Request validation in Flask - Mini Project: Build & Apply

Request Validation in Flask

📖 Scenario: You are building a simple web application using Flask. Users will submit their name and age through a form. You want to make sure the data received is valid before processing it.

🎯 Goal: Build a Flask app that validates incoming request data for name and age. The app should check that name is a non-empty string and age is a positive integer.

📋 What You'll Learn

Create a Flask app with a route to accept POST requests

Extract name and age from the request JSON

Add validation to check name is not empty and age is a positive integer

Return a JSON response indicating success or error

💡 Why This Matters

🌍 Real World

Validating user input is essential in web apps to prevent errors and security issues. This project shows how to check data before using it.

💼 Career

Backend developers often write request validation to ensure APIs receive correct data. This skill is fundamental for building reliable web services.

1

Set up Flask app and route

Import Flask and request from flask. Create a Flask app called app. Define a route /submit that accepts POST requests and a function submit() that returns a JSON response with {"message": "Ready"}.

Flask

# Import Flask and request
# Create Flask app called app
# Define POST route /submit with function submit
# Return JSON {"message": "Ready"}

Need a hint?

Use Flask(__name__) to create the app. Use @app.route decorator with methods=['POST']. Return JSON using jsonify.

2

Extract data from request JSON

Inside the submit() function, get the JSON data from the request using request.get_json() and store it in a variable called data. Then extract name and age from data.

Flask

from flask import Flask, request, jsonify

app = Flask(__name__)

@app.route('/submit', methods=['POST'])
def submit():
    data = request.get_json()
    # Extract name and age from data

Need a hint?

Use request.get_json() to get JSON data. Use data.get('name') and data.get('age') to extract values.

3

Add validation for name and age

Add validation inside submit() to check if name is a non-empty string and age is an integer greater than 0. If validation fails, return JSON with {"error": "Invalid input"} and status code 400.

Flask

from flask import Flask, request, jsonify

app = Flask(__name__)

@app.route('/submit', methods=['POST'])
def submit():
    data = request.get_json()
    name = data.get('name')
    age = data.get('age')
    # Validate name and age

Need a hint?

Use isinstance() to check types. Use name.strip() to check for empty strings. Return JSON error with status 400 if invalid.

4

Return success message with validated data

If validation passes, return a JSON response with {"message": "Data received", "name": name, "age": age}.

Flask

from flask import Flask, request, jsonify

app = Flask(__name__)

@app.route('/submit', methods=['POST'])
def submit():
    data = request.get_json()
    name = data.get('name')
    age = data.get('age')
    if not isinstance(name, str) or not name.strip():
        return jsonify({"error": "Invalid input"}), 400
    if not isinstance(age, int) or age <= 0:
        return jsonify({"error": "Invalid input"}), 400
    # Return success JSON with name and age

Need a hint?

Return JSON with keys message, name, and age using jsonify.