A simple Flask hello world application.

• Python3.8

Create a project folder and create a virtual environment and activate it.

mkdir flask-hello-world && cd $_
python3 -m venv .venv
source .venv/bin/activate

First create a file to save dependencies (only Flask dependency in this case).
The must common name for this file is requirements.txt, so:

touch requirements.txt

Now you need to define your dependencies, and the file looks like this:

flask==1.1.2

You can check Flask releases in: https://github.com/pallets/flask/releases.
To install it, you just to need run:

pip install -r requirements.txt

Now, you can check installed dependencies (Flask and its dependencies).

pip freeze

And the output should be like this:

click==7.1.2
Flask==1.1.2
itsdangerous==1.1.0
Jinja2==2.11.2
MarkupSafe==1.1.1
Werkzeug==1.0.1

WOW Flask has very few dependencies to be a web application framework,
it is because Flask is a microframework

Create a main file call main.py (The name does not matter, it could be another).

touch main.py

And then put the next code:

from flask import Flask

app = Flask(__name__)


@app.route('/')
def hello():
  return 'Hello world with Flask'

To run your app, first you need to set a environment variable called FLASK_APP
and its value is the name of your file where your app is (main.py in this case).
Optionally you can set the debug mode for developing suing FLASK_DEBUG env var to 1.

export FLASK_APP=main.py
export FLASK_DEBUG=1

Check the env var:

echo $FLASK_APP
$ main.py

And then you can run your app:

flask run

And it outputs something like:

* Serving Flask app "main.py"
* Environment: production
  WARNING: This is a development server. Do not use it in a production deployment.
  Use a production WSGI server instead.
* Debug mode: on
* Running on http://127.0.0.1:5000/ (Press CTRL+C to quit)

By default Flask app run a server in the 5000 port in localhost.
Now you can send a request to your app:

curl http://127.0.0.1:5000/

The output should be like:

Hello world with Flask

To stop your new app, just press CTRL + c

MIT. Copyright (c)