Testing forms and POST data in Flask - Mini Project: Build & Apply

Testing forms and POST data

📖 Scenario: You are building a simple Flask web app that has a form where users can submit their name. You want to test that the form correctly sends the data using POST and that your Flask route processes it properly.

🎯 Goal: Build a Flask app with a form that submits a user's name via POST. Then write a test that sends POST data to the route and checks the response.

📋 What You'll Learn

Create a Flask app with a route /submit that accepts POST requests

Add a simple HTML form with a text input named username and a submit button

Write a test function using Flask's test client to send POST data with username

Verify the response contains the submitted username

💡 Why This Matters

🌍 Real World

Web developers often need to test that forms correctly send data and that backend routes process it as expected.

💼 Career

Testing form submissions is a common task in backend web development roles to ensure reliable user input handling.

1

Create the Flask app and route

Create a Flask app instance called app. Define a route /submit that accepts POST requests and returns the text Received: plus the value of username from the form data.

Flask

from flask import Flask, request

# Create Flask app instance

# Define /submit route that accepts POST

# Return 'Received: ' plus username from form data

Need a hint?

Use request.form.get('username') to get the form data inside the route.

2

Add a simple HTML form

Add a route / that returns an HTML form with method POST and action /submit. The form should have a text input with name username and a submit button.

Flask

from flask import Flask, request

app = Flask(__name__)

@app.route('/submit', methods=['POST'])
def submit():
    username = request.form.get('username', '')
    return f"Received: {username}"

# Add route / that returns HTML form

Need a hint?

Use triple quotes ''' to return multi-line HTML string in the route.

3

Write a test function to POST data

Write a test function called test_submit_post that uses app.test_client() to send a POST request to /submit with form data {'username': 'Alice'}. Store the response in a variable called response.

Flask

from flask import Flask, request

app = Flask(__name__)

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

@app.route('/submit', methods=['POST'])
def submit():
    username = request.form.get('username', '')
    return f"Received: {username}"

# Write test function test_submit_post

Need a hint?

Use with app.test_client() as client: to create a test client context.

4

Check the response contains the username

Inside the test_submit_post function, add an assertion that checks response.data contains the bytes string b'Received: Alice'.

Flask

from flask import Flask, request

app = Flask(__name__)

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

@app.route('/submit', methods=['POST'])
def submit():
    username = request.form.get('username', '')
    return f"Received: {username}"

def test_submit_post():
    with app.test_client() as client:
        response = client.post('/submit', data={'username': 'Alice'})
        # Add assertion to check response data

Need a hint?

Use assert b'Received: Alice' in response.data to check the response content.