Build a Web Scraper & Alert System

What you’ll build

A program that watches any website for changes and alerts you. When a price drops, a product comes back in stock, or a news article appears — you’ll know about it before your friends do.

Before you start: Think about something you check every day — a store for a restock, a job site for new listings, a news site for breaking stories. What if a little robot checked it for you every 15 minutes and texted you when something changed? That’s exactly what you’re building.

🤔 Challenge: Find one website you want to monitor. Open it in your browser. Where does the information you care about appear? Is it in the price tag, the date, a status label? Identifying this is half the job.

What you need

No starter kits, no physical hardware! Just:

• A Linux computer (Raspberry Pi, old laptop, or even a free cloud instance)
• Python 3.7+ (most Linux systems already have it)
• Internet connection
• Patience for the first run (it takes 10 minutes to set up)

💡 Don’t have Linux? You can use Windows with WSL (Windows Subsystem for Linux) or install Python directly. The guide works on any system with Python 3.

Step 1: Understand the concept

Your scraper does 4 things:

1. Download the web page HTML
2. Extract just the part you care about (using CSS selectors)
3. Compare this version with the last version
4. Alert you if anything changed

  Every 15 minutes:
  ┌──────────────┐
  │ Download HTML │
  └──────┬───────┘
         ↓
  ┌──────────────┐
  │ Extract data  │ ← Use CSS selectors (like .price, #stock)
  └──────┬───────┘
         ↓
  ┌──────────────┐
  │ Compare with  │
  │ last version  │
  └──────┬───────┘
         ↓
  ┌──────────────┐
  │ Changed?      │
  │ YES → Alert!  │
  │ NO → Wait...  │
  └──────────────┘

🤔 Why not download the whole page? Most web pages are 500KB+ with ads, tracking and styling. If you just extract the price (say 20 bytes), comparison is instant and uses almost no bandwidth.

Step 2: Install Python tools

Open a terminal and type these commands one at a time:

# Create a project folder
mkdir web-monitor && cd web-monitor

# Create a virtual environment (isolated Python installation)
python3 -m venv venv

# Activate it (Linux/Mac)
source venv/bin/activate

# Install the tools you need
pip install requests beautifulsoup4 python-dotenv

What each command does:

• mkdir web-monitor — creates a folder for your project
• python3 -m venv venv — creates an isolated Python environment (so your system Python stays clean)
• source venv/bin/activate — activates the virtual environment (your terminal prompt will change to show (venv))
• pip install ... — installs Python packages (like apps for Python)

🤔 Why a virtual environment? Without one, if you install something for one project, it might break another project. Virtual environments keep everything separate. Think of it like having separate toolboxes for each project.

💡 Test it: Type python in your terminal. If you see >>>, you’re in Python. Type print("hello") — if it prints “hello”, Python works. Type exit() to leave Python and return to your normal terminal.

Step 3: Write the scraper

Create a file called scraper.py and paste this code:

import requests
from bs4 import BeautifulSoup
from datetime import datetime
import hashlib
import os
from pathlib import Path

HISTORY_DIR = Path("history")
HISTORY_DIR.mkdir(exist_ok=True)

def scrape(url, selector="body"):
    """Download a page and extract the target element."""
    response = requests.get(url, timeout=30, headers={
        "User-Agent": "Mozilla/5.0 (Web Monitor)"
    })
    response.raise_for_status()

    soup = BeautifulSoup(response.text, "html.parser")

    if selector and selector != "body":
        element = soup.select_one(selector)
        content = str(element) if element else "NOT FOUND"
    else:
        content = soup.get_text(strip=True)

    return content

def has_changed(site_name, content):
    """Check if the content changed since last time."""
    content_hash = hashlib.sha256(content.encode()).hexdigest()

    # Find previous snapshots for this site
    files = sorted(HISTORY_DIR.glob(f"{site_name}_*.html"))
    if len(files) >= 2:
        with open(files[-2], 'r') as f:
            previous = f.read()
        previous_hash = hashlib.sha256(previous.encode()).hexdigest()
        return content_hash != previous_hash

    return True  # First time checking — consider it "changed"

def save_snapshot(site_name, content):
    """Save the current version with a timestamp."""
    now = datetime.now()
    filename = f"{site_name}_{now.strftime('%Y%m%d_%H%M')}.html"
    filepath = HISTORY_DIR / filename
    with open(filepath, 'w') as f:
        f.write(content)

What each part does:

• requests.get(url) — downloads the web page (like your browser does)
• BeautifulSoup(text, "html.parser") — turns HTML into something Python can search through
• soup.select_one(".price") — finds the FIRST element with class “price” (like CSS selectors in developer tools)
• hashlib.sha256() — creates a unique fingerprint (hash) of the content. If the content changes, the hash changes too.

🤔 How do I know what CSS selector to use? Right-click the element on the webpage → “Inspect” → look at the HTML. The class="price" part means the selector is .price. The id="stock" part means the selector is #stock.

💡 Try this: Open a product page in your browser. Right-click the price → Inspect. Look for class or id. Try that selector in the code. If it returns “NOT FOUND”, try a different one.

Step 4: Set up notifications

Create .env (this file stores secrets — never share it):

# Email alerts
[email protected]
[email protected]
SMTP_HOST=smtp.gmail.com
SMTP_PORT=587
[email protected]
SMTP_PASS=your_app_password

# WhatsApp alerts (Twilio)
TWILIO_SID=ACxxxxxxxxxxxxx
TWILIO_TOKEN=your_twilio_token
TWILIO_FROM=+1234567890
TWILIO_TO=+2782xxxxxxx

🤔 Twilio? Twilio is a service that sends SMS/WhatsApp messages via API. You can use their free trial (sends ~100 messages). For WhatsApp, you need the WhatsApp Business API. For personal projects, email alerts are easier to set up.

💡 Gmail setup: You need an “App Password” for Gmail, not your normal password. Go to Google Account → Security → App Passwords → Generate one for “Mail”.

Step 5: Run it

Create monitor.py:

from scraper import check_site
from notifications import send_email, send_whatsapp
import yaml

def run_checks():
    with open("config.yaml") as f:
        config = yaml.safe_load(f)

    for site in config["sites"]:
        print(f"Checking: {site['name']}...")
        result = check_site(site)

        if result["changed"]:
            print(f"  → CHANGE DETECTED!")
            message = f"ALERT: {site['name']} changed!\nURL: {site['url']}"
            if site.get("alert") == "whatsapp":
                send_whatsapp(message)
            elif site.get("alert") == "email":
                send_email(f"Change: {site['name']}", message)
        else:
            print(f"  → No change")

if __name__ == "__main__":
    run_checks()

Run it:

python monitor.py

🎉 You just built a website monitor! It checks your site, detects changes, and sends alerts.

Step 6: Automate it

Make it run every 15 minutes without you doing anything:

# Open your cron schedule editor
crontab -e

# Add this line (adjust the path):
*/15 * * * * cd /home/pi/web-monitor && venv/bin/python monitor.py >> monitor.log 2>&1

Troubleshooting

“Module not found” errors?

• Make sure you activated the virtual environment: source venv/bin/activate
• Or use the full path: venv/bin/python monitor.py

Scraper returns “NOT FOUND”?

• The selector is wrong — use browser DevTools to find the right one
• Some sites block bots — add headers={"User-Agent": "Mozilla/5.0"} (already in the code)

“Connection timed out”?

• Check your internet connection
• The target site might be down or blocking your IP
• Try adding a longer timeout: timeout=60

What’s next?

Build the Digital Pet — your first hardware project.