**Course Title:** Mastering Flask Framework: Building Modern Web Applications **Section Title:** Routing, Views, and Templates **Topic:** Defining routes and URL building in Flask ### Introduction to Routing in Flask Routing is a crucial aspect of building web applications. It involves defining URLs and mapping them to specific functions or views in your application. In Flask, routing is handled using the `@app.route()` decorator. This decorator takes in a URL path as a string and maps it to a specific function in your application. ### Defining Routes in Flask To define a route in Flask, you use the `@app.route()` decorator followed by the URL path. Here's an example of defining a simple route: ```python from flask import Flask app = Flask(__name__) @app.route('/') def home(): return 'Welcome to the homepage!' if __name__ == '__main__': app.run(debug=True) ``` In this example, the `@app.route('/')` decorator defines a route for the root URL of your application (`/`). When a user accesses this URL, the `home()` function is called, which returns a simple string. ### Route Variables Flask allows you to pass variables in your route paths using angle brackets (`<>`). Here's an example of defining a route with a variable: ```python @app.route('/hello/<name>') def hello(name): return f'Hello, {name}!' ``` In this example, the `@app.route('/hello/<name>')` decorator defines a route that accepts a variable `name` in the URL path. The `name` variable is then passed to the `hello()` function, which returns a string with the variable inserted. ### Route Variable Types Flask supports several types of route variables, including: * `string`: The default type, which accepts any string. * `int`: Accepts integers only. * `float`: Accepts floating-point numbers only. * `path`: Accepts any path string. * `any`: Accepts any of the above types. Here's an example of defining a route with an integer variable: ```python @app.route('/user/<int:user_id>') def user(user_id): return f'Welcome, user {user_id}!' ``` ### URL Building with `url_for()` Flask provides a function called `url_for()` to build URLs based on your route definitions. This function takes in the function name of your route as a string and returns the corresponding URL. Here's an example of using `url_for()`: ```python from flask import Flask, url_for app = Flask(__name__) @app.route('/') def home(): return 'Welcome to the homepage!' @app.route('/hello/<name>') def hello(name): return f'Hello, {name}!' @app.route('/dynamic') def dynamic(): url = url_for('hello', name='John') return f'The URL for the hello route is: {url}' if __name__ == '__main__': app.run(debug=True) ``` In this example, the `url_for()` function is used in the `dynamic()` function to generate the URL for the `hello()` route with a variable `name` set to `'John'`. ### Key Concepts and Takeaways * Use the `@app.route()` decorator to define routes in Flask. * Use angle brackets (`<>`) to pass variables in your route paths. * Use route variable types to restrict input types. * Use `url_for()` to build URLs based on your route definitions. ### Additional Resources * [Flask Documentation: Routing](https://flask.palletsprojects.com/en/2.2.x/quickstart/#routing) * [Flask Documentation: Variable Rules](https://flask.palletsprojects.com/en/2.2.x/quickstart/#variable-rules) ### What's Next? In the next topic, we will cover creating views and rendering templates with Jinja2. You can move on to that topic once you feel comfortable with defining routes and building URLs in Flask. Leave a comment below if you have any questions or need help with the concepts covered in this topic.