How Flask processes HTTP requests

Introduction

Flask processes HTTP requests to let your web app respond to users. It listens for requests and sends back the right answers.

When you want to build a simple website that shows different pages.

When you need to handle form submissions from users.

When you want to create an API that other programs can talk to.

When you want to respond differently based on the URL or request method.

When you want to serve files or data based on user requests.

Syntax

Flask

from flask import Flask, request

app = Flask(__name__)

@app.route('/path', methods=['GET', 'POST'])
def handler():
    # code to handle request
    return 'response'

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

@app.route tells Flask which URL to listen to.

The methods list tells Flask which HTTP methods to accept (like GET or POST).

Examples

This example handles a GET request to the home page and returns a greeting.

Flask

from flask import Flask

app = Flask(__name__)

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

This example handles a POST request with form data and responds with a message including the submitted name.

Flask

from flask import Flask, request

app = Flask(__name__)

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

Sample Program

This Flask app shows how it processes different HTTP requests:

• GET request to '/' returns a welcome message.
• GET request to '/greet/<name>' returns a personalized greeting.
• POST request to '/data' reads JSON data and responds with the message received.

Flask

from flask import Flask, request

app = Flask(__name__)

@app.route('/')
def index():
    return 'Welcome to the homepage!'

@app.route('/greet/<name>')
def greet(name):
    return f'Hello, {name}!'

@app.route('/data', methods=['POST'])
def data():
    info = request.json
    return f'Received: {info.get("message", "No message")}'

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

Important Notes

Flask matches the URL path to the function decorated with @app.route.

The request object holds all info about the incoming HTTP request.

Flask automatically converts the function's return value into an HTTP response.

Summary

Flask listens for HTTP requests and matches them to functions using routes.

Each route can handle different HTTP methods like GET or POST.

The request object lets you access data sent by the user.

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