Simon Willison’s Weblog

snoop. Neat Python debugging utility by Alex Hall: snoop lets you “import snoop” and then add “@snoop” as a decorator to any function, which causes that function’s source code to be output directly to the console with details of any variable state changes that occur while it’s running.

I didn’t know you could make a Python module callable like that—turns out it’s running “sys.modules[’snoop’] = snoop” in the __init__.py module! # 31st January 2024, 6:19 pm

Appending the request URL to SQL statements in Django. A clever frame-walking monkey-patch which pulls the most recent HttpRequest object out of the Python stack and adds the current request.path to each SQL query as an SQL comment, so you can see it in debugging tools such as slow query logs and the PostgreSQL “select * from pg_stat_activity” query. # 2nd June 2010, 9:09 am

Fun with TextMate and PDB. TextMate bookmarks (against lines in a file) are stored as OS X extended attributes, which can be accessed from Python using the xattr module. Here’s a clever piece of code that uses bookmarks to set breakpoints in the command-line pdb debugger. # 23rd March 2010, 9:48 am

Presenting django-devserver, a better runserver. I really like this—it’s a Django management command (./manage.py rundevserver) which adds SQL logging and cache access summaries to the console output of Django’s local development server. It solves a similar set of problems to the debug toolbar, but requires slightly less setup and doesn’t inject additional HTML in to your pages. You can add your own custom modules to it as well. # 10th February 2010, 11:33 am

Debugging in Python. The missing manual for Python’s powerful pdb debugger. # 18th November 2009, 12:34 pm

Debugging Django in Production Revisited. Eric Holscher expands his show-technical-errors-to-superusers middleware to only show them to users in the group named “Technical Errors”. # 7th September 2009, 5:21 am

Finding and fixing memory leaks in Python. Using Dozer, a clever piece of WSGI middleware which displays sparklines of Python object counts and allows you to introspect them, using the gc module under the hood. # 22nd April 2009, 12:16 pm

Announcing django-viewtools. A really excellent idea—run ./manage.py viewtools --pdb /path/on/site/ to debug a view in your Django project that is raising an error using the Python debugger, or use --profile to run the full request cycle for that URL through the profiler. # 17th February 2009, 9:35 pm

Django snippets: server with debugging backdoor. Six lines of code that uses spawning to fire up a Django server on port 8000 and a remote interactive interpreter backdoor on port 8001, so you can interogate the state of your server within the same process. # 8th September 2008, 10:15 pm

Super User Conditional Page Exception Reporting. The name is almost as long as the code snippet: this serves Django’s debug page to logged in super-users, falling back to the default 500 template for everyone else. # 31st July 2008, 9:06 pm

modswgi: Debugging Techniques. mod_wsgi is excellent software, and the documentation is equally superb. I used these instructions recently to run the Python debugger inside a running instance of Apache, which helped my track down some import errors that weren’t occurring with Django’s development server. # 25th May 2008, 1:34 pm

The joy of pdb.set_trace(). I use nosetests --pdb-failures as my main entrypoint for Python debugging—it starts the debugger at the first failing test. # 25th April 2007, 11:37 pm