the after_request function is missing the @app.route decorator.

💡 Hint

Check what type the after_request function returns.

The after_request hook must return a valid response object. Returning a string causes Flask to raise a TypeError.

📝 Syntax

advanced

Which after_request hook syntax is correct?

Choose the correct syntax for an after_request hook that adds a header 'X-Test' with value 'Yes'.

A

@app.after_request
def add_header():
    response.headers['X-Test'] = 'Yes'
    return response

B

@app.after_request()
def add_header():
    response.headers['X-Test'] = 'Yes'
    return response

C

@app.after_request
def add_header(response):
    response['X-Test'] = 'Yes'
    return response

D

@app.after_request
def add_header(response):
    response.headers['X-Test'] = 'Yes'
    return response

💡 Hint

Remember the after_request function must accept the response argument and return it.

Option D correctly defines the after_request hook with the response parameter and modifies headers properly.

❓ state_output

expert

What is the final response header after multiple after_request hooks?

Given this Flask app with two after_request hooks, what is the final value of the 'X-Count' header in the response?

Flask

from flask import Flask, Response
app = Flask(__name__)

@app.after_request
def first_hook(response: Response):
    response.headers['X-Count'] = '1'
    return response

@app.after_request
def second_hook(response: Response):
    current = int(response.headers.get('X-Count', '0'))
    response.headers['X-Count'] = str(current + 1)
    return response

@app.route('/')
def index():
    return 'Test'

A 'X-Count' header will be '2'.

B 'X-Count' header will be '1'.

C 'X-Count' header will be '0'.

D The code will raise a KeyError.

💡 Hint

Consider the order in which after_request hooks run and how headers are updated.

Flask calls after_request hooks in the order they are registered. The first sets 'X-Count' to '1', the second increments it to '2'.