How to inject pdb to failing Python scripts?

Question

I'm working on a django project that has a large fixture which does not load:

$ python manage.py loaddata apps/mainsite/fixtures/test_auctions.json 
/Users/cp/bidsite/.ve/lib/python2.6/site-packages/django/db/models/fields/subclassing.py:80: DeprecationWarning: A Field class whose get_db_prep_save method hasn't been updated to take a `connection` argument.
  new_class = super(SubfieldBase, cls).__new__(cls, name, bases, attrs)
/Users/cp/bidsite/.ve/lib/python2.6/site-packages/django/db/models/fields/subclassing.py:80: DeprecationWarning: A Field class whose get_db_prep_lookup method hasn't been updated to take `connection` and `prepared` arguments.
  new_class = super(SubfieldBase, cls).__new__(cls, name, bases, attrs)
/Users/cp/bidsite/.ve/lib/python2.6/site-packages/celery/task/schedules.py:5: DeprecationWarning: celery.task.schedules is deprecated and renamed to celery.schedules
  "celery.task.schedules is deprecated and renamed to celery.schedules"))
Problem installing fixture 'apps/mainsite/fixtures/test_auctions.json': Traceback (most recent call last):
  File "/Users/cp/bidsite/.ve/lib/python2.6/site-packages/django/core/management/commands/loaddata.py", line 174, in handle
    obj.save(using=using)
  File "/Users/cp/bidsite/.ve/lib/python2.6/site-packages/django/core/serializers/base.py", line 165, in save
    models.Model.save_base(self.object, using=using, raw=True)
  File "/Users/cp/bidsite/.ve/lib/python2.6/site-packages/django/db/models/base.py", line 570, in save_base
    created=(not record_exists), raw=raw, using=using)
  File "/Users/cp/bidsite/.ve/lib/python2.6/site-packages/django/dispatch/dispatcher.py", line 172, in send
    response = receiver(signal=self, sender=sender, **named)
  File "/Users/cp/bidsite/apps/mainsite/models.py", line 257, in update_auction_details
    auction_json = instance.as_json()
  File "/Users/cp/bidsite/apps/mainsite/models.py", line 1110, in as_json
    'product': self.product.as_json(),
  File "/Users/cp/bidsite/.ve/lib/python2.6/site-packages/django/db/models/fields/related.py", line 315, in __get__
    rel_obj = QuerySet(self.field.rel.to).using(db).get(**params)
  File "/Users/cp/bidsite/.ve/lib/python2.6/site-packages/django/db/models/query.py", line 349, in get
    % self.model._meta.object_name)
DoesNotExist: Product matching query does not exist.

The problem is that the stacktrace gives me no clue as to what line on the fixture is causing this error. How can I debug this? The only thing I can think of is there is a feature in ipythgon where whenever you execute something and it raises an exception, ipython automatically injects a pdb prompt so you can step around to figure out what happened. How can I do that with this? Is there a command line switch for python that does that? What could I do here to debug this?

Answer 1

This Python Recipe will install a debugger that starts on uncaught exceptions:

http://code.activestate.com/recipes/65287-automatically-start-the-debugger-on-an-exception/

The gist of it is to install an exception hook in sys.excepthook that calls pdb.pm() when called (though it's slightly more complicated than that).