NoReverseMatchError, path, and args vs. kwargs

Using Django
1

Hi everyone,

I have the following view: 
    def update_cart(request, id):
    cart = Cart.objects.all()
    product = Product.objects.get(pk=id)
    cart.products.add(product)
    return HttpResponseRedirect(reverse("update_cart_url", args=(product.id)))

And URL pattern in my project level urls.py (after importing update_cart from carts.views):
        path('cart/<int:product.id>/', update_cart, name="update_cart_url")

Which are called in a template: 
    <h3>{{ product.name}}<a href=" {%  url 'update_cart_url' product.id %}">Add to Cart</a></h3>

When rendering the template, I receive the following error: 
    NoReverseMatch at /webCart/detail/6/
Reverse for 'update_cart_url' with arguments '(6,)' not found. 1 pattern(s) tried: ['cart/<int:product\\.id>/$']

Note that the argument 6 above correctly matches the id of the item that the template was attempting to render.

There are lots of resources online in respect to this exception, and reading through them I believe I narrowed down my problem to several possibilities:

  1. I am using args when I should be using kwargs or vice versa

  2. I am attempting to reverse a URL by using path instead of url in urls.py

  3. Should the url pattern be defined within the app directory rather than the project directory?

Does anyone have any ideas?

2

Dots aren’t supported in url pattern variable names. Use an underscore instead and your problem should be solved.

1 Like
3

I believe the issue may be here:

The url just contains an integer, which you can then use as the parameter being passed to your view to retrieve the specific object to which it refers.

try:

path('cart/<int:id>/', update_cart, name="update_cart_url")

Also, and I’m not sure if it makes a difference here, but:

I believe args needs to be an iterable. This is one of those python gotchas that gets me all-too-often. Try either:

return HttpResponseRedirect(reverse("update_cart_url", args=(product.id,)))

or

return HttpResponseRedirect(reverse("update_cart_url", args=[product.id]))

In the first example, adding the trailing comma forces args to be a tuple and not just the singular value. In the second, you’re definitely passing a list - both of which are iterables.

Ken

1 Like
5

Thank you both, this is super helpful. The he tutorial is helpful as well.

6

So Django wasn’t parsing your URL attempt correctly - it would see the dot in <int:product.id> and “skip over” trying to convert it. It therefore didn’t hit its built-in warning that you’ve typed something that’s not a valid identifier.

I’ve made a ticket and PR to try fix this for future versions in Django, so others won’t hit the same problem again :slight_smile:

@KenWhitesell was right about the brackets for the tuple as well. I prefer using lists wherever possible for this reason. See my post.

1 Like