? Flask Tutorial

? Quick Overview

Flask is a lightweight Python web framework used to build web applications quickly. It provides the essential tools, libraries, and features you need to create simple single-page apps or larger, more complex web applications.

Flask is often called a micro-framework because it keeps the core minimal and lets you add only the components you need, making it easy to learn and flexible to use.

? Key Concepts

Flask application object – created using Flask(__name__).
Routes – URLs in your app mapped to Python functions using @app.route().
View functions – Python functions that return what the user sees in the browser.
Development server – built-in server started using app.run().
Debug mode – use debug=True for automatic reload and better error messages during development.
Virtual environment – recommended to isolate dependencies per project.

? Installing Flask

Install Flask using pip from your terminal or command prompt:

? View Installation Command

# Install Flask using pip
pip install Flask

? Syntax / Theory

At the core of a Flask app, you usually have these pieces:

Create a Flask app object using the current module name: app = Flask(__name__).
Use the @app.route() decorator to connect a URL (like '/') to a Python function.
The function (called a view) returns a response, often a string or HTML.
Use the if __name__ == '__main__': block to run the app when the file is executed directly.
app.run(debug=True) starts the development server with debug mode enabled.

?‍? Creating a Basic Flask App

Here is a minimal Flask application that returns a simple message at the home page:

? View Code Example

# Import the Flask class
from flask import Flask

# Create the Flask application instance
app = Flask(__name__)

# Define a route for the homepage
@app.route('/')
def home():
    # Return a simple response
    return "Hello, Flask!"

# Run the app only if this file is executed directly
if __name__ == '__main__':
    # Start the development server in debug mode
    app.run(debug=True)

? Running the App

Save the file as something like app.py, then run it from your terminal:

? View Run Command

# Run the Flask app
python app.py

After running, Flask starts a development server (by default on port 5000). Open your browser and visit:

http://127.0.0.1:5000/

? Adding More Routes

You can define additional routes to serve different pages, like an About page:

? View Route Example

# Define a new route for the About page
@app.route('/about')
def about():
    # Return a simple text response for this route
    return "This is the About page."

Now, visiting http://127.0.0.1:5000/about will show the text from the about() function.

? Live Output / Explanation

? What You Will See in the Browser

When you open http://127.0.0.1:5000/, the browser displays: Hello, Flask!
When you open http://127.0.0.1:5000/about, the browser displays: This is the About page.

Each route corresponds to a Python function. Flask calls the function when the URL is requested and sends its return value back to the browser as the response.

? Use Cases / When to Use Flask

Building small to medium-sized web applications and APIs.
Creating quick prototypes for ideas or projects.
Building RESTful APIs for frontend frameworks or mobile apps.
Learning web development concepts with minimal boilerplate.

? Tips & Best Practices

Use virtual environments to manage Flask and other project-specific dependencies.
Enable debug=True only in development, not in production.
Keep routes and view functions small and focused for cleaner, more maintainable code.
Use meaningful route names and URLs to make your app easier to understand.
Organize larger apps using Blueprints and separate files as the project grows.

? Try It Yourself / Practice Tasks

Create a Flask app with multiple routes like /contact and /services.
Modify your routes to return simple HTML content (e.g., <h1>Welcome</h1>) instead of plain text.
Experiment with URL parameters (e.g., /hello/<name>) and display the name in the response.
Add a new route that returns information about your favorite programming language.