**Course Title:** Mastering Flask Framework: Building Modern Web Applications **Section Title:** Introduction to Flask and Development Environment **Topic:** Creating your first Flask application **Objective:** By the end of this topic, you will have created and run your first Flask web application. You will understand the basic components of a Flask application and how to structure and initialize it. **Creating your first Flask application** In this topic, we will guide you through the process of creating your first Flask web application. We will cover the basic components of a Flask application and demonstrate how to structure and initialize it. ### Step 1: Install Flask Before we start creating our Flask application, we need to install Flask. If you haven't installed Flask yet, you can do so by running the following command in your terminal: ```bash pip install flask ``` ### Step 2: Create a new project directory Create a new directory for your project and navigate into it: ```bash mkdir my_flask_app cd my_flask_app ``` ### Step 3: Create a new Flask application Create a new file called `app.py` and add the following code: ```python from flask import Flask app = Flask(__name__) @app.route('/') def index(): return 'Hello, World!' if __name__ == '__main__': app.run() ``` This code creates a new Flask application and defines a single route for the root URL ('/'). The `index()` function returns a simple 'Hello, World!' message. ### Step 4: Initialize the Flask application To initialize the Flask application, we need to set the `FLASK_APP` environment variable to the name of our application, and then run the application using the `flask run` command. Here's how you can do it: **Windows** ```bash set FLASK_APP=app.py flask run ``` **Mac/Linux** ```bash export FLASK_APP=app.py flask run ``` **Using a virtual environment (recommended)** ```bash FLASK_APP=app.py flask run ``` ### Step 5: Run your Flask application Once you've initialized your Flask application, you can run it by navigating to `http://localhost:5000` in your web browser. You should see the 'Hello, World!' message displayed on the page. **Debug mode** Flask provides a debug mode that automatically reloads the application when changes are detected in the code. To enable debug mode, you can set the `FLASK_DEBUG` environment variable to `True`: ```bash export FLASK_DEBUG=True ``` Alternatively, you can set the `debug` parameter to `True` when creating the Flask application: ```python app = Flask(__name__) app.debug = True ``` **Key Concepts** * **Flask application instance**: The Flask application instance is the central component of a Flask application. You create it by passing the `__name__` attribute of the current module to the `Flask` class. * **Route decorator**: The `@app.route()` decorator is used to define a route for a specific URL. The function decorated with this decorator is called the view function. * **View function**: The view function is responsible for handling the request and returning a response to the client. * **Request object**: The request object is an instance of the `Request` class and provides information about the incoming request. **Practical Takeaways** * You can create a new Flask application by instantiating the `Flask` class and passing the `__name__` attribute of the current module to it. * You can define routes for specific URLs using the `@app.route()` decorator. * You can run the Flask application using the `flask run` command. **External Resources** * [Flask Tutorial](https://flask.palletsprojects.com/en/2.0.x/tutorial/) * [Flask Documentation](https://flask.palletsprojects.com/en/2.0.x/) **What's Next?** In the next topic, we will cover 'Defining routes and URL building in Flask.' We will discuss how to define routes for specific URLs, create URLs, and navigate between routes. Do you have any questions or need help with this topic? Leave a comment below!