Error Handling and Debugging

In this lesson, you'll explore error handling and debugging in Python web development. Learn to manage errors and fix bugs by creating a Flask app, introducing errors, and using try-except blocks and VS Code debugging tools to resolve them.

Learning Goals Learning Outcomes Teacher Notes

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1 Introduction
2 Set Up Your Project
3 Create Sample Database
4 Build Basic Flask App
5 What are Errors?
6 Introduce a Common Error
7 Error Handling with Try-Except
8 What is Debugging?
9 Debugging in VS Code
10 Challenge
11 Wrap Up

1 - Introduction

In this lesson, you will learn about error handling and debugging in Python web development. Understanding how to manage errors and fix bugs is crucial for building reliable applications. You will create a simple Flask app that interacts with a database, intentionally introduce errors, and learn how to handle and debug them.

By the end, you will know how to use try-except blocks for error handling, debug code in VS Code, and avoid common pitfalls like incorrect database connections or unhandled exceptions.

2 - Set Up Your Project

First, you need to create a new folder for this lesson's project and set up the environment.

Open VS Code.
Create a new folder on your computer called 'FlaskErrorHandling'.
Open this folder in VS Code (File > Open Folder).

Next, create a virtual environment to manage your project's dependencies.

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

Next, Open a terminal in VS Code to install Flask.

Go to View > Terminal.
Run: pip install flask.
Wait for the installation to complete.

3 - Create Sample Database

You will create a sample SQLite database with a 'users' table. This will be used in your Flask app.

Create a new file called 'create_db.py' in your project folder and add the following code:

import sqlite3

conn = sqlite3.connect('users.db')
c = conn.cursor()

c.execute('''CREATE TABLE IF NOT EXISTS users
             (id INTEGER PRIMARY KEY,
              name TEXT NOT NULL,
              age INTEGER NOT NULL)''')

# Insert sample data
c.execute("INSERT INTO users (name, age) VALUES ('Alice', 25)")
c.execute("INSERT INTO users (name, age) VALUES ('Bob', 30)")

conn.commit()
conn.close()
print('Database created successfully!')

Run this script in the terminal: python create_db.py.

You should see 'Database created successfully!' and a new file 'users.db' in your folder. This database has a users table with two sample records.

4 - Build Basic Flask App

Now, create a basic Flask app to display users from the database. This will be the foundation where we introduce errors.

Create a file called 'app.py' and add the following code:

from flask import Flask, render_template
import sqlite3

app = Flask(__name__)

@app.route('/')
def home():
    conn = sqlite3.connect('users.db')
    c = conn.cursor()
    c.execute("SELECT * FROM users")
    users = c.fetchall()
    conn.close()
    return render_template('users.html', users=users)

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

Create a 'templates' folder and then create the 'users.html' in it with:

<!DOCTYPE html>
<html>
<head><title>Users</title></head>
<body>
<h1>Users</h1>
<table border="1">
    <tr><th>ID</th><th>Name</th><th>Age</th></tr>
    {% for user in users %}
    <tr><td>{{ user[0] }}</td><td>{{ user[1] }}</td><td>{{ user[2] }}</td></tr>
    {% endfor %}
</table>
</body>
</html>

Run the app: python app.py. Visit http://127.0.0.1:5000/ to see the users list.

You should see a table with Alice and Bob. Next, we will introduce an error.

5 - What are Errors?

In programming, errors (often called bugs) are problems in your code that stop it from working as expected. They can cause your program to crash, produce wrong results, or behave unexpectedly. Understanding errors is the first step to fixing them and making your apps reliable.

There are three main types of errors you might encounter in Python:

Syntax Errors: These happen when your code breaks the rules of Python, like forgetting a colon after an 'if' statement or mismatched brackets. Python will spot these before running the code and show an error message in your IDE (Integrated Development Environment i.e. VS Code).
Runtime Errors: These occur while your program is running. For example, trying to divide by zero, accessing a file that doesn't exist, or connecting to a database with the wrong name. In web apps like Flask, these can cause pages to fail to load.
Logic Errors: Your code runs without crashing, but the output is incorrect because of a mistake in your thinking, like using the wrong formula in a calculation.

In this lesson on error handling and debugging for Flask web development, we'll focus on runtime errors, especially those related to databases, and how to handle them using tools like try-except blocks.

Remember, everyone makes errors – even experienced developers! The key is learning how to find and fix them.

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