How Flask processes HTTP requests - Practice Exercises

Challenge - 5 Problems

❓ component_behavior

intermediate

What does this Flask route return?

Consider this Flask app code. What is the output when you visit /hello in a browser?

Flask

from flask import Flask
app = Flask(__name__)

@app.route('/hello')
def hello():
    return 'Hello, Flask!'

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

A The browser shows a blank page

B The browser shows a 404 Not Found error

C The browser shows a 500 Internal Server Error

D The browser shows the text: Hello, Flask!

💡 Hint

Check the route decorator and the return value of the function.

The route decorator @app.route('/hello') connects the URL path '/hello' to the hello() function. When you visit '/hello', Flask calls hello() and returns its string, which the browser displays.

❓ state_output

intermediate

What is the value of request.method inside this route?

Given this Flask route, what will request.method be if you send a POST request to /submit?

Flask

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

@app.route('/submit', methods=['GET', 'POST'])
def submit():
    return f'Method used: {request.method}'

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

A PUT

B GET

C POST

D DELETE

💡 Hint

Look at the HTTP method used in the request and the allowed methods in the route.

The route allows GET and POST. If you send a POST request, request.method will be 'POST'.

📝 Syntax

advanced

Which option correctly defines a Flask route that accepts a dynamic integer parameter?

You want a route that matches URLs like /user/42 where 42 is a number. Which code snippet correctly defines this route?

A

@app.route('/user/<id:int>')
def user(id):
    return f'User {id}'

B

@app.route('/user/<int:id>')
def user(id):
    return f'User {id}'

C

@app.route('/user/<int>')
def user(id):
    return f'User {id}'

D

@app.route('/user/<id>')
def user(int):
    return f'User {int}'

💡 Hint

Flask uses syntax for dynamic parts in routes.

Flask route parameters use syntax. means convert the URL part to int and assign to id.

🔧 Debug

advanced

Why does this Flask app raise a RuntimeError?

This Flask app code raises a RuntimeError: Working outside of application context. Why?

Flask

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

print(current_app.name)

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

A Because current_app is accessed outside a request or app context

B Because Flask app is not created correctly

C Because print statement is inside the if __name__ == '__main__' block

D Because app.run() is missing parentheses

💡 Hint

current_app needs an active application context to work.

current_app only works inside an application context, which Flask creates during requests or when pushed manually. Accessing it at the top level causes RuntimeError.

🧠 Conceptual

expert

What is the correct order Flask processes an incoming HTTP request?

Arrange these steps in the order Flask processes an HTTP request internally.

A 2,1,3,4

B 1,2,3,4

C 2,3,1,4

D 1,3,2,4

💡 Hint

Flask needs a context before matching routes and running code.

Flask first creates a request context, then matches the URL to a route, runs the route function, then sends the response.

Practice

1. What does Flask use to decide which function should handle an incoming HTTP request?

easy

A. The route URL pattern defined with @app.route

B. The order in which functions are defined in the code

C. The size of the HTTP request body

D. The IP address of the client

Solution

1. Step 1: Understand Flask routing

   Flask uses route decorators like @app.route to map URLs to functions.

2. Step 2: Match incoming request URL

   When a request comes, Flask matches its URL to the route patterns to find the right function.

3. Final Answer:

   The route URL pattern defined with @app.route -> Option A

4. Quick Check:

   Routing = @app.route [OK]

Hint: Routes connect URLs to functions in Flask [OK]

Common Mistakes:

• Thinking function order matters
• Confusing client IP with routing
• Using request size to route

2. Which of the following is the correct way to define a Flask route that only accepts POST requests?

easy

A. @app.route('/submit', methods=['GET'])

B. @app.route('/submit', accept='POST')

C. @app.route('/submit', methods=['POST'])

D. @app.route('/submit', method='POST')

Solution

1. Step 1: Check correct syntax for methods argument

   Flask route decorator uses methods=['POST'] to specify allowed HTTP methods.

2. Step 2: Identify the correct option

   @app.route('/submit', methods=['POST']) uses methods=['POST'], which is the correct syntax.

3. Final Answer:

   @app.route('/submit', methods=['POST']) -> Option C

4. Quick Check:

   methods=['POST'] sets POST only [OK]

Hint: Use methods=['POST'] to allow only POST requests [OK]

Common Mistakes:

• Using method instead of methods
• Using accept instead of methods
• Allowing GET instead of POST

3. Given this Flask code snippet:

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

@app.route('/greet', methods=['POST'])
def greet():
    name = request.form.get('name', 'Guest')
    return f'Hello, {name}!'

What will be the response if a POST request is sent to '/greet' with form data name='Alice'?

medium

A. Hello, Guest!

B. Hello, Alice!

C. Hello, None!

D. Error: Missing name parameter

Solution

1. Step 1: Understand request.form.get usage

   The code tries to get 'name' from form data; if missing, defaults to 'Guest'.

2. Step 2: Analyze the POST request data

   The POST request includes name='Alice', so request.form.get('name') returns 'Alice'.

3. Final Answer:

   Hello, Alice! -> Option B

4. Quick Check:

   Form data name='Alice' returns 'Hello, Alice!' [OK]

Hint: request.form.get returns form value or default [OK]

Common Mistakes:

• Assuming default 'Guest' always used
• Confusing GET and POST data
• Expecting error if parameter missing

4. What is wrong with this Flask route code?

@app.route('/data', methods=['GET', 'POST'])
def data():
    if request.method == 'POST'
        return 'Posted data'
    else:
        return 'Get data'

medium

A. Missing colon after if statement

B. request.method is not valid in Flask

C. Route decorator should not have methods argument

D. Function name cannot be 'data'

Solution

1. Step 1: Check syntax of if statement

   The if statement lacks a colon at the end, causing a syntax error.

2. Step 2: Verify other parts

   request.method is valid, methods argument is correct, function name is allowed.

3. Final Answer:

   Missing colon after if statement -> Option A

4. Quick Check:

   if statement needs colon [OK]

Hint: Check colons after if/else statements [OK]

Common Mistakes:

• Forgetting colon after if
• Thinking request.method is invalid
• Misunderstanding methods argument

5. You want to create a Flask route '/submit' that accepts both GET and POST requests. When accessed by GET, it should show a form. When accessed by POST, it should process form data and return a confirmation message. Which code snippet correctly implements this behavior?

hard

A. from flask import Flask, request
app = Flask(__name__)

@app.route('/submit', methods=['GET', 'POST'])
def submit():
return 'Submit page'

B. from flask import Flask, request
app = Flask(__name__)

@app.route('/submit')
def submit():
if request.method == 'POST':
return 'Data received'
else:
return ''

C. from flask import Flask
app = Flask(__name__)

@app.route('/submit', methods=['POST'])
def submit():
return 'Only POST allowed'

D. from flask import Flask, request
app = Flask(__name__)

@app.route('/submit', methods=['GET', 'POST'])
def submit():
if request.method == 'POST':
data = request.form.get('data')
return f'Data received: {data}'
else:
return ''''''

Solution

1. Step 1: Check methods argument

   from flask import Flask, request app = Flask(__name__) @app.route('/submit', methods=['GET', 'POST']) def submit(): if request.method == 'POST': data = request.form.get('data') return f'Data received: {data}' else: return '''''' correctly uses methods=['GET', 'POST'] to accept both request types.

2. Step 2: Verify request method handling

   from flask import Flask, request app = Flask(__name__) @app.route('/submit', methods=['GET', 'POST']) def submit(): if request.method == 'POST': data = request.form.get('data') return f'Data received: {data}' else: return '''''' checks request.method and returns form HTML for GET and processes form data for POST.

3. Step 3: Compare other options

   The other options either miss methods argument, do not handle both methods properly, or do not return form on GET.

4. Final Answer:

   The code snippet that uses methods=['GET', 'POST'], checks request.method, returns form for GET and processes data for POST -> Option D

5. Quick Check:

   methods=['GET','POST'] + if request.method [OK]

Hint: Use methods=['GET','POST'] and check request.method [OK]

Common Mistakes:

• Not specifying methods argument
• Not checking request.method
• Returning same response for all methods