Fernando Correia’s Weblog

August 8, 2008

Debugging Google App Engine applications

Filed under: Software Development — Fernando Correia @ 8:08 pm

I admit: until now I was using just a text editor to create my Google App Engine (GAE) Web service.

But I know that as it gets more complex I will need a debugger. So I went after instructions and I found a great article by Joscha Feth that explains how to configure Eclipse and PyDev for debugging with GAE.

I was able to follow the instructions and debug a sample Hello World GAE application. It was easy.

There were just a couple caveats:

First, some commands seem to be different in Eclipse 3.4 than in the version that was used in the article. For instance, in Help -> Software Updates I couldn’t find a Find and Install command, but I was able to use the Add Site button instead. Also, there isn’t a Run -> Open Run Dialog command. I used Run -> Run Configurations.

Second, when I tried to debug I got this error: Variable references empty selection: ${project_loc}. I found out that I had to click on the project on the left panel and then click on the debug button and select my run configuration.

After that, I could set breakpoints on my Python code and inspect local variables when the breakpoints were hit. Nice integration of Python within Eclipse!

I just miss the vertical lines that show the indentation levels. It seems PyDev lacks this feature…



  1. Also, google appengine – instructions for pydev on eclipse mentions about ‘arguments’ port=9999, – this doesn’t work if you do not have it right format, which is “–port=9999”

    Comment by pv — August 19, 2008 @ 7:48 pm

  2. The above posting shows ‘- -‘ as ‘-‘ . so it is ‘- – port=nnnn” . no space between ‘-‘s.

    Comment by pv — August 19, 2008 @ 7:57 pm

  3. Thanks! I think that is shown in this image: http://code.google.com/appengine/articles/eclipse_img/pargs.png

    Comment by fernandoacorreia — August 19, 2008 @ 8:04 pm

  4. Variable references empty selection: ${project_loc}.

    when it occured,you can

    1.select your project
    2.F5 refresh
    3.then debug it again

    Comment by reedfly — October 16, 2008 @ 8:34 pm

  5. It didn’t like when I tried to set a port. But now that I look at post 2, I think I know why. There’s two hyphens! But I’m ok with default port. That’s really tricky having to select your src folder for it to work. Ouch! I was almost ready to give up.

    It’d be nice of google to update their instruction page here:
    titled “Configuring Eclipse on Windows to Use With Google App Engine”

    To incorporate all of these issues, and how the latest eclipse version has different things to click. I sort of stumbled through the setup, finding things that seemed right even though they weren’t named how google said they would be. But it’s working!

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