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Getting Started

Flask is a lightweight WSGI web application framework designed to make getting started quick and easy, with the ability to scale up to complex applications.

Prerequisites​

  • Python: Version 3.10 or newer is required.
  • Package Manager: This guide assumes you are using uv, but you can also use pip.

Installation​

The recommended way to install Flask is using uv. This will install Flask and its core dependencies: Werkzeug (WSGI), The Flask Jinja Environment (templating), Getting Started with the Flask CLI (CLI), and others.

uv add flask

Optional Dependencies​

Flask supports several optional features that can be installed as extras:

  • Async: Adds support for async routes and handlers. 
    uv add flask[async]
  • Dotenv: Automatically loads environment variables from .env and .flaskenv files. 
    uv add flask[dotenv]

Hello World / Quick Start​

Create a file named app.py with the following content:

from flask import Flask

# Create the application instance
app = Flask(__name__)

# Define a route and its handler
@app.route("/")
def hello_world():
    return "<p>Hello, World!</p>"

Run the Server​

Use the flask CLI to start the development server:

flask run

The application will be available at http://127.0.0.1:5000/.

Configuration​

Flask can be configured via environment variables or by loading configuration files.

Environment Variables​

  • FLASK_APP: Specifies the application to load. If not set, Flask looks for app.py or wsgi.py.
  • FLASK_DEBUG: Set to 1 to enable debug mode, which provides an interactive debugger and reloads the server on code changes.
export FLASK_APP=hello
export FLASK_DEBUG=1
flask run

Instance Folders​

For configuration that should not be committed to version control (like secret keys), Flask supports an instance folder.

app = Flask(__name__, instance_relative_config=True)
app.config.from_pyfile("config.py", silent=True)

Verify Installation​

Confirm that Flask is installed correctly by checking its version:

flask --version

This will output the versions of Python, Flask, and Werkzeug.

Next Steps​

Troubleshooting​

  • Command not found: If the flask command is not found, ensure your virtual environment is activated or use python -m flask.
  • Address already in use: If port 5000 is occupied, you can specify a different port: flask run --port 8080.
  • ImportError: If you encounter an ImportError when running flask run, ensure FLASK_APP is set correctly to the module containing your Flask instance.