# Web Development with Flask

Flask is a lightweight WSGI web application framework. It is designed to make getting started quick and easy, with the ability to scale up to complex applications. It began as a simple wrapper around Werkzeug and Jinja and has become one of the most popular Python web application frameworks.

## 1. Installation

You can install Flask using pip:

```bash
pip install Flask
```

## 2. A Minimal Application

A minimal Flask application looks something like this:

```python
from flask import Flask

app = Flask(__name__)

@app.route("/")
def hello_world():
    return "<p>Hello, World!</p>"

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

## 3. Routing

Modern web frameworks use routing techniques to help a user remember application URLs. You can use the `route()` decorator to bind a function to a URL.

```python
@app.route('/')
def index():
    return 'Index Page'

@app.route('/hello')
def hello():
    return 'Hello, World'
```

## 4. Variable Rules

You can add variable sections to a URL by marking sections with `<variable_name>`. Your function then receives the `<variable_name>` as a keyword argument.

```python
@app.route('/user/<username>')
def show_user_profile(username):
    # show the user profile for that user
    return f'User {username}'

@app.route('/post/<int:post_id>')
def show_post(post_id):
    # show the post with the given id, the id is an integer
    return f'Post {post_id}'
```

[[programming/python/python]]