This cartoon wrote a sweary word on your toilet wall

I don't normally blog cartoons but this one made me pause for thought and laugh.

Django - Named URLS Gotcha

A post in two parts.

First of all - thanks to Magus on #django for pointing out one of my errors to me. I've been working through Practical Django Projects over the last couple of days and it talks about having named urls. Take a look at these three one liners:

# from urls.links.py 

    (r'^$', 'archive_index',link_info_dict, 'coltrane_link_archive_index'), 

# from urls.py

    (r'^weblog/links/', include('coltrane.urls.links')),

# In my template I get the url using 

  {% url coltrane_link_archive_index  %}

We give our view a name (coltrane_link_archive_index), then we call that view with urls.py so it's context will be /weblog/links - and then in our template we can just use the url tag to call that named url. If the url gets moved, your templates will still work - and of course it saves a lot of tedious typing. Great.

So working through the book, the blog application requires a lot of templates and views and stuff. So I thought I'd just do the main one and get the minor ones working later. But I did check to make sure that links to those views were working and they were not.

Written like this, the problem is obvious, a named view won't work if the view it's pointing to doesn't work. So if your url tag's don't function - make sure what they should be taking you to are ok.

But there's more. Because when I wrote the underlying views it still wasn't working. And I discovered something interesting. I had a problem last night after restarting the web server - The first time I'd go to a page I'd get

ImproperlyConfigured: Error while importing URLconf 'coltrane.urls.tags': name 'Link' is not defined

For some reason I couldn't work out why that was happening last night. I have no idea why I could not - it's blatantly obvious. I wasn't doing an import of the Link model for my tags url file. But I was ignoring it yesterday because I found that if you just asked for the page again it then displays (not the tags page, I hadn't written that, but the rest of the site seemed to be working. I put it down to some strange 'glitch'. It wasn't.

I'm guessing here somewhat but I think what happens is the first time you load your site up, Django precompiles the regular expressions used for your urls. During that compile if it hits an error you'll have some urls working (up to where it failed) and some not. The second time you hit the site that doesn't happen (it thinks the regular expression compilation has completed) and doesn't raise the error again. But of course all the urls that failed won't be working (hence why url tag wasn't working).

On a final note, you'd think by now that I would have learnt that when you have a 'strange glitch' there's something bigger lurking there that you really need to understand.

NewForms Admin - Flatpages

Practical Django Projects (Chapter three), talks about defining an admin interface which is all well and good unless you're using the NewForms Admin branch (which is intended to become part of Release 1.0). So if you're an early adopter or reading this after the big change, you'll be needing to do the following.

The reason I bothered writing this was I couldn't work out how to override the fact that FlatPage had already registered a ModelAdmin (try to register yours throws an error). A pointer from #django on irc and a look at the source code revealed that unregister was the baby you wanted.

from django.contrib import admin
from search.models import SearchKeyword
from django.contrib.flatpages.models import FlatPage

class SearchKeywordInline(admin.TabularInline):
    model = SearchKeyword
    
class FlatPageAdmin(admin.ModelAdmin):
    inlines = [
        SearchKeywordInline,
        ]

# We have to unregister it, and then reregister
admin.site.unregister(FlatPage)
admin.site.register(FlatPage, FlatPageAdmin)