Custom middleware creation in Flask - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What does this Flask middleware do to the response?

Consider this Flask middleware that modifies the response body. What will be the final response content when a client requests the root URL?

Flask

from flask import Flask, request, Response

app = Flask(__name__)

@app.before_request
def before():
    print('Before request')

@app.after_request
def after(response):
    response.set_data(response.get_data(as_text=True) + ' World!')
    return response

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

if __name__ == '__main__':
    app.run()

A "Hello World!"

B "Hello"

C " World!"

D Empty response

💡 Hint

Look at how the after_request function changes the response data.

The after_request middleware appends ' World!' to the original response 'Hello', so the final output is 'Hello World!'.

❓ lifecycle

intermediate

When is Flask middleware executed in the request lifecycle?

Given Flask middleware functions before_request and after_request, when are they executed relative to the route handler?

A before_request runs after the route handler; after_request runs before the route handler

B before_request runs before the route handler; after_request runs after the route handler

C Both run before the route handler

D Both run after the route handler

💡 Hint

Think about the names and purpose of these middleware hooks.

before_request runs before the route handler to prepare or check the request. after_request runs after the route handler to modify the response.

📝 Syntax

advanced

Which middleware code snippet correctly adds a custom header to all responses?

Choose the correct Flask middleware code that adds the header X-Custom: True to every response.

A

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

B

@app.before_request
def add_header():
    response.headers['X-Custom'] = 'True'
    return response

C

@app.before_request
def add_header(response):
    response.headers['X-Custom'] = 'True'
    return response

D

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

💡 Hint

Remember which middleware hook receives the response object.

The after_request function receives the response object and can modify headers. The before_request does not receive the response.

🔧 Debug

advanced

What does this Flask middleware do when a request is made?

Examine the middleware code below. What happens when a request is made?

Flask

from flask import Flask
app = Flask(__name__)

@app.before_request
def check():
    return 'Blocked'

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

if __name__ == '__main__':
    app.run()

A The route handler is never called because before_request raises an exception

B The before_request must return None or a Response object, returning a string causes a TypeError

C The before_request returns a string which stops the request and returns 'Blocked' instead of calling the route

D The middleware is missing a return statement, causing a runtime error

💡 Hint

Check what returning a string from before_request does in Flask.

Returning a string from before_request stops further processing and sends that string as the response. This is valid and does not cause an error.

❓ state_output

expert

What is the final response content after multiple middleware modify it?

Given these two Flask middleware functions, what is the final response content when requesting '/'?

Flask

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

@app.after_request
def add_hello(response):
    response.set_data(response.get_data(as_text=True) + ' Hello')
    return response

@app.after_request
def add_world(response):
    response.set_data(response.get_data(as_text=True) + ' World')
    return response

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

if __name__ == '__main__':
    app.run()

A "Start World Hello"

B "Start Hello"

C "Start Hello World"

D "Start World"

💡 Hint

Consider the order in which multiple after_request functions run (reverse registration order).

Flask runs multiple after_request functions in reverse order of registration. add_world (later registered) runs first, appending ' World', then add_hello appends ' Hello'.