Using pdb to debug management commands and unit tests (Debugging Django Series,Part 4)

Screencast 4

Today’s screencast is about pdb again. This time we are going to be debugging management commands, and unit tests for django. This is a little bit more powerful than the previous screencast which just introduced the basic debugging commands.

Setup

This screencast uses a couple of really handy pieces of software. iPython is a wonderful piece of software for all python developers. It gives you handy things like tab completion, syntax highlighting, and all the modern amenities that we’re used to in our editor from the python shell. Django’s manage.py shell even uses ipython if it detects it, that’s an endorsement if i’ve ever heard one.

ipdb is a simple wrapper around pdb that allows you to use ipython when you are doing your debugging. This is really handy as well. To get the code for these projects, go to their websites linked above or use the following code:

bzr branch lp:ipython
easy_install ipdb

Download and Video

You can download the video here (20MB mov)

Debugging management commands and unit tests from Eric Holscher on Vimeo.

Writeup

We start the screencast by breaking the testmaker management command that I’ve written. We call it like this:

python -i ~/EH/manage.py testmaker 67.207.139.9:8000

The import thing to note is the -i, which tells python to drop into the python (>>>) shell after the command is run. I then show how to use pdb postmordem command to go back into the crashed management command. This is called like so:

import pdb
pdb.pm()

and this allows you to actually go back into the previous command! Even if pdb isn’t currently loaded at the time. This is a really neat feature of the debugger, and incredibly useful for diagnosing breakage that is hard to reproduce. You can go back up into the application and see the actual state of the variables at that time.

Next I introduce ipython which is a really nice python distribution. It has a really nice debugger, called idpb, which gives you all the ipython commands inside the debugger.

Next we go on to run testmaker with valid input after showing how to do a simple fix to check if the input was correct. We call ipython with an app passed in:

ipython ~/EH/manage.py testmaker 67.207.139.9:8000 -- -a mine

Note that – is meant to tell bash that the input is done for ipython, and the rest will actually go to manage.py and into your python code. This is good to know for trying to pass things into management commands in ipython on the command line. This code will generate tests and fixtures for the application inside of the mine/ directory. Once we browser around a little on the test server, we have generated a unit test based on what we have done.

Assuming you have ipdb installed from Pypi, you can include ipdb inside of your unit test (or any python file being executed) and get the ipython debugger instead of vanilla pdb:

import ipdb
ipdb.set_trace()

Although I don’t expound on it inside of the screencast, getting inside of tests is probably one of the more useful things you can do with the debugger. Trying to debug tests is really difficult, and sometimes they return really strange errors that are hard to get a handle on. Sometimes the line numbers are also off, and debugging doctests are notoriously hard to debug. You can debug doctests just as easily, with the following code (using ipdb if preferred):

>>> import pdb
>>> pdb.set_trace()

I figured out that the error in the unit test was actually due to a stale fixture left over from a previous run of the testmaker app. It didn’t have enough data to return a paginated list, so has_next was false instead of true like when we ran it against the live database.

In related news, searching for ipdb on google made me stumble onto the The Internet Pinball Machine Database, which I didn’t know existed previously. Yess!

This Post

Published 2008-09-02

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