Why does this URLConf/reverse() combination, with duplicate named url entries, work? What is the correct way to do this?

Question

• URLConf:

  url(r'^body/(?P<id>\d+)/$', 'body_part_detail', name='body-part-detail'),
  url(r'^body/(?P<id>\d+)/(?P<slug>[-\w\d]+)/$', 'body_part_detail', name='body-part-detail'),
  

• get_absolute_url() implementation:

  def get_absolute_url(self):
      kwargs = {
          'id' : self.id,
          'slug' : slugify(self.name)
      }
  
      return reverse('body-part-detail', kwargs=kwargs)
  

Now, I've got two urls where name=body-part-detail (which may be wrong?). Nevertheless, reverse succeeds in the sense that it uses the second url and returns the desired URL with the slug component. Why does this work? Am I just "lucky" in this case (quotes because programs shouldn't be lucky!)?

Finally, there are several places on SO and the Web where people give examples of URLConfs with urls that share name keyword arg. For example, Visgean Skeloru's answer here: Optional get parameters in django?. Is this wrong/bad?

Answer 1

The reverse function is identifying that you have the kwargs id and slug along with a pattern name body-part-detail, and returning the only url that corresponds to those kwargs and that name. You're not lucky, that's what reverse is designed to do.

You may be able to use the same name for numerous patterns but I don't think that's how it was designed to work (I can see a very old bug ticket that suggested implementing a check to ensure there aren't any duplicates). I suspect that your app will be easier to maintain if you use a unique name for each pattern. Have a look at the docs for me more info.

Answer 2

In your reverse() call you're passing two arguments. So Django discovers that it can't match it to the first URL because, although the name is correct, that URL only has one URL parameter. So Django necessarily matches it to the second URL.

Optional URL patterns are okay but it's best to give each URL a unique name and use that to reverse URLs. That removes any ambiguity when resolving URLs and it also makes the code easier to follow for yourself and other developers.