Warning message while running Flask

While I am running Flask code from my command line, a warning is appearing:

Serving Flask app "hello_flask" (lazy loading)
* Environment: production
  WARNING: Do not use the development server in a production environment.
  Use a production WSGI server instead.

What does this mean?

As stated in the Flask documentation:

While lightweight and easy to use, Flask’s built-in server is not suitable for production as it doesn’t scale well and by default serves only one request at a time.

Given that a web application is expected to handle multiple concurrent requests from multiple users, Flask is warning you that the development server will not do this (by default). It recommends using a Web Server Gateway Interface (WSGI) server (numerous possibilities are listed in the deployment docs with further instructions for each) that will function as your web/application server and call Flask as it serves requests.

Try gevent:

from flask import Flask
from gevent.pywsgi import WSGIServer

app = Flask(__name__)

@app.route('/api', methods=['GET'])
def index():
    return "Hello, World!"

if __name__ == '__main__':
    # Debug/Development
    # app.run(debug=True, host="0.0.0.0", port="5000")
    # Production
    http_server = WSGIServer(('', 5000), app)
    http_server.serve_forever()

Note: Install gevent using pip install gevent

As of Flask 1.x, the default environment is set to production.

To use the development environment, create a file called .flaskenv and save it in the top-level (root) of your project directory. Set the FLASK_ENV=development in the .flaskenv file. You can also save the FLASK_APP=myapp.py.

Example:

myproject/.flaskenv:

FLASK_APP=myapp.py
FLASK_ENV=development

Then you just execute this on the command line:

flask run

That should take care of the warning.

To remove the "Do not use the development server in a production environment." warning, run:

export FLASK_ENV=development

before flask run.