Django app throws spurious exception when importing views from third-party app

Question

I'm working on a Django app that occasionally throws a ViewDoesNotExist exception when trying to import modules from a third-party app (Solango, to be specific). By "occasionally", I mean often enough to be annoying, but definitely a minority of requests. Solango is on the app's PYTHONPATH and can be imported reliably through the console. The error also never happens during local development, so maybe it has something to do with the server setup (the app uses Apache + mod_wsgi in daemon mode).

Here's a stack trace showing the error occurring in the admin (although it occurs on pretty much every page on the site):

Traceback:
File "/home/nybooks/ve/lib/python2.5/site-packages/django/core/handlers/base.py" in get_response
  92.                 response = callback(request, *callback_args, **callback_kwargs)
File "/home/nybooks/ve/lib/python2.5/site-packages/django/contrib/admin/sites.py" in root
  445.             return self.index(request)
File "/home/nybooks/ve/lib/python2.5/site-packages/django/views/decorators/cache.py" in _wrapped_view_func
  44.         response = view_func(request, *args, **kwargs)
File "/home/nybooks/ve/lib/python2.5/site-packages/django/contrib/admin/sites.py" in index
  342.             context_instance=template.RequestContext(request)
File "/home/nybooks/ve/lib/python2.5/site-packages/django/shortcuts/__init__.py" in render_to_response
  20.     return HttpResponse(loader.render_to_string(*args, **kwargs), **httpresponse_kwargs)
File "/home/nybooks/ve/lib/python2.5/site-packages/django/template/loader.py" in render_to_string
  108.     return t.render(context_instance)
File "/home/nybooks/ve/lib/python2.5/site-packages/django/template/__init__.py" in render
  178.         return self.nodelist.render(context)
File "/home/nybooks/ve/lib/python2.5/site-packages/django/template/__init__.py" in render
  778.                 bits.append(self.render_node(node, context))
File "/home/nybooks/ve/lib/python2.5/site-packages/django/template/__init__.py" in render_node
  791.         return node.render(context)
File "/home/nybooks/ve/lib/python2.5/site-packages/django/template/loader_tags.py" in render
  97.         return compiled_parent.render(context)
File "/home/nybooks/ve/lib/python2.5/site-packages/django/template/__init__.py" in render
  178.         return self.nodelist.render(context)
File "/home/nybooks/ve/lib/python2.5/site-packages/django/template/__init__.py" in render
  778.                 bits.append(self.render_node(node, context))
File "/home/nybooks/ve/lib/python2.5/site-packages/django/template/__init__.py" in render_node
  791.         return node.render(context)
File "/home/nybooks/ve/lib/python2.5/site-packages/django/template/loader_tags.py" in render
  97.         return compiled_parent.render(context)
File "/home/nybooks/ve/lib/python2.5/site-packages/django/template/__init__.py" in render
  178.         return self.nodelist.render(context)
File "/home/nybooks/ve/lib/python2.5/site-packages/django/template/__init__.py" in render
  778.                 bits.append(self.render_node(node, context))
File "/home/nybooks/ve/lib/python2.5/site-packages/django/template/__init__.py" in render_node
  791.         return node.render(context)
File "/home/nybooks/ve/lib/python2.5/site-packages/django/template/defaulttags.py" in render
  245.                     return self.nodelist_true.render(context)
File "/home/nybooks/ve/lib/python2.5/site-packages/django/template/__init__.py" in render
  778.                 bits.append(self.render_node(node, context))
File "/home/nybooks/ve/lib/python2.5/site-packages/django/template/__init__.py" in render_node
  791.         return node.render(context)
File "/home/nybooks/ve/lib/python2.5/site-packages/django/template/defaulttags.py" in render
  255.             return self.nodelist_true.render(context)
File "/home/nybooks/ve/lib/python2.5/site-packages/django/template/__init__.py" in render
  778.                 bits.append(self.render_node(node, context))
File "/home/nybooks/ve/lib/python2.5/site-packages/django/template/__init__.py" in render_node
  791.         return node.render(context)
File "/home/nybooks/ve/lib/python2.5/site-packages/django/template/loader_tags.py" in render
  24.         result = self.nodelist.render(context)
File "/home/nybooks/ve/lib/python2.5/site-packages/django/template/__init__.py" in render
  778.                 bits.append(self.render_node(node, context))
File "/home/nybooks/ve/lib/python2.5/site-packages/django/template/__init__.py" in render_node
  791.         return node.render(context)
File "/home/nybooks/ve/lib/python2.5/site-packages/django/template/defaulttags.py" in render
  372.             url = reverse(self.view_name, args=args, kwargs=kwargs)
File "/home/nybooks/ve/lib/python2.5/site-packages/django/core/urlresolvers.py" in reverse
  265.             *args, **kwargs)))
File "/home/nybooks/ve/lib/python2.5/site-packages/django/core/urlresolvers.py" in reverse
  238.         possibilities = self.reverse_dict.getlist(lookup_view)
File "/home/nybooks/ve/lib/python2.5/site-packages/django/core/urlresolvers.py" in _get_reverse_dict
  165.                     for name in pattern.reverse_dict:
File "/home/nybooks/ve/lib/python2.5/site-packages/django/core/urlresolvers.py" in _get_reverse_dict
  173.                     lookups.appendlist(pattern.callback, (bits, p_pattern))
File "/home/nybooks/ve/lib/python2.5/site-packages/django/core/urlresolvers.py" in _get_callback
  134.             raise ViewDoesNotExist, "Could not import %s. Error was: %s" % (mod_name, str(e))

Exception Type: ViewDoesNotExist at /admin/
Exception Value: Could not import solango.views. Error was: cannot import name settings

Any ideas on what's causing the problem, or at least how I can go about debugging it?

Answer 1

Your web server is out of file descriptors. Reconfigure mod_wsgi for daemon mode.

Answer 2

You could have order dependencies in what code is imported. This doesn't show up in development servers as a lot of stuff is preloaded which isn't under production WSGI server.

So ultimately it is like how you are using Django. In the interim you can try advice in:

http://blog.dscpl.com.au/2010/03/improved-wsgi-script-for-use-with.html

for alternate content for WSGI script file and see if problem goes away.