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Fixing the TemplateDoesNotExist error AppEngine/Django 1.2

When I converted my blogging application from Django 0.96 templates to 1.2, I encountered three problems that set me back a few days: my custom template tag libraries stopped working, all of the output had its HTML content escaped and all of my templates that used extends or include broke. I described how I resolved the first two issues in my most recent two posts, and I have been planning to cover the third issue, but I procrastinated.

Yesterday, a fellow AppEngine programmer posted a question on StackOverflow about the exact problem that I had fixed, so I bit the bullet and wrote up a lengthy answer describing my solution. I hope that someday soon I might have the time required to cover the subject matter more comprehensively, but for the time being, here is the question with my answer on StackOverflow.

You'll know that you care about this if your AppEngine application has suddenly started throwing TemplateDoesNotExist exceptions.

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Be Safe with Django 1.2

 

Transitioning to using Django 1.2 templates with AppEngine has been a real adventure, and this is just the second in a series of posts about my experience with it. I am glad to have the increased power that is in the newest version of the template language, but I have paid a steep price in hours spent making changes to be compatible with it.

For a good chunk of last night and spots of time here-and-there today, I have been going through my code and adding the |safe filter to variables that might contain HTML that needs to be rendered. Django 1.2 assumes that you want your variables to be escaped. While that is a good practice to prevent spammers and script kiddies from abusing your site, I always assume that I need to sanitize on input rather than output, and I consider all of my stored data to be safely renderable.

There are a couple of ways to prevent Django from escaping your variables. I have chosen to append the |safe filter. Why? Well, it is safer. As I said, I sanitize on input, but there’s always a chance that I will have overlooked something, and ...

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Fixing Custom Tags with Django 1.2

When I upgraded to version 1.4.2 of the AppEngine SDK, I was both intrigued and alarmed to see the warning message that I ought to be using use_library() to specify a version of Django that my application would use. On one hand, I was glad that I could switch to version 1.2, as it provides dramatically-improved if tags over 0.96. On the other hand, I knew that I would be in for a certain amount of pain because things were going to break. The first thing that broke for me was the call to template.register_template_library that my code uses to load a library of custom template tags. It crashed with the extraordinary helpful stack trace:

  webapp.template.register_template_library('MVCTags')
File "C:\google_appengine\google\appengine\ext\webapp\template.py", line 269, in register_template_library
   django.template.add_to_builtins(package_name) 
File "C:\google_appengine\lib\django_1_2\django\template\__init__.py", line 1049, in add_to_builtins
   builtins.append(import_library(module))
File "C:\google_appengine\lib\django_1_2\django\template\__init__.py", line 984, in import_library
   app_path, taglib = taglib_module.rsplit('.',1)
ValueError: need more than 1 value to unpack 

After a great deal of stepping through code in Wing IDEs superb Debugger, I figured out ...

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