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Mar 30, 2010

Mixed Jython/Python development and Google AppEngine oddities

Recently, I worked on a Python project, which I developed as a stand-alone WSGI application. I also experimented with running this app in Jython. In case you don't know: Jython is a neat Python implementation, which runs in the Java VM. The advantage of this is that you have access to all the usual Java infrastructure, framework and libraries, while being able to rapidly develop your program in the Python language. I ended up designing my application so that it could run both as Python as well as Jython project.

Shortly afterwards, I moved my WSGI application to Google's AppEngine, which is is a hosted environment for Python and Java apps. AppEngine can run pure WSGI applications quite easily, so this shouldn't be too much of a problem. Using Google's local development server (, I was able to run my application on my desktop in the emulated Google environment without problem.

I then used the provided command line utility ( to move my app to Google's actual servers... and nothing worked. I received a "500 Internal Server Error". Looking at the logs I found that my application was not able to import any of my own modules. While standard library items were imported without an issue, any module I had provided myself resulted in an import error.

What had happened? It took me a long time to figure this out, so I thought I share it with you: When running the Python application via Jython, a bunch of .class files were created, which is normal. However, when the application was uploaded via, those class files were uploaded as well (files ending in *.pyc and a few other extensions are ignored, but apparently anything else is considered to be part of your application).

So then, for reasons that aren't quite clear, the presence of .class files in your Python application seems to be really confusing for Google AppEngine.

The lesson? If you are developing an application that can run locally in Jython as well as on Google's AppEngine, make sure to clear out your *.class files before uploading. It can save you hours of grief.

Oh, and by the way, you should follow me on twitter here.

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