Python Computer Science

Beginner

80 mins

Teacher/Student led

What you need:
Chromebook/Laptop/PC

Advanced Debugging and Testing

In this lesson, you'll explore advanced debugging and testing techniques for software development. Learn the key stages of testing—unit, integration, and system—and apply them to debug and test a Flask web app with SQLite in VS Code.

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1 Introduction
2 Understanding Testing Stages
3 Create your environment and install Flask
4 Set Up the Buggy Flask App
5 Identify the Bugs
Quiz

1 - Introduction

In this lesson, you will explore advanced debugging and testing techniques in software engineering. You will learn about the stages of testing: unit testing, integration testing, and system testing. Then, you will apply these concepts by debugging and testing a Flask web application that uses SQLite for data storage. Finally, you will reflect on the code's limitations and suggest improvements.

By the end, you will be able to:

Explain the stages of testing.
Debug a Flask app with SQLite in VS Code.
Perform unit, integration, and system tests.

You will work in VS Code. Ensure you have Python, Flask, and SQLite installed from previous lessons. Create a new folder called 'DebugFlaskApp' for this lesson.

2 - Understanding Testing Stages

Before debugging, let's review the key stages of software testing:

Unit Testing: Tests individual components or functions in isolation to ensure they work as expected. For example, testing a single function that adds data to a database.
Integration Testing: Tests how different components work together, such as how a Flask route interacts with a database query.
System Testing: Tests the entire application as a whole to verify it meets requirements, including user interfaces and end-to-end functionality.

Debugging involves identifying and fixing errors (bugs) in the code, often using tools like print statements, breakpoints in VS Code, or error messages.

In the next steps, you will debug a provided Flask app that has intentional bugs, then apply these testing stages.

3 - Create your environment and install Flask

Create a virtual environment to manage your project's dependencies separately.

Open the Command Palette: View > Command Palette.
Type 'Python: Create Environment' and select it.
Choose 'Venv' as the type.
Select your installed Python version.
VS Code will create a '.venv' folder in 'DebugFlaskApp'.

Now, open a terminal in VS Code to install Flask if it's not already installed.

Go to the menu: View > Terminal
This opens the Terminal panel at the bottom of VS Code.
In the terminal, run: pip install flask
Wait for the installation to complete.

4 - Set Up the Buggy Flask App

Create the initial structure for a simple Flask app that manages a user database with SQLite. This code has bugs that you will fix later.

First, in your 'DebugFlaskApp' folder, create a file called 'app.py' and add this complete code:

from flask import Flask, render_template, request
import sqlite3

app = Flask(__name__)

def init_db():
    conn = sqlite3.connect('users.db')
    c = conn.cursor()
    c.execute('''CREATE TABLE IF NOT EXISTS users
        (id INTEGER PRIMARY KEY, name TEXT, age INTEGER CHECK(age = CAST(age AS INTEGER)))''')
    conn.commit()
    conn.close()

init_db()

@app.route('/')
def home():
    return render_template('index.html')

@app.route('/add', methods=['POST'])
def add_user():
    name = request.form['name']
    age = request.form['age']  # Bug: Not converting to int
    conn = sqlite3.connect('users.db')
    c = conn.cursor()
    c.execute("INSERT INTO users (name, age) VALUES (?, ?)", (name, age))
    conn.commit()
    conn.close()
    return 'User added!'

@app.route('/users')
def list_users():
    conn = sqlite3.connect('users.db')
    c = conn.cursor()
    c.execute("SELECT * FROM userss")  # Bug: Table name misspelled
    users = c.fetchall()
    conn.close()
    return render_template('users.html', users=users)

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

Create a 'templates' folder inside 'DebugFlaskApp'. Inside 'templates', create 'index.html':

<!DOCTYPE html>
<html>
<head><title>Add User</title></head>
<body>
<h1>Add User</h1>
<form method="post" action="/add">
    <label>Name:</label><input type="text" name="name"><br>
    <label>Age:</label><input type="text" name="age"><br>
    <button type="submit">Add</button>
</form>
<a href="/users">View Users</a>
</body>
</html>

And 'users.html' file in the 'templates':

<!DOCTYPE html>
<html>
<head><title>Users</title></head>
<body>
<h1>Users</h1>
<ul>
{% for user in users %}
    <li>{{ user[1] }} (Age: {{ user[2] }})</li>
{% endfor %}
</ul>
<a href="/">Back</a>
</body>
</html>

Run the app by opening a terminal in VS Code and typing: python app.py. Visit http://127.0.0.1:5000/. Try adding a user and viewing the list – you will encounter errors. These are the bugs to fix next.

5 - Identify the Bugs

Now, lets debug the app.

There are 2 bugs in the code:

In add_user: Age is not converted to int, causing a type error.
In list_users: Table name is misspelled as 'userss' instead of 'users'.

Use VS Code's debugger:

Set breakpoints by clicking in the gutter next to line numbers
Run with debugging (F5 or Run > Start Debugging)
Try adding a user with 'fifteen' as the age
Try navigating to the View Users screen

Using the debugger, step through the code line by line until it throws an error. Do this for both bugs.

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