🔗 URL Shortener

A simple, beginner-friendly URL shortener built with Python, Flask, and SQLite.

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Features

Feature	Description
Shorten URLs	Paste any http:// or https:// URL → get a 6-char short link
Redirect	Visit /<short_id> → automatically redirected to the original URL
Duplicate detection	Same URL submitted twice? Reuses the existing short code
URL validation	Rejects empty or malformed input with clear error messages
Recent links list	Homepage shows all previously shortened URLs
SQLite storage	Zero-config, single-file database (database.db)

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Project Structure

URL shortener/
├── app.py              # Main Flask application
├── database.db         # SQLite database (auto-created on first run)
├── requirements.txt    # Python dependencies
├── test_app.py         # Quick smoke tests
├── README.md
├── static/
│   └── style.css       # Dark glassmorphism UI styles
└── templates/
    └── index.html      # Jinja2 homepage template

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How to Run Locally

1. Install dependencies

pip install -r requirements.txt

2. Start the development server

python app.py

3. Open in your browser

http://127.0.0.1:5000

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Sample Test URLs

URL	Purpose
https://www.google.com/search?q=flask+tutorial	Long query string
https://github.com/pallets/flask	Clean GitHub URL
https://en.wikipedia.org/wiki/URL_shortening	Wikipedia article
https://docs.python.org/3/library/sqlite3.html	Python docs

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How It Works (Key Logic)

1. User submits a URL via the form on the homepage.
2. Validation — a regex checks the URL starts with http:// or https://.
3. Duplicate check — if the URL was shortened before, return the existing short code.
4. Generate short ID — pick 6 random alphanumeric characters; retry on collision.
5. Store the mapping (short_id → original_url) in SQLite.
6. Redirect — when someone visits /<short_id>, look it up and issue a 302 redirect.

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Running Tests

With the server running in another terminal:

python test_app.py

Expected output:

[PASS] Redirect test: status=302, location=https://www.google.com/search?q=flask+tutorial
[PASS] Invalid ID test: status=302 (redirect to homepage)
[PASS] Duplicate URL reuse test: 2 rows in DB (expected 2)

All tests passed!