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

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

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.

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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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