How to Prevent Gunicorn from Starting Flask’s Development Server

Discover how to troubleshoot and fix the issue of Gunicorn invoking Flask's development server, ensuring a smooth deployment for your applications.
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This video is based on the question https://stackoverflow.com/q/77304625/ asked by the user 'Anthr. Line' ( https://stackoverflow.com/u/14818359/ ) and on the answer https://stackoverflow.com/a/77304686/ provided by the user 'Yuri R' ( https://stackoverflow.com/u/11025602/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Introduction

Running a Flask application using Gunicorn is a common practice among developers looking to deploy web applications efficiently. However, a problem can arise when Gunicorn unintentionally triggers Flask's development server. This can be quite perplexing, especially if you've successfully set up other servers without any issue. In this post, we’ll dive into this problem and guide you through the steps to ensure that your application runs as intended.

The Problem

When you execute a Gunicorn command such as:

[[See Video to Reveal this Text or Code Snippet]]

You might see output similar to the following:

[[See Video to Reveal this Text or Code Snippet]]

As the output suggests, Flask's development server starts up, which you explicitly do not want when utilizing Gunicorn. This can happen despite your attempts to change configurations like the port, host, and other variables.

Solution

To resolve this issue, we’ll explore a few key adjustments to make sure Flask’s development server does not run and your Gunicorn setup is correctly configured.

1. Check Your Script

Look for the following lines in your main application script:

[[See Video to Reveal this Text or Code Snippet]]

If these lines are present, they are responsible for starting the Flask development server. Here’s what you can do:

Comment Out the Block:
Simply put a # in front of these lines to prevent them from executing:

[[See Video to Reveal this Text or Code Snippet]]

This adjustment ensures that the development server does not launch inadvertently when you start Gunicorn.

2. Environment Variables

Ensure that you do not have any environment variables set that could influence Flask to run in development mode. Specifically, check for:

FLASK_ENV=development

FLASK_APP=app.py

You can verify and unset these variables with the following commands:

[[See Video to Reveal this Text or Code Snippet]]

3. Verify Your Application Reference

It’s crucial to confirm that your command accurately points to your Flask application object. For example, in the command:

[[See Video to Reveal this Text or Code Snippet]]

Make sure that main is the correct module name and that app refers to your Flask application instance.

Conclusion

By following these steps, you should be able to configure your Gunicorn server to run your Flask application without inadvertently triggering the development server. Properly structuring your application and managing environment variables is key to a successful deployment.

If you continue to experience issues, consider reviewing your project’s directory structure and dependencies. Remember, optimizing your deployment environment is essential to ensuring that your application runs smoothly and securely.

Feel free to reach out if you have any further questions or need assistance with your Flask and Gunicorn setup!