Emacs: methods for debugging python

Question

I use emacs for all my code edit needs. Typically, I will use M-x compile to run my test runner which I would say gets me about 70% of what I need to do to keep the code on track however lately I've been wondering how it might be possible to use M-x pdb on occasions where it would be nice to hit a breakpoint and inspect things.

In my googling I've found some things that suggest that this is useful/possible. However I have not managed to get it working in a way that I fully understand.

I don't know if it's the combination of buildout + appengine that might be making it more difficult but when I try to do something like

M-x pdb
Run pdb (like this): /Users/twillis/projects/hydrant/bin/python /Users/twillis/bin/pdb /Users/twillis/projects/hydrant/bin/devappserver /Users/twillis/projects/hydrant/parts/hydrant-app/

Where .../bin/python is the interpreter buildout makes with the path set for all the eggs.

~/bin/pdb is a simple script to call into pdb.main using the current python interpreter

HellooKitty:hydrant twillis$ cat ~/bin/pdb
#! /usr/bin/env python

if __name__ == "__main__":
    import sys
    sys.version_info
    import pdb
    pdb.main()
HellooKitty:hydrant twillis$ 

.../bin/devappserver is the dev_appserver script that the buildout recipe makes for gae project and .../parts/hydrant-app is the path to the app.yaml

I am first presented with a prompt

Current directory is /Users/twillis/bin/
C-c C-f

Nothing happens but

HellooKitty:hydrant twillis$ ps aux | grep pdb
twillis    469 100.0  1.6   168488  67188 s002  Rs+   1:03PM   0:52.19 /usr/local/bin/python2.5 /Users/twillis/projects/hydrant/bin/python /Users/twillis/bin/pdb /Users/twillis/projects/hydrant/bin/devappserver /Users/twillis/projects/hydrant/parts/hydrant-app/
twillis    477   0.0  0.0  2435120    420 s000  R+    1:05PM   0:00.00 grep pdb
HellooKitty:hydrant twillis$ 

something is happening

C-x [space]

will report that a breakpoint has been set. But I can't manage to get get things going.

It feels like I am missing something obvious here. Am I?

So, is interactive debugging in emacs worthwhile? is interactive debugging a google appengine app possible? Any suggestions on how I might get this working?

Answer 1

Hmm. You're doing this a little differently than I do. I haven't experimented with your method. I use the pdb library module directly, with no wrapper script, just using the "-m" python command-line option to tell python to run the module as a script.

To be excessively thorough, here's my sequence of operations:

1. I hit Alt-X in EMACS, type "pdb", then return.
2. EMACS prompts me with "Run pdb (like this):" and I type "python -m pdb myprogram.py".
3. EMACS creates a debugger mode window for pdb, where I can give the debugger commands, and tracks the execution of the program in the source code.

I suppose it's possible there's some reason this doesn't work well with the appengine. I recommend getting it working first with a trivial python program and once you know that's working, try stepping up to the full app.

In practice, I don't do much python debugging with pdb. Most of my debugging is essentially "printf debugging", done inserting print statements into my unit tests and (occasionally) into the actual code.