Getting started with Flask

Hello World


from flask import Flask

app = Flask(__name__)

def hello():
    return 'Hello, World!'

Then run it with:

flask run
 * Running on http://localhost:5000/

Adding the code below will allow running it directly with python

if __name__ == '__main__':

Installation - Stable

Use pip to install Flask in a virtualenv.

pip install flask

Step by step instructions for creating a virtualenv for your project:

mkdir project && cd project
python3 -m venv env
# or `virtualenv env` for Python 2
source env/bin/activate
pip install flask

Never use sudo pip install unless you understand exactly what you’re doing. Keep your project in a local virtualenv, do not install to the system Python unless you are using the system package manager.

Installation - Latest

If you want to use the latest code, you can install it from the repository. While you potentially get new features and fixes, only numbered releases are officially supported.

pip install

Installation - Development

If you want to develop and contribute to the Flask project, clone the repository and install the code in development mode.

git clone ssh://
cd flask
python3 -m venv env
source env/bin/activate
pip install -e .

There are some extra dependencies and tools to be aware of as well.


Used to build the documentation.

pip install sphinx
cd docs
make html
firefox _build/html/index.html


Used to run the test suite.

pip install pytest
py.test tests


Used to run the test suite against multiple Python versions.

pip install tox

Note that tox only uses interpreters that are already installed, so if you don’t have Python 3.3 installed on your path, it won’t be tested.