Reverse for '*' with arguments '()' and keyword arguments '{}' not found

Caught an exception while rendering:

Reverse for 'products.views.'filter_by_led' with arguments '()' and keyword arguments '{}' not found.

I was able to successfully import products.views.filter_by_led from the shell and it worked so the path should be correct.

Here is the urls.py:

(r'^led-tv/$', filter_by_led ),

This is where the error is being generated:

href="{% url products.views.filter_by_led %}">

Which I can't understand because this works fine from the same file:

{% url products.views.lcd_screen_size screen_size=50 %}

Here is the function definition:

def filter_by_led(request):

I don't understand why Django would think that the function would not be able to find the Reverse for that function.

I deleted all the *.pyc files and restarted Apache.

What am I doing wrong?

There are 3 things I can think of off the top of my head:

1. Just used named urls, it's more robust and maintainable anyway

2. Try using django.core.urlresolvers.reverse at the command line for a (possibly) better error

   >>> from django.core.urlresolvers import reverse
   >>> reverse('products.views.filter_by_led')
   

3. Check to see if you have more than one url that points to that view

Shell calls to reverse (as mentioned above) are very good to debug these problems, but there are two critical conditions:

• you must supply arguments that matches whatever arguments the view needs,
• these arguments must match regexp patterns.

Yes, it's logical. Yes, it's also confusing because reverse will only throw the exception and won't give you any further hints.

An example of URL pattern:

url(r'^cookies/(?P<hostname>[^/]+)/(?P<url_id>\d+)/$', 'register_site.views.show_cookies', name='show_cookies'),

And then what happens in shell:

>>> from register_site.views import show_cookies
>>> reverse(show_cookies)
NoReverseMatch: Reverse for 'register_site.views.show_cookies' with arguments '()' and keyword arguments '{}' not found.

It doesn't work because I supplied no arguments.

>>> reverse('show_cookies', kwargs={'url_id':123,'hostname': 'aaa'})
'/cookies/aaa/123'

Now it worked, but...

>>> reverse('show_cookies', kwargs={'url_id':'x','hostname': 'www.dupa.com'})
NoReverseMatch: Reverse for 'show_cookies' with arguments '()' and keyword arguments '{'url_id': 'x', 'hostname': 'www.dupa.com'}' not found.

Now it didn't work because url_id didn't match the regexp (expected numeric, supplied string).

You can use reverse with both positional arguments and keyword arguments. The syntax is:

reverse(viewname, urlconf=None, args=None, kwargs=None, prefix=None, current_app=None)

As it comes to the url template tag, there's funny thing about it. Django documentation gives example of using quoted view name:

{% url 'news.views.year_archive' yearvar %}

So I used it in a similar way in my HTML template:

{% url 'show_cookies' hostname=u.hostname url_id=u.pk %}

But this didn't work for me. But the exception message gave me a hint of what could be wrong - note the double single quotes around view name:

Reverse for ''show_cookies'' with arguments...

It started to work when I removed the quotes:

{% url show_cookies hostname=u.hostname url_id=u.pk %}

And this is confusing.