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Introduction to Flask

Learn Flask, the lightweight Python web framework. Build your first web application, understand Flask

Flask is a lightweight, flexible Python web framework that gives you the tools to build web applications quickly and efficiently. Known as a "micro-framework," Flask doesn't include a database abstraction layer, form validation, or other features that third-party libraries provide — but it's incredibly easy to extend and learn.

Why Flask?

Flask is one of the most popular Python frameworks for several reasons:

Minimal and Lightweight — No forced dependencies or project structure
Flexible — Build anything from a simple API to a full web app
Pythonic — Clean, readable code that follows Python conventions
Extensive Ecosystem — Hundreds of Flask extensions available
Great for APIs — Perfect for RESTful API development
Excellent Documentation — One of the best-documented frameworks

Prerequisites

Before learning Flask, ensure you're comfortable with:

Python 3.8+ basics (variables, functions, classes)
Basic HTML/CSS knowledge
Understanding of HTTP (GET, POST requests)
Virtual environments and pip

Installation

Setting Up a Virtual Environment

Always use virtual environments to isolate your project dependencies:

bash example

# Create a new project directory
mkdir my-flask-app
cd my-flask-app

# Create a virtual environment
python -m venv venv

# Activate the virtual environment
# On Windows:
venv\Scripts\activate

# On macOS/Linux:
source venv/bin/activate

Installing Flask

bash example

pip install flask

Verify Installation

python example

python -c "import flask; print(flask.__version__)"
# Output: 3.0.x

Full Requirements File

bash example

# requirements.txt
flask>=3.0.0
python-dotenv>=1.0.0

Install from requirements:

bash example

pip install -r requirements.txt

Your First Flask Application

The Minimal App

Create a file called app.py:

python example

from flask import Flask

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

# Define a route and view function
@app.route('/')
def hello_world():
    return 'Hello, World!'

# Run the app
if __name__ == '__main__':
    app.run(debug=True)

Run your app:

bash example

python app.py

Visit http://127.0.0.1:5000 in your browser and you'll see "Hello, World!"

Understanding the Code

Let's break down each part:

python example

from flask import Flask

This imports the Flask class from the flask module.

python example

app = Flask(__name__)

Creates a Flask application instance. __name__ tells Flask the name of the module — this helps Flask find templates and static files.

python example

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

The @app.route('/') decorator tells Flask that when someone visits the root URL /, call the hello_world() function. This function returns the response to send back to the browser.

python example

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

Only runs the development server when the script is executed directly (not imported). debug=True enables the debugger and auto-reloader.

Flask Project Structure

For small projects, a single file works fine. For larger projects, use this structure:

my-flask-app/ │ ├── app.py # Application entry point ├── config.py # Configuration settings ├── requirements.txt # Dependencies ├── .env # Environment variables (don't commit!) ├── .gitignore │ ├── templates/ # HTML templates (Jinja2) │ ├── base.html │ ├── index.html │ └── about.html │ ├── static/ # Static files │ ├── css/ │ │ └── style.css │ ├── js/ │ │ └── main.js │ └── images/ │ └── venv/ # Virtual environment (don't commit!)

Running Flask

Development Server

bash example

# Method 1: Run directly
python app.py

# Method 2: Using Flask CLI
flask --app app run

# Method 3: With debug mode enabled
flask --app app run --debug

# Method 4: Specify host and port
flask --app app run --host=0.0.0.0 --port=8080

Environment Variables

bash example

# Set Flask environment variables
# Windows (Command Prompt):
set FLASK_APP=app.py
set FLASK_DEBUG=1
flask run

# Windows (PowerShell):
$env:FLASK_APP = "app.py"
$env:FLASK_DEBUG = "1"
flask run

# macOS/Linux:
export FLASK_APP=app.py
export FLASK_DEBUG=1
flask run

Using python-dotenv

Create a .env file:

Example

FLASK_APP=app.py FLASK_DEBUG=1 SECRET_KEY=your-secret-key-here DATABASE_URL=sqlite:///database.db

Load it in your app:

Example

from dotenv import load_dotenv import os load_dotenv() app = Flask(__name__) app.config['SECRET_KEY'] = os.environ.get('SECRET_KEY', 'fallback-key')

Flask Configuration

Example

from flask import Flask import os app = Flask(__name__) # Basic Configuration app.config['DEBUG'] = True app.config['TESTING'] = False app.config['SECRET_KEY'] = 'your-secret-key-change-in-production' # Using a Config Class (recommended) class Config: DEBUG = False TESTING = False SECRET_KEY = os.environ.get('SECRET_KEY') or 'hard-to-guess-string' class DevelopmentConfig(Config): DEBUG = True DATABASE_URI = 'sqlite:///dev.db' class ProductionConfig(Config): DATABASE_URI = os.environ.get('DATABASE_URL') # Apply configuration app.config.from_object(DevelopmentConfig)

Multiple Routes

python example

from flask import Flask

app = Flask(__name__)

@app.route('/')
def home():
    return '<h1>Home Page</h1>'

@app.route('/about')
def about():
    return '<h1>About Page</h1><p>This is my Flask app.</p>'

@app.route('/contact')
def contact():
    return '<h1>Contact Us</h1>'

@app.route('/services')
def services():
    return '''
    <h1>Our Services</h1>
    <ul>
        <li>Web Development</li>
        <li>API Development</li>
        <li>Consulting</li>
    </ul>
    '''

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

Returning Different Responses

Flask can return various types of responses:

Example

from flask import Flask, jsonify, make_response app = Flask(__name__) # Return plain text @app.route('/text') def text_response(): return 'Plain text response' # Return HTML @app.route('/html') def html_response(): return '<h1>HTML Response</h1><p>With <strong>formatting</strong></p>' # Return JSON @app.route('/json') def json_response(): data = { 'name': 'Flask', 'version': '3.0', 'type': 'Micro Framework' } return jsonify(data) # Return with status code @app.route('/created') def created_response(): return 'Resource created!', 201 # Return with custom headers @app.route('/custom') def custom_response(): response = make_response('Custom Response', 200) response.headers['X-Custom-Header'] = 'MyValue' response.headers['Content-Type'] = 'text/plain' return response

Flask Application Factory Pattern

For larger applications, use the Application Factory pattern:

Example

# app/__init__.py from flask import Flask def create_app(config_name='development'): """Application factory function.""" app = Flask(__name__) # Load configuration if config_name == 'development': app.config['DEBUG'] = True app.config['SECRET_KEY'] = 'dev-secret-key' elif config_name == 'production': app.config['DEBUG'] = False app.config['SECRET_KEY'] = 'prod-secret-key-very-long-and-random' # Register blueprints from .main import main as main_blueprint app.register_blueprint(main_blueprint) return app

python example

# run.py
from app import create_app

app = create_app('development')

if __name__ == '__main__':
    app.run()

Flask Application Context

Flask uses application and request contexts to manage state:

python example

from flask import Flask, g, current_app

app = Flask(__name__)

@app.before_request
def before_request():
    """Runs before every request."""
    g.user = None  # Initialize user as None
    print("Before request hook called")

@app.after_request
def after_request(response):
    """Runs after every request."""
    print(f"Response status: {response.status_code}")
    return response

@app.teardown_request
def teardown_request(exception):
    """Runs at the end of each request."""
    if exception:
        print(f"Error occurred: {exception}")

@app.route('/context')
def context_demo():
    # Access current app config
    debug_mode = current_app.config.get('DEBUG')
    return f'Debug mode: {debug_mode}'

Error Handling

python example

from flask import Flask, render_template, jsonify

app = Flask(__name__)

@app.errorhandler(404)
def not_found(error):
    return jsonify({'error': 'Page not found', 'status': 404}), 404

@app.errorhandler(500)
def internal_error(error):
    return jsonify({'error': 'Internal server error', 'status': 500}), 500

@app.errorhandler(403)
def forbidden(error):
    return jsonify({'error': 'Forbidden', 'status': 403}), 403

# Custom exception
class APIError(Exception):
    def __init__(self, message, status_code=400):
        self.message = message
        self.status_code = status_code

@app.errorhandler(APIError)
def handle_api_error(error):
    return jsonify({'error': error.message}), error.status_code

@app.route('/test-error')
def test_error():
    raise APIError('Something went wrong', 400)

Practice Exercises

Exercise 1: Personal Portfolio App

Create a Flask app with the following routes:

/ — Home page with your name and a brief introduction
/projects — List of your projects
/skills — Your technical skills
/contact — Contact information

Exercise 2: Mini Calculator API

Build a Flask API that accepts two numbers and an operation:

/add?a=5&b=3 — Returns {"result": 8}
/subtract?a=10&b=4 — Returns {"result": 6}
/multiply?a=3&b=7 — Returns {"result": 21}
/divide?a=15&b=3 — Returns {"result": 5.0}

Exercise 3: Configuration Practice

Create a Flask app that:

Reads configuration from a .env file
Uses the Application Factory pattern
Has separate DevelopmentConfig and ProductionConfig classes
Prints the active config name on startup

Summary

In this lesson, you learned:

✅ What Flask is and why it's popular
✅ How to install Flask and set up a virtual environment
✅ Creating your first Flask application
✅ Understanding routes and view functions
✅ Organizing a Flask project structure
✅ Running the development server
✅ Working with Flask configuration
✅ Using the Application Factory pattern
✅ Basic error handling

Next Steps

Flask Routing — Learn about dynamic URLs, URL converters, and HTTP methods
Flask Templates — Use Jinja2 to create dynamic HTML pages
Flask Forms — Handle user input with WTForms
Flask Database — Connect to SQLite/PostgreSQL with SQLAlchemy

*Next Lesson: Flask Routing →*

Exam Focus

Revise definitions, diagrams, examples, and short-answer points for Introduction to Flask.

Interview Use

Prepare one clear explanation, one practical example, and one common mistake for this Python Master Course topic.

Search Terms

python-master-course, python master course, python, master, course, web, development, flask

Continue learning this concept

Web DevelopmentIntroduction to DjangoLearn Django, Python Web DevelopmentFlask RoutingMaster Flask URL routing — dynamic routes, URL converters, HTTP methods, redirects, URL building with url_for, and RESTful route design patterns in Python Flask.Web DevelopmentFlask Templates with Jinja2Learn Flask templates using Jinja2 templating engine. Master template inheritance, variables, control flow, filters, macros, and dynamic HTML rendering in Python Flask applications.Web DevelopmentDjango Models and ORMMaster Django models and the ORM. Learn to define database models, field types, relationships (ForeignKey, ManyToMany, OneToOne), migrations, QuerySets, and advanced database queries in Django.Web DevelopmentDjango REST API with DRFBuild professional REST APIs with Django REST Framework (DRF). Learn serializers, ViewSets, routers, authentication, permissions, filtering, pagination, and testing REST APIs in Django.Web DevelopmentDjango ViewsMaster Django views — function-based views (FBVs), class-based views (CBVs), generic views, mixins, authentication, form handling, and REST-style view patterns with full practical examples.

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Python Notes

Introduction to Flask

Learn Flask, the lightweight Python web framework. Build your first web application, understand Flask

Why Flask?

Flask is one of the most popular Python frameworks for several reasons:

Minimal and Lightweight — No forced dependencies or project structure
Flexible — Build anything from a simple API to a full web app
Pythonic — Clean, readable code that follows Python conventions
Extensive Ecosystem — Hundreds of Flask extensions available
Great for APIs — Perfect for RESTful API development
Excellent Documentation — One of the best-documented frameworks

Prerequisites

Before learning Flask, ensure you're comfortable with:

Python 3.8+ basics (variables, functions, classes)
Basic HTML/CSS knowledge
Understanding of HTTP (GET, POST requests)
Virtual environments and pip

Installation

Setting Up a Virtual Environment

Always use virtual environments to isolate your project dependencies:

bash example

# Create a new project directory
mkdir my-flask-app
cd my-flask-app

# Create a virtual environment
python -m venv venv

# Activate the virtual environment
# On Windows:
venv\Scripts\activate

# On macOS/Linux:
source venv/bin/activate

Installing Flask

bash example

pip install flask

Verify Installation

python example

python -c "import flask; print(flask.__version__)"
# Output: 3.0.x

Full Requirements File

bash example

# requirements.txt
flask>=3.0.0
python-dotenv>=1.0.0

Install from requirements:

bash example

pip install -r requirements.txt

Your First Flask Application

The Minimal App

Create a file called app.py:

python example

from flask import Flask

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

# Define a route and view function
@app.route('/')
def hello_world():
    return 'Hello, World!'

# Run the app
if __name__ == '__main__':
    app.run(debug=True)

Run your app:

bash example

python app.py

Visit http://127.0.0.1:5000 in your browser and you'll see "Hello, World!"

Understanding the Code

Let's break down each part:

python example

from flask import Flask

This imports the Flask class from the flask module.

python example

app = Flask(__name__)

Creates a Flask application instance. __name__ tells Flask the name of the module — this helps Flask find templates and static files.

python example

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

The @app.route('/') decorator tells Flask that when someone visits the root URL /, call the hello_world() function. This function returns the response to send back to the browser.

python example

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

Only runs the development server when the script is executed directly (not imported). debug=True enables the debugger and auto-reloader.

Flask Project Structure

For small projects, a single file works fine. For larger projects, use this structure:

Running Flask

Development Server

bash example

# Method 1: Run directly
python app.py

# Method 2: Using Flask CLI
flask --app app run

# Method 3: With debug mode enabled
flask --app app run --debug

# Method 4: Specify host and port
flask --app app run --host=0.0.0.0 --port=8080

Environment Variables

bash example

# Set Flask environment variables
# Windows (Command Prompt):
set FLASK_APP=app.py
set FLASK_DEBUG=1
flask run

# Windows (PowerShell):
$env:FLASK_APP = "app.py"
$env:FLASK_DEBUG = "1"
flask run

# macOS/Linux:
export FLASK_APP=app.py
export FLASK_DEBUG=1
flask run

Using python-dotenv

Create a .env file:

Example

FLASK_APP=app.py FLASK_DEBUG=1 SECRET_KEY=your-secret-key-here DATABASE_URL=sqlite:///database.db

Load it in your app:

Example

from dotenv import load_dotenv import os load_dotenv() app = Flask(__name__) app.config['SECRET_KEY'] = os.environ.get('SECRET_KEY', 'fallback-key')

Flask Configuration

Example

Multiple Routes

python example

from flask import Flask

app = Flask(__name__)

@app.route('/')
def home():
    return '<h1>Home Page</h1>'

@app.route('/about')
def about():
    return '<h1>About Page</h1><p>This is my Flask app.</p>'

@app.route('/contact')
def contact():
    return '<h1>Contact Us</h1>'

@app.route('/services')
def services():
    return '''
    <h1>Our Services</h1>
    <ul>
        <li>Web Development</li>
        <li>API Development</li>
        <li>Consulting</li>
    </ul>
    '''

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

Returning Different Responses

Flask can return various types of responses:

Example

Flask Application Factory Pattern

For larger applications, use the Application Factory pattern:

Example

python example

# run.py
from app import create_app

app = create_app('development')

if __name__ == '__main__':
    app.run()

Flask Application Context

Flask uses application and request contexts to manage state:

python example

from flask import Flask, g, current_app

app = Flask(__name__)

@app.before_request
def before_request():
    """Runs before every request."""
    g.user = None  # Initialize user as None
    print("Before request hook called")

@app.after_request
def after_request(response):
    """Runs after every request."""
    print(f"Response status: {response.status_code}")
    return response

@app.teardown_request
def teardown_request(exception):
    """Runs at the end of each request."""
    if exception:
        print(f"Error occurred: {exception}")

@app.route('/context')
def context_demo():
    # Access current app config
    debug_mode = current_app.config.get('DEBUG')
    return f'Debug mode: {debug_mode}'

Error Handling

python example

from flask import Flask, render_template, jsonify

app = Flask(__name__)

@app.errorhandler(404)
def not_found(error):
    return jsonify({'error': 'Page not found', 'status': 404}), 404

@app.errorhandler(500)
def internal_error(error):
    return jsonify({'error': 'Internal server error', 'status': 500}), 500

@app.errorhandler(403)
def forbidden(error):
    return jsonify({'error': 'Forbidden', 'status': 403}), 403

# Custom exception
class APIError(Exception):
    def __init__(self, message, status_code=400):
        self.message = message
        self.status_code = status_code

@app.errorhandler(APIError)
def handle_api_error(error):
    return jsonify({'error': error.message}), error.status_code

@app.route('/test-error')
def test_error():
    raise APIError('Something went wrong', 400)

Practice Exercises

Exercise 1: Personal Portfolio App

Create a Flask app with the following routes:

/ — Home page with your name and a brief introduction
/projects — List of your projects
/skills — Your technical skills
/contact — Contact information

Exercise 2: Mini Calculator API

Build a Flask API that accepts two numbers and an operation:

/add?a=5&b=3 — Returns {"result": 8}
/subtract?a=10&b=4 — Returns {"result": 6}
/multiply?a=3&b=7 — Returns {"result": 21}
/divide?a=15&b=3 — Returns {"result": 5.0}

Exercise 3: Configuration Practice

Create a Flask app that:

Reads configuration from a .env file
Uses the Application Factory pattern
Has separate DevelopmentConfig and ProductionConfig classes
Prints the active config name on startup

Summary

In this lesson, you learned:

✅ What Flask is and why it's popular
✅ How to install Flask and set up a virtual environment
✅ Creating your first Flask application
✅ Understanding routes and view functions
✅ Organizing a Flask project structure
✅ Running the development server
✅ Working with Flask configuration
✅ Using the Application Factory pattern
✅ Basic error handling

Next Steps

Flask Routing — Learn about dynamic URLs, URL converters, and HTTP methods
Flask Templates — Use Jinja2 to create dynamic HTML pages
Flask Forms — Handle user input with WTForms
Flask Database — Connect to SQLite/PostgreSQL with SQLAlchemy

*Next Lesson: Flask Routing →*

Exam Focus

Revise definitions, diagrams, examples, and short-answer points for Introduction to Flask.

Interview Use

Prepare one clear explanation, one practical example, and one common mistake for this Python Master Course topic.

Search Terms

python-master-course, python master course, python, master, course, web, development, flask

Continue learning this concept

PreviousDjango Views NextFlask Routing