Resolving NumPy Sub-Interpreter Warnings in WSGI Applications

If you’ve encountered a warning like this while running a WSGI application with Apache and mod_wsgi:

UserWarning: NumPy was imported from a Python sub-interpreter but NumPy does not properly support sub-interpreters. This will likely work for most users but might cause hard to track down issues or subtle bugs. A common user of the rare sub-interpreter feature is wsgi which also allows single-interpreter mode.

You’re not alone. This warning indicates that NumPy was imported in a sub-interpreter, which can lead to subtle bugs and hard-to-track-down issues. Fortunately, there’s a straightforward solution.

Why This Happens

Apache’s mod_wsgi can run WSGI applications in sub-interpreters to isolate different applications. However, some libraries, like NumPy, do not fully support this feature. Running NumPy in a sub-interpreter can lead to unpredictable behavior and warnings.

Solution: Use Single-Interpreter Mode

To prevent these issues, configure mod_wsgi to run in single-interpreter mode. This forces your application to run in the main interpreter, avoiding the problems associated with sub-interpreters.

Here’s how you can do it:

Step-by-Step Guide

1. Edit Your Apache Configuration:Open your Apache configuration file (e.g., 000-default.conf or your specific virtual host configuration file) and add the WSGIApplicationGroup directive.

<VirtualHost *:80>
    ServerName yourdomain.com
    DocumentRoot /path/to/your/project

    WSGIDaemonProcess your_app_name python-path=/path/to/your/project python-home=/path/to/your/virtualenv
    WSGIProcessGroup your_app_name
    WSGIScriptAlias / /path/to/your/project/wsgi.py

    # Add this line to force the use of the main interpreter
    WSGIApplicationGroup %{GLOBAL}

    <Directory /path/to/your/project>
        <Files wsgi.py>
            Require all granted
        </Files>
    </Directory>

    ErrorLog ${APACHE_LOG_DIR}/error.log
    CustomLog ${APACHE_LOG_DIR}/access.log combined
</VirtualHost>

Explanation:

• WSGIApplicationGroup %{GLOBAL}: This directive tells mod_wsgi to run the WSGI application in the main interpreter of the process, avoiding sub-interpreters.

Restart Apache:

After making the changes, restart Apache to apply the new settings.

sudo systemctl restart apache2

or

sudo systemctl restart httpd

Additional Tips

• Monitor Logs: Keep an eye on your logs to ensure that the warning is gone and no other issues arise.
• Check for Other Libraries: If you use other libraries that might not support sub-interpreters, running in single-interpreter mode should help with those as well.

Conclusion

By setting WSGIApplicationGroup %{GLOBAL}, you can avoid the NumPy warning and prevent potential issues associated with sub-interpreters in your WSGI application. This simple configuration change ensures that your application runs smoothly and reliably.

If you found this post helpful, feel free to share it with others who might be facing the same issue. For more tips and solutions, visit my blog.

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I hope this guide helps you resolve the NumPy sub-interpreter warning in your WSGI applications. If you have any questions or need further assistance, please leave a comment below!

Amit Mittal

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