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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: uv is the recommended tool for installation and dependency management, though pip is also supported.

Installation​

Install Flask using uv:

uv pip install Flask

Optional Dependencies​

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

  • Async support: For using async routes and handlers.
  • Dotenv support: For automatically loading environment variables from .env and .flaskenv files.
uv pip install "Flask[async,dotenv]"

Hello World / Quick Start​

Create a file named app.py:

from flask import Flask

app = Flask(__name__)

@app.route("/")
def hello_world():
    return "<p>Hello, World!</p>"

Run the Application​

Use the flask CLI to start the development server:

flask run

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

Application Factory Pattern​

For larger applications, it is recommended to use the The Flask Application Object pattern. Create a directory for your package and an __init__.py file:

# myapp/__init__.py
from flask import Flask

def create_app(test_config=None):
    app = Flask(__name__, instance_relative_config=True)
    
    @app.route("/hello")
    def hello():
        return "Hello, World!"

    return app

Run it by specifying the factory:

flask --app myapp:create_app run

Configuration​

Flask can be configured via environment variables or configuration files.

  • FLASK_APP: Specifies the application to load.
  • FLASK_DEBUG: Set to 1 or true to enable debug mode (includes interactive debugger and reloader).

Example using environment variables:

export FLASK_APP=app.py
export FLASK_DEBUG=1
flask run

If python-dotenv is installed, Flask will automatically load variables from .env or .flaskenv in the project root.

Verify Installation​

Confirm Flask is installed correctly by checking the version:

flask --version

To run the project's test suite (if you have cloned the repository):

pytest

Other Install Options​

Using pip​

If you prefer standard pip:

python -m venv .venv
source .venv/bin/activate  # Linux/macOS
# or .venv\Scripts\activate on Windows
pip install Flask

Next Steps​