here is my code(btw, i was completed fork by your digital book )
and it occured the error like the bottom
django.core.urlresolvers.NoReverseMatch: Reverse for 'show_category' with arguments '('',)' and keyword arguments '{}' not found. 1 pattern(s) tried: ['rango/category/(?P<category_name_slug>[\w\-]+)/$']
it confused me three days, i don't know why it happend, and i hope the writer can see it.
thanks anyway