What is reverse()?

django

When I read Django code sometimes, I see in some templates reverse(). I am not quite sure what this is but it is used together with HttpResponseRedirect. How and when is this reverse() supposed to be used?

reverse() | Django documentation

Let's suppose that in your urls.py you have defined this:

url(r'^foo$', some_view, name='url_name'),

In a template you can then refer to this url as:

<!-- django <= 1.4 -->
<a href="{% url url_name %}">link which calls some_view</a>

<!-- django >= 1.5 or with {% load url from future %} in your template -->
<a href="{% url 'url_name' %}">link which calls some_view</a>

This will be rendered as:

<a href="/foo/">link which calls some_view</a>

Now say you want to do something similar in your views.py - e.g. you are handling some other URL (not /foo/) in some other view (not some_view) and you want to redirect the user to /foo/ (often the case on successful form submission).

You could just do:

return HttpResponseRedirect('/foo/')

But what if you want to change the URL in the future? You'd have to update your urls.py and all references to it in your code. This violates the DRY (Don't Repeat Yourself) principle and the whole idea of editing in one place only - which is something to strive for.

Instead, you can say:

from django.urls import reverse
return HttpResponseRedirect(reverse('url_name'))

This looks through all URLs defined in your project for the URL defined with the name url_name and returns the actual URL /foo/.

This means that you refer to the URL only by its name attribute - if you want to change the URL itself or the view it refers to you can do this by editing one place only - urls.py.

Existing answers did a great job at explaining the what of this reverse() function in Django.

However, I'd hoped that my answer shed a different light at the why: why use reverse() in place of other more straightforward, arguably more pythonic approaches in template-view binding, and what are some legitimate reasons for the popularity of this "redirect via reverse() pattern" in Django routing logic.

One key benefit is the reverse construction of a url, as others have mentioned. Just like how you would use {% url "profile" profile.id %} to generate the url from your app's url configuration file: e.g. path('<int:profile.id>/profile', views.profile, name="profile").

But as the OP have noted, the use of reverse() is also commonly combined with the use of HttpResponseRedirect. But why?

I am not quite sure what this is but it is used together with HttpResponseRedirect. How and when is this reverse() supposed to be used?

Consider the following views.py:

from django.http import HttpResponseRedirect
from django.urls import reverse

def vote(request, question_id):
    question = get_object_or_404(Question, pk=question_id)
    try:
        selected = question.choice_set.get(pk=request.POST['choice'])
    except KeyError:
        # handle exception
        pass
    else:
        selected.votes += 1
        selected.save()
        return HttpResponseRedirect(reverse('polls:polls-results',
                                    args=(question.id)
        ))

And our minimal urls.py:

from django.urls import path
from . import views

app_name = 'polls'
urlpatterns = [
    path('<int:question_id>/results/', views.results, name='polls-results'),
    path('<int:question_id>/vote/', views.vote, name='polls-vote')
]

In the vote() function, the code in our else block uses reverse along with HttpResponseRedirect in the following pattern:

HttpResponseRedirect(reverse('polls:polls-results',
                                        args=(question.id)

This first and foremost, means we don't have to hardcode the URL (consistent with the DRY principle) but more crucially, reverse() provides an elegant way to construct URL strings by handling values unpacked from the arguments (args=(question.id) is handled by URLConfig). Supposed question has an attribute id which contains the value 5, the URL constructed from the reverse() would then be:

'/polls/5/results/'

In normal template-view binding code, we use HttpResponse() or render() as they typically involve less abstraction: one view function returning one template:

def index(request):
    return render(request, 'polls/index.html')

But in many legitimate cases of redirection, we typically care about constructing the URL from a list of parameters. These include cases such as:

HTML form submission through POST request
User login post-validation
Reset password through JSON web tokens

Most of these involve some form of redirection, and a URL constructed through a set of parameters. Hope this adds to the already helpful thread of answers!

The existing answers are quite clear. Just in case you do not know why it is called reverse: It takes an input of a url name and gives the actual url, which is reverse to having a url first and then give it a name.

This is an old question, but here is something that might help someone.

From the official docs:

Django provides tools for performing URL reversing that match the different layers where URLs are needed: In templates: Using the url template tag. In Python code: Using the reverse() function. In higher level code related to handling of URLs of Django model instances: The get_absolute_url() method.

Eg. in templates (url tag)

<a href="{% url 'news-year-archive' 2012 %}">2012 Archive</a>

Eg. in python code (using the reverse function)

return HttpResponseRedirect(reverse('news-year-archive', args=(year,)))

The function supports the dry principle - ensuring that you don't hard code urls throughout your app. A url should be defined in one place, and only one place - your url conf. After that you're really just referencing that info.

Use reverse() to give you the url of a page, given either the path to the view, or the page_name parameter from your url conf. You would use it in cases where it doesn't make sense to do it in the template with {% url 'my-page' %}.

There are lots of possible places you might use this functionality. One place I've found I use it is when redirecting users in a view (often after the successful processing of a form)-

return HttpResponseRedirect(reverse('thanks-we-got-your-form-page'))

You might also use it when writing template tags.

Another time I used reverse() was with model inheritance. I had a ListView on a parent model, but wanted to get from any one of those parent objects to the DetailView of it's associated child object. I attached a get__child_url() function to the parent which identified the existence of a child and returned the url of it's DetailView using reverse().

What is reverse()?

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