Why middleware extends functionality in Flask - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add middleware that logs each request in Flask.

Flask

from flask import Flask, request

app = Flask(__name__)

@app.before_request
def [1]():
    print(f"Request to: {request.path}")

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

Drag options to blanks, or click blank then click option'

Ahandle_request

Bbefore_request

Clog_request

Dprocess_request

Attempts:

3 left

💡 Hint

Common Mistakes

Using a function name that doesn't describe its purpose

Not using the @app.before_request decorator

The function decorated with @app.before_request runs before each request. Naming it log_request makes its purpose clear.

2fill in blank

medium

Complete the code to create middleware that modifies the response headers.

Flask

from flask import Flask, Response

app = Flask(__name__)

@app.after_request
def add_header(response: Response):
    response.headers['X-Custom-Header'] = [1]
    return response

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

Drag options to blanks, or click blank then click option'

A'Middleware-Active'

B'Active-Middleware'

C'Custom-Header'

D'Header-Set'

Attempts:

3 left

💡 Hint

Common Mistakes

Using a header value that doesn't clearly indicate middleware

Forgetting to return the response object

The middleware adds a custom header with the value 'Middleware-Active' to the response headers.

3fill in blank

hard

Fix the error in the middleware function that blocks requests to '/secret'.

Flask

from flask import Flask, request, abort

app = Flask(__name__)

@app.before_request
def block_secret():
    if request.path == [1]:
        abort(403)

@app.route('/secret')
def secret():
    return 'Top secret!'

Drag options to blanks, or click blank then click option'

A'/secret'

B'secret/'

C'/Secret'

D'secret'

Attempts:

3 left

💡 Hint

Common Mistakes

Using a path without the leading slash

Using incorrect casing in the path string

The path must exactly match '/secret' to block access properly.

4fill in blank

hard

Fill both blanks to create middleware that measures request processing time and adds it to response headers.

Flask

import time
from flask import Flask, g, request

app = Flask(__name__)

@app.before_request
def start_timer():
    g.start = [1]

@app.after_request
def add_duration(response):
    duration = time.time() - g.start
    response.headers['X-Duration'] = str(duration) + [2]
    return response

Drag options to blanks, or click blank then click option'

Atime.time()

B' seconds'

C' ms'

Dtime.perf_counter()

Attempts:

3 left

💡 Hint

Common Mistakes

Using time.perf_counter() which is more precise but less common here

Forgetting to convert duration to string before concatenation

The middleware uses time.time() to record the start time and adds the duration in seconds as a string with ' seconds' suffix to the response header.

5fill in blank

hard

Fill all three blanks to create middleware that modifies request data and response data.

Flask

from flask import Flask, request, jsonify

app = Flask(__name__)

@app.before_request
def modify_request():
    if request.is_json:
        data = request.get_json()
        data[[1]] = [2]
        request.json = data

@app.after_request
def modify_response(response):
    if response.is_json:
        json_data = response.get_json()
        json_data[[3]] = 'modified by middleware'
        response.set_data(jsonify(json_data).get_data())
    return response

Drag options to blanks, or click blank then click option'

A'middleware_flag'

BTrue

C'middleware_status'

D'status'

Attempts:

3 left

💡 Hint

Common Mistakes

Using keys that don't match between request and response

Not updating the response data properly after modification

The middleware adds a 'middleware_flag' with value True to the request JSON, and adds 'middleware_status' to the response JSON to show it was modified.

Practice

1. What is the main purpose of middleware in a Flask application?

easy

A. To add extra processing steps before or after the main app handles a request

B. To replace the Flask app's routing system

C. To directly modify the database schema

D. To serve static files like images and CSS

Solution

1. Step 1: Understand middleware role

   Middleware acts as a layer that processes requests or responses around the main app logic.

2. Step 2: Compare options

   Replacing routing, modifying database schema, or serving static files are not middleware tasks.

3. Final Answer:

   To add extra processing steps before or after the main app handles a request -> Option A

4. Quick Check:

   Middleware extends app functionality = C [OK]

Hint: Middleware adds steps around app requests and responses [OK]

Common Mistakes:

• Thinking middleware replaces routing
• Confusing middleware with static file serving
• Assuming middleware changes database structure

2. Which of the following is the correct way to add middleware in a Flask app?

easy

A. app = Middleware(app)

B. app.middleware = Middleware()

C. Middleware(app).run()

D. app.add_middleware(Middleware)

Solution

1. Step 1: Recall Flask middleware pattern

   Middleware wraps the Flask app by passing the app instance to the middleware constructor.

2. Step 2: Analyze options

   Only app = Middleware(app) correctly wraps the app instance. Others use invalid syntax or methods not in Flask.

3. Final Answer:

   app = Middleware(app) -> Option A

4. Quick Check:

   Middleware wraps app instance = A [OK]

Hint: Middleware wraps app like a gift box around it [OK]

Common Mistakes:

• Trying to assign middleware as an attribute
• Calling run() on middleware instead of app
• Using non-existent add_middleware method

3. Given this middleware code, what will be printed when a request is made?

class SimpleMiddleware:
    def __init__(self, app):
        self.app = app
    def __call__(self, environ, start_response):
        print('Before app')
        response = self.app(environ, start_response)
        print('After app')
        return response

app = SimpleMiddleware(app)

medium

A. No output printed

B. Only Before app printed

C. Only After app printed

D. Before app printed, then After app printed

Solution

1. Step 1: Understand __call__ method flow

   When a request comes, 'Before app' prints, then the wrapped app runs, then 'After app' prints.

2. Step 2: Confirm both prints execute

   Both print statements are before and after calling the app, so both appear in order.

3. Final Answer:

   Before app printed, then After app printed -> Option D

4. Quick Check:

   Middleware prints before and after app = A [OK]

Hint: Middleware prints before and after calling app [OK]

Common Mistakes:

• Thinking only one print runs
• Assuming prints happen in reverse order
• Forgetting __call__ runs on each request

4. This middleware code causes an error. What is the problem?

class BrokenMiddleware:
    def __init__(self, app):
        self.app = app
    def __call__(self, environ, start_response):
        # Forgot to call the wrapped app
        return 'Hello'

medium

A. Middleware must return a number, not a string

B. Middleware must call the wrapped app to return a valid response

C. Middleware should not have a __call__ method

D. Middleware __init__ method is missing

Solution

1. Step 1: Check __call__ method behavior

   The middleware returns a string directly without calling the wrapped app, breaking the request flow.

2. Step 2: Understand Flask middleware contract

   Middleware must call the wrapped app to get a valid response object, then return it.

3. Final Answer:

   Middleware must call the wrapped app to return a valid response -> Option B

4. Quick Check:

   Middleware must call app = D [OK]

Hint: Middleware must call wrapped app to work properly [OK]

Common Mistakes:

• Returning a string instead of calling app
• Missing __call__ method entirely
• Thinking middleware returns numbers

5. You want to add middleware that logs request paths and modifies the response to add a custom header. Which approach correctly extends functionality using middleware?

hard

A. Modify the Flask app routes to log and add headers directly inside each route function

B. Use middleware that only logs requests but does not call the app

C. Create middleware that logs environ['PATH_INFO'], calls the app, then adds header to response before returning

D. Add a middleware that replaces the response with a fixed string ignoring the app

Solution

1. Step 1: Understand middleware responsibilities

   Middleware should handle logging before calling the app and modify the response after.

2. Step 2: Evaluate options

   Create middleware that logs environ['PATH_INFO'], calls the app, then adds header to response before returning correctly logs the request path, calls the app, then modifies the response headers. Others either bypass app or do tasks incorrectly.

3. Final Answer:

   Create middleware that logs environ['PATH_INFO'], calls the app, then adds header to response before returning -> Option C

4. Quick Check:

   Middleware logs and modifies response = B [OK]

Hint: Middleware wraps app: log before, modify response after [OK]

Common Mistakes:

• Modifying routes instead of middleware
• Not calling the wrapped app
• Replacing response ignoring app output