WARNING: This is a development server. Do not use it in a production deployment

# WARNING: This is a development server. Do not use it in a production deployment

The warning "This is a development server. Do not use it in a production deployment" occurs when we start a Flask application using the development server.

To resolve the issue, install and use the waitress module to host your Flask application in production.

warning this is development server do not use it in production

You can use the development server when working on your project on localhost.

For example, if you have the following code stored in a file app.py.

app.py

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from flask import Flask

app = Flask(__name__)

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

You can run the development server with the following command.

cmd

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flask run

python -m flask run
python3 -m flask run
py -m flask run

# Using a production-ready server

If you need a production-ready server, install the waitress module.

Open your terminal and run the following command.

shell

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pip install waitress

# 👇️ For Python 3
pip3 install waitress

# 👇️ If you don't have pip in your PATH environment variable
python -m pip install waitress

# 👇️ For Python 3
python3 -m pip install waitress

# 👇️ Using py alias
py -m pip install waitress

# 👇️ If you get a permissions error
pip install waitress --user

# 👇️ For Anaconda
conda install -c conda-forge waitress

Once you install the waitress module, you can adjust your code to the following.

app.py

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from flask import Flask

app = Flask(__name__)

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

if __name__ == "__main__":
    from waitress import serve
    serve(app, host="0.0.0.0", port=8080)

Now you can start your production-ready server with python app.py.

shell

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python app.py

The server will be accessible at http://localhost:8080.

The default Flask server is not suitable for production, but the WSGI server from the waitress module is.

This section of the Flask documentation has more information on the recommended options for self-hosting.

# Creating a virtual environment

If you don't already have a virtual environment, you can create one with the following commands.

PowerShell

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# 👇️ Might also be: "python3 -m venv venv"
python -m venv venv

# 👇️ Activate on Windows (PowerShell)
venv\Scripts\Activate.ps1

# 👇️ Activate on Windows (cmd.exe)
venv\Scripts\activate.bat

# 👇️ Activate on Unix or MacOS
source venv/bin/activate

# 👇️ Install Flask in your virtual environment
pip install flask
pip install waitress

If the python -m venv venv command doesn't work, try the following 2 commands:

py -m venv venv.
python3 -m venv venv

Now you can start your server with python app.py.

# Using the `gevent` module

Alternatively, you can use the gevent module.

First, install gevent by running the following command in your virtual environment.

shell

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pip install gevent

# 👇️ For Python 3
pip3 install gevent

# 👇️ If you don't have pip in your PATH environment variable
python -m pip install gevent

# 👇️ For Python 3
python3 -m pip install gevent

# 👇️ Using py alias
py -m pip install gevent

# 👇️ If you get a permissions error
pip install gevent --user

# 👇️ For Anaconda
conda install -c anaconda gevent

Now, tweak your app.py file to the following.

app.py

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from flask import Flask
from gevent.pywsgi import WSGIServer

app = Flask(__name__)

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


if __name__ == '__main__':
    http_server = WSGIServer(("127.0.0.1", 8080), app)
    http_server.serve_forever()

You can start your server with the python app.py command.

shell

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python app.py

The server will be accessible at http://localhost:8080.

You can check out the other options for production-ready WSI servers in this section of the docs.

# Additional Resources

You can learn more about the related topics by checking out the following tutorials:

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WARNING: This is a development server. Do not use it in a production deployment