Friday, May 16, 2008

Python: Debugging Google App Engine Apps Locally

Python has a wonderful interactive interpreter (i.e. shell). However, sometimes you need more setup before you can start coding. Previously, I wrote Python: Coding in the Debugger for Beginners.

Google App Engine works a bit like CGI in that output to STDOUT goes to the browser. This breaks my normal "import pdb; pdb.set_trace()" trick. However, it's not hard to put STDOUT, etc. back so that you can use pdb in the way you normally would:
for attr in ('stdin', 'stdout', 'stderr'):
setattr(sys, attr, getattr(sys, '__%s__' % attr))
import pdb
pdb.set_trace()

12 comments:

Ian Bicking said...

Or use weberror's magical new pdb middleware! Well... I haven't tried it with appengine, so there's a decent chance it doesn't work there (but patches welcome).

metapundit.net said...

Heh - I wondered about that today at the hackathon. I tried import pdb;pdb.set_trace() and got a weird error; now I guess i know why...

Shannon -jj Behrens said...

> Or use weberror's magical new pdb middleware! Well... I haven't tried it with appengine, so there's a decent chance it doesn't work there (but patches welcome).

Haha, as usual, you're way ahead of me.

whit537 said...

You can also pass std{in,out} arguments to pdb.Pdb, like so:

def set_trace():
    import pdb, sys
    debugger = pdb.Pdb(stdin=sys.__stdin__,
        stdout=sys.__stdout__)
    debugger.set_trace(sys._getframe().f_back)

This messes with Google's sandbox less.

Shannon -jj Behrens said...

> This messes with Google's sandbox less.

Very helpful. Thanks!

Stuart Grimshaw said...

I've tried the various methods mentioned here, but I always get errors displayed in the browser.

Using whit537's method I get this shown in the browser:

BdbQuit at /stop/37023745/
Request Method: GET
Request URL: http://localhost:8080/stop/37023745/
Exception Type: BdbQuit
Exception Value:
Exception Location: /System/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/bdb.py in dispatch_call, line 80

Shannon -jj Behrens said...

It sounds like you're not successfully pointing stdout at the actual terminal. I'm surprised that my technique (in the main body of this blog post) doesn't work.

Stuart Grimshaw said...

Actually, that does look like it works, I just can't work out how to attach to it, I'm running it through the Google App Engine Launcher on my mac if that makes any difference.

Shannon -jj Behrens said...

You need to figure out how to start the server from a shell. The start server script (whatever it's called) is on your Mac somewhere ;)

Stuart Grimshaw said...

Turns out "dev_appserver.py" is on the defaultpath, probably set up by the installer, or the "Make Symlinks" option off the gui apps menu.

Go to your apps parent directory and run "dev_appserver.py yourappname"

Jeff Enderwick said...

I tried:
pdb.Pdb(stdin=getattr(sys,'__stdin__'),stdout=getattr(sys,'__stderr__')).set_trace(sys._getframe().f_back)

- the idea being leave stdout alone. Seems to work. I did not have any crap heading back toward the browser. Is there any best soln out there from someone who works on GAE (as a full time thing)?

Shannon -jj Behrens said...

> the idea being leave stdout alone. Seems to work.

Thanks.

> Is there any best soln out there from someone who works on GAE (as a full time thing)?

I have no clue.