1. What is Flask?

Flask is a lightweight web framework for Python. It provides the essentials to build web applications, such as routing, templates, and handling requests, while being minimalistic and flexible for developers to build their own features.

2. Setting Up Your Environment

To set up Flask, install it via pip. Make sure you have Python installed on your system.

pip install flask

Verify the installation by running the following command:

python -m flask --version

3. Your First Flask Application

Start by creating a simple Flask application that returns a "Hello, World!" message on the root route.

# app.py
from flask import Flask
app = Flask(__name__)

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

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

Run the app using:

python app.py

4. Routes and Views

Flask uses routes to map URLs to functions, called view functions, which generate the response for the client. The @app.route decorator is used to define these routes.

# Example:
@app.route('/hello')
def hello_page():
    return "Hello from Flask!"

5. Templates

Flask uses Jinja2 for rendering templates. Templates allow you to generate HTML dynamically based on the data passed to them from the server.

# Example:
from flask import render_template

@app.route('/greet')
def greet():
    return render_template('greet.html', name="Alice")

Create the greet.html file inside the templates folder with content like:

    
    


    
Hello, {{ name }}!

6. Forms and Handling Requests

Flask allows you to handle form submissions and access form data using the request object.

# Example:
from flask import request

@app.route('/submit', methods=['POST'])
def submit():
    name = request.form['name']
    return f"Hello, {name}!"

HTML form example:

7. Static Files

Flask can serve static files (such as images, CSS, and JavaScript) from a special static folder. Simply place static files inside this folder, and access them using the /static/ URL prefix.

# Example of linking a static CSS file:

8. Redirects and Error Handling

Flask provides utilities for redirecting users to different URLs and handling errors.

# Example:
from flask import redirect, url_for

@app.route('/old-page')
def old_page():
    return redirect(url_for('hello'))

@app.errorhandler(404)
def page_not_found(error):
    return "Page not found", 404

9. Database Integration

Flask can integrate with various databases using extensions such as Flask-SQLAlchemy. This allows you to interact with relational databases through models and queries.

# Example using SQLAlchemy:
from flask_sqlalchemy import SQLAlchemy

app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///database.db'
db = SQLAlchemy(app)

class User(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(80))

@app.route('/add_user')
def add_user():
    new_user = User(name='Alice')
    db.session.add(new_user)
    db.session.commit()
    return 'User added!'

10. Flask Extensions

Flask has many extensions that provide additional functionality. Common extensions include Flask-WTF (forms), Flask-Login (authentication), Flask-Mail (email), and Flask-RESTful (building APIs).

# Example with Flask-WTF:
from flask_wtf import FlaskForm
from wtforms import StringField, SubmitField

class MyForm(FlaskForm):
    name = StringField('Name')
    submit = SubmitField('Submit')