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Django/python - render from template throws NoReverseMatch exception from statement {% url views.hello_world %}

I'm following along in the course but for curiosities sake I skipped over to the "final" version of the files as distributed and ease of discussion of this problem.

in home.html

{% extends "layout.html" %}

{% block title %}Well hello there!{% endblock %}

{% block content %}
<h1>Welcome to python h%&&! Prepare to die()</h1>

{% endblock %}

in layout.html

{% load static from staticfiles %}
<!doctype html>
        <title>{% block title %}{% endblock %}</title>
        <link rel="stylesheet" href="{% static 'css/layout.css' %}">
        <div class="site-container">
              <a href="{% url views.hello_world %}">Home</a> 
               <a href="{% url 'courses:list' %}">Courses</a>
            {% block content %}{% endblock %}
from django.shortcuts import render
import pdb

def hello_world(r):
    return render(r, "home.html")

in urls,

from django.conf.urls import include, url
from django.contrib import admin
from django.contrib.staticfiles.urls import staticfiles_urlpatterns

#from . import views
from learning_site import views
from learning_site import urls
from learning_site.views import hello_world

urlpatterns = [
    url(r'^$', views.hello_world )

urlpatterns += staticfiles_urlpatterns()

in views.py

from django.shortcuts import render

import pdb

def hello_world(request):
    return render(request, "home.html" )

browsing to localhost:8000, I get a

NoReverseMatch at / Reverse for " with arguments '()' and keyword argument '{}' not found. 0 patterns tried

Looking at the stack trace it looks like it's failing on render in resolvers.py in _reverse_with_prefix()

(see http://dpaste.com/0S3XK24)

Hence after hitting the first breakpoint I added the next one with

from django.core.urlresolvers import reverse
from django.core.urlresolvers import RegexURLResolver
break RegexURLResolver._reverse_with_prefix

In _reverse_with_prefix() they are doing something like

foo = self.reverse_dict.getlist(view)

however it returns an empty list

calling this manually after the breakpoint I see that I can try

foo = self.reverse_dict.getlist(view.hello_world) 

which seems to do the same thing (IE empty set back)

on the other hand, if I first do a

from learning_app.views import views 

while in pdb at the breakpoint

I see that it appears to return the correct list

Searching a bit on the web, I see that the suggested answer for this is to include a namespace in the original template

something like {%s url namespace:view %}

but what namespace ?? learning_app:view ? do I need to say {%s url 'learning_app:views.hello_world' %s}

I'm using python 3.5, with django -1.10a1-py3.5.egg in ubunto 16.04 latest which is in a vm (hence direct cut and paste are broken back to host o/s per vmware)

Nursultan Bolatbayev
Nursultan Bolatbayev
16,774 Points

Could u use Markdown Cheatsheet as ur code is unreadable.

2 Answers

Myers Carpenter
Myers Carpenter
6,421 Points

I didn't try to copy and paste your code and run it, but reading over it I would guess that

<a href="{% url 'courses:list' %}">Courses</a>

is causing the problem, because you don't have "courses:list" in the urls.py file. I don't know why you aren't getting a more clear error message.

If you want to get more help I would suggest putting your code up on a github, so that others can clone your repo and run the code and see the same error you are getting.

I solved it by changing the home view to have a name so instead of {% url views.hello_world %} I used {% url 'home' %}

working my way through the stack, It looks like the argument to url does not seem to make it all the way down when using the first form of the {% url object %} expression

the problem is present in the original .zip file for the class in "Final" so perhaps some semantics have changed in the version of Django I happened to use