Passing Pk or Slug to Generic DetailView in Django?

url patterns are checked in the order you define them

so here:

urlpatterns = [
        url(r'(?P<post_id>[^/]+)', GenreDetail.as_view(), name = 'post'),
        url(r'(?P<post_id>[^/]+)/(?P<slug>[-\w]+)$', GenreDetail.as_view()),
    ] 

...the first pattern is getting matched (because it does not end with $ so the extra segment is just ignored)

...and that pattern only passes a single keyword arg

Generally it is a bad idea to have multiple url patterns pointing to the same view. If possible you should try and make a single regex (eg using optional groups) which handles the various cases of the url for a particular view. It's more explicit that way.

On the other hand, simply reversing the order of your patterns to put the more explicit one first would also work and be correct (this is the Django rule of urlpatterns!)

urlpatterns = [
        url(r'(?P<post_id>[^/]+)/(?P<slug>[-\w]+)$', GenreDetail.as_view()),
        url(r'(?P<post_id>[^/]+)', GenreDetail.as_view(), name = 'post'),
    ] 

As @ozgur mentions you also need to tell the view to use post_id instead of pk by setting pk_url_kwarg

If you want to fetch details using either post_id or slug then your urls should be like this

url(r'post/(?P<post_id>\d+)/$', GenreDetail.as_view(), name = 'post_detail'),
url(r'post/(?P<slug>[-\w]+)/$', GenreDetail.as_view(), name = 'post_detail_slug'),

And your view should be like this

from django.views.generic import DetailView 

class GenreDetail(DetailView):
    model = Post
    template_name = "post.html"
    pk_url_kwarg = "post_id"
    slug_url_kwarg = 'slug'
    query_pk_and_slug = True

For more details please read the docs.