Google App Engine & eclipse (PyDev)

Once you have configured your run environment his setup also allows for you to debug from the same launch profile. To debug you just use the Debug version of the launch profile, no changes have to be made! There are a few things to note about this configuration:

• When you start a run session this only starts the Google App Engine server. Unlike using Flex Builder or Aptana, this does not launch a browser window to your application path. You will need to open a browser window and then navigate to the localhost:[port] that you have configured. If someone knows how to directly launch to a specified path let me know and I will update this post.
• Since you are starting a server with your run session you need to stop the session before starting a new debug session (or visa versa). If you do not stop (terminate) the previous session you will get an error in the Eclipse console.
• The Google App Engine is smart enough to check touch times so when you update a file you can just refresh the browser (’course you probably know that if you did the gApp tutorial). This makes the workflow with PyDev really easy. Keep you browser window open and keep hitting refresh after any changes. Unless of course you are using a lot of JavaScript then you may need to clear you cache.

Have fun and let me know if you have any tips on improving the workflow!

During the weekend I decided to begin playing with the Google App Engine and see what all the hubbub was about. Like most developers I couldn’t get into the preview (yet) but I could download and run the SDK. The SDK is all Python, a language I knew a little about but never delved into. Actually, the only thing I knew about it was that whitespace matters and having a good IDE is critical. Knowing this I looked up IDE options and found a plugin called PyDev for my favorite IDE, Eclipse.

After taking a brief look at the feature set I saw the important ones; code completion, debugging, refactoring and I was sold. I downloaded the plugin into Eclipse 3.3.2 and fired it up. The fist first thing I tried to do was create a new project, when I tried that I got a message informing me that I had to define the Python interpreter settings before I could create a project. Okay, makes sense to me.

I looked up the help docs and it said to open Eclipse > preferences > PyDev > Interpreter – Python, choose new… from the Interpreters section and point to my Python install directory. I knew that OS X Leopard has Python built in so I look into the usual locations such as ~/Library/, /Library/ and /System/. Nothing obvious jumped out so I did a quick search on the web and find the default install location is:

/System/Library/Frameworks/Python.framework/

Awesome, so I point the Interpreter to the Python exec yet it won’t let me choose it. Well, it is a symlink so maybe I need to go to the actual exec. I know that OS X has 2.5 installed so I navigate down to:

/System/Library/Frameworks/Python.framework/Versions/2.5/

and try to choose the Python exec located there. It accepts this location and then when I select it I get the “Error getting info on interpreter” dialog. What the hell does that mean? It has some notes in the dialog that says that common errors include specifying an invalid interpreter and/or spaces in the Eclipse path. Oops. I have have a space in my Eclipse path, I fix it then try again. Same error. Hmmmm….

There is also a bug documentation URL referenced in dialog so I do some searching and it tells me that if you get this error you are pointing to an invalid interpreter. Well duh, the dialog told me that much. What frustrates me is that the bug URL has the people responding back with “Hey, I fixed it.” with no statement on how, just that they fixed it! Okay, that’s not helping… time to do some testing.

I try using 2.3 to see if that works. Nope, same error. Okay, what other paths are there? I do some more digging and find out there is actually an APP file located at:

/System/Library/Frameworks/Python.framework/Versions/2.5/Resources/

I try both the Python.app and Python Launcher.app. This time a get a different error:

Check your error log for more details.

More info can also be found at the bug report:
http://sourceforge.net/tracker/index.php?func=detail&aid=1523582&group_id=85796&atid=577329

java.io.IOException: /System/Library/Frameworks/Python.framework/Versions/2.5/Resources/Python.app: cannot execute

WTF!?!? I do more digging and people are having the same issue but NO WHERE IS IT DOCUMENTED ON HOW TO FIX IT!?!?! Okay, deep breaths… I want to use this plugin and people have it working in OS X and swear by it, I must prevail!

I do some more searching and stumble upon a forum that talks about getting PyDev running with Maya.

– SOLUTION HERE –

Update: Gabriel Lamounier provided the direct path to Python for Leopard in the comments section for my original post. He points out that the correct path is:

/System/Library/Frameworks/Python.framework/Versions/2.5/bin/python2.5

I verified it and PyDev does like this path. Thanks Gabriel, I swore I tried this path during my testing but I must not have, after multiple hours of digging it’s sometimes easy to miss the semi-obvious. So, the easy answer is… use this path when setting up your Python interpreter. For grins, read the rest of the post to find out the “hack” way I came upon.

It seems that you cannot point to the APP but you have to point to the contents in the APP which contains the Python interpreter. sigh Seriously, who would have thunk it? So, the path for the Python interpreter in OS X 10.5 is:

/System/Library/Frameworks/Python.framework/Versions/2.5/Resources/Python.app/
Contents/MacOS

Remember, an OS X APP is really just a fancy Directory. But wait, there’s more! You can’t actually navigate to the contents of an APP file from your standard OS X open dialog. Well thank god for kessaris who posted the Maya solution because he mentions that when you are in the open dialog you can press SHIT-CMD-G SHIFT-CMD-G to open a URL dialog that allows you to paste a path in. In most cases you have to Right click (CMD-Click) > Paste… to get the path in, but once you do you will see the Python interpreter in the MacOS folder. Select it and it works! You are now off to the Python races.

I want to give a big shout out to kessaris, whoever you are, for posting this incredibly helpful reply. Also, I hope this post helps someone install PyDev, because it really, really is a great IDE. More on PyDev and the Google App Engine coming soon!!