The problem is, when Django attempts to load a custom tag library (in source:django/trunk/django/template/__init__.py, in the get_library() function), it traps all ImportError's, regardless of where they are thrown from:
try:
mod = __import__(module_name, {}, {}, [''])
except ImportError, e:
raise InvalidTemplateLibrary, "Could not load template library from %s, %s" % (module_name, e)
An example...
If I have a "view" like so:
def my_view(request):
return render_to_response('my.tpl', {})
...And a corresponding template:
{% load tag_lib %}
<h1>my template</h1>
...
...Now, here's the tag library tag_lib:
import notthere
...
...Invoking our view, now, will yield a not-very-easy-to-understand (unless you already know the problem) error message:
TemplateSyntaxError at /my_view
'tag_lib' is not a valid tag library: Could not load template library from django.templatetags.tag_lib, No module named notthere
Request Method: GET
Request URL: http://localhost:8000/my_view
Exception Type: TemplateSyntaxError
Exception Value: 'tag_lib' is not a valid tag library: Could not load template library from django.templatetags.tag_lib, No module named notthere
Exception Location: /usr/lib/python2.4/site-packages/Django-0.95-py2.4.egg/django/template/defaulttags.py in load, line 692
You may need to look closely to notice that the root of the problem was that our tag library tried to load a module notthere; the problem was not that we tried to load a non-existant tag library.
This is essentially the same message that you will get when trying to load a tag library that is completely non-existant - i.e., if our template were:
{% load not_tag_lib %}
<h1>my template</h1>
...
We would now see the error message:
TemplateSyntaxError at /my_view
'not_tag_lib' is not a valid tag library: Could not load template library from django.templatetags.not_tag_lib, No module named not_tag_lib
...
The type of error message that I would like to see would be one that is not obfuscated and wrapped in a TemplateSyntaxError exception. Such a useful error message will be displayed if some other problem occurs in our tag library; for instance, if our tag library looks like
blablabla
...
then we will be greeted with the slightly-more-decipherable error message,
NameError at /my_view
name 'blablabla' is not defined
Request Method: GET
Request URL: http://localhost:8000/my_view
Exception Type: NameError
Exception Value: name 'blablabla' is not defined
Exception Location: /usr/lib/python2.4/site-packages/Django-0.95-py2.4.egg/django/template/__init__.py in get_library, line 880
I'm not certain that there's a way around this (I'm relatively new to Python), but I suspect it is possible to resolve by (somehow) determining which file/line the ImportError? originated from/on, and adjusting the error message that is raised in the get_library() function. I'd be willing to spend time on a patch if I you're all willing to accept one. :)
